|Author:||Edwin Dunkin, Assistant at the Royal Observatory|
|Title:||A day at the Royal Observatory & A night at the Royal Observatory|
|About:||These two accounts were both published in The Leisure Hour, Vol XI (London), 1862. A day at the Royal Observatory was published in two parts, the first in edition No.524, 9 Jan 1862, pp. 22–26 and the second in edition No.525, 16 Jan 1862, pp.39–43. A night at the Royal Observatory was published in edition No. 526, 23 Jan 1862, pp.55–60. Dunkin's articles followed on directly from one on the Observatory's Origin and History, which had been written by James Glaisher and published in edition No.523, 2 Jan 1862, pp. 7–11. Dunkin's account was later partially updated and published in 1891 in a revised and extended edition of his book The Midnight Sky.|
[From The Leisure Hour, Vol XI (London), No.524, Jan 9,1862. pp. 22–26, Note: the illustrations will be added when time allows]
THE ROYAL OBSERVATORY GREENWICH.
A DAY AT THE OBSERVATORY.
BY EDWIN DUNKIN ESQ., F.R.A.S.
Few visitors of intelligence to the Royal Park of Greenwich, after enjoying the picturesque grassy slopes, and shaded avenues of elms and chestnuts, for which the park is so justly celebrated, have, we venture to assert, returned to the busy scents of the metropolis, without having felt some desire to become acquainted with the scientific processes carried on within the walls of the Royal Observatory. True, there is nothing imposing in the external appearance of this noble institution, with its architecture of brick towers and minarets, of sufficient novelty to attract the attention of the visitor; but, not withstanding the superior education of modern times, the almost superstitious feeling in the mind of the public which still attaches to the labours of the astronomer, will always involuntarily draw upon the imagination, which can only be satisfied by some kind of initiation into the mysteries performed in the Observatory.
So far, then, as regards the interior of the Royal Observatory, we will endeavour to describe popularly what may be seen by a visitor; the use made of some of the principal instruments; and also to state a few of the results to science, and to the nation, which have been obtained from the small but busy staff, whose occupation, by one or other of its members, is carried on throughout the twenty-four hours.
It may be proper to mention in this place, that the admission of visitors to the Observatory is necessarily placed under very strict rules; in fact, no person, unless of some scientific reputation, or by an introduction by a well-known astronomer, can ever hope to be admitted within the entrance gate. For this reason, therefore, it is hoped that the reader will accompany us through the establishment, relying upon the information which we are enabled to give, which will afford him some comprehension for the exclusive rules which it is necessary to enforce, for the preservation of order in the institution.
At what object is the attention of that group of Greenwich Park visitors so earnestly directed, near the entrance gate? A clock in the wall? Surely there is nothing extraordinary in looking at a clock. But the curiosity of the group is somewhat excited because the hours on the dial are marked continuously from 0 to 24, and because on its face it is called a "galvano-magnetic clock." It is really oftentimes very amusing to hear the voluntary explanations of some one of the group more learned than the rest, endeavouring to impress on his attentive audience the real meaning and use of this mystical time-piece.
However, we may as well inform those of our readers who intend visiting Greenwich Park, that this public clock is only one of a number of sympathetic galvanic clocks in the Observatory, kept in motion and regulated by a motor or parent clock, with which they are connected by wires through which the galvanic or electric current passes. These clocks will be further explained when we have to speak specially on this branch of the establishment.
It is now time to ring the bell at the humble entrance gate. It is opened by the porter, who, for a considerable period, has always been a pensioner from the sister institution, the Royal Hospital for Seamen.
On entering into the courtyard, the first objects presented to the eye are the Astronomer-Royal's private apartments, and the more ancient part of the Observatory, consisting of the octagon room, etc., originally constructed after the designs of Sir Christopher Wren; to the left is the range of low buildings, whose official character is marked by the rails which separate them from the more common potions of the court. The oldest of these latter buildings dates from about one hundred and twenty years back, the others being added occasionally at a later period, as the increase of the instruments required. In these unpretending rooms most of the astronomical observations are made, reduced, and prepared for the press.
Before entering, into the separate apartments, we will ascend to the leads on the highest part of the Observatory, in order to obtain at one view the extent of the establishment. From this spot the locality of each instrument can be pointed out-in fact, the ground plan of the building can be seen to great advantage. But what would probably interest the visitor at this moment far more than gazing down upon the roofs of certain old-fashioned dwellings, is the magnificent landscape visible on all sides. If we cast our eyes towards the north, the magnitude of the great metropolis, enveloped in the produce of its own chimneys, the noble hospital, and the winding Thames, which is visible for miles, covered with its shipping, are all seen together. If we turn to the south and west, the beautiful hills of Kent and Surrey form a pleasing variety to the scene; whilst in the east, Shooter's Hill, with its mimic fortress of Severndroog, is a conspicuous object; and last, but not least, immediately below us lies the park itself; yielding to none in freshness and beauty.
After enjoying this panoramic view, it is time to descend and commence our inspection of the different apartments. The first to engage our attention is the transit-circle room, in which is mounted the principal instrument in daily use. This room is the first on the left after entering the Observatory gate. Before looking at the transit-circle, let us cast a glance on some of those ancient instruments suspended on the walls; for most of those formerly used in various astronomical researches, but now, of course, out of use, and obsolete, are exhibited in this room. Indeed, the history of the improvement in the construction of these instruments is very strikingly illustrated. For example, look at the curious transit of Dr. Halley; mark the great improvement of that of Dr. Bradley; then that great stride made soon after the accession of Mr. Pond; and lastly, look around at the noble transit-circle now in use, and we must exclaim that in this science, like every other, the nineteenth century has witnessed improvement, of which our forefathers never bad any conception. But in a historical view, perhaps the most interesting or these astronomical curiosities is the zenith sector of Bradley, which was made for him in 1727, by Graham, and fixed at Wanstead. It was purchased by the Government in 1749. Every person who claims even the slightest acquaintance with astronomy, must possess some veneration for an instrument, by the aid of which Bradley was enabled to discover the aberration of light, and the nutation of the earth's axis.
The transit-circle is one of the finest meridional instruments in the world. Those which can compete with it are simply copies of itself – one at the Cape of Good Hope, and another at Cadiz.
This instrument is placed in the meridian of Greenwich, and is capable of being moved only in the direction of north and south; in fact, the middle wire in the eye-piece of the telescope is the meridian line from which the arcs of longitudes on British maps are reckoned. In illustration we may notice that one half of the observing room is in the eastern hemisphere, and the other half in the western.
The transit-circle is an embodiment of two well-known astronomical instruments, as its name implies – viz., the transit and the mural circle. These two instruments were in use at the Observatory till the end of the year 1850, the first observation with the transit-circle being made in January, 1851. The observations are therefore of two kinds – transits of stars, and measures of angular distance in declination. The first consists in noting or observing, the clock time when a celestial object passes the system of wires, which are simply lines of spiders' web stretched across the view in the eye-piece. By these observations the observer is enabled to find the exact error of the clock which has been employed, from which can be deduced correct time. He is thus able to communicate, by means of the electric wires, exact Greenwich time to the world. But there are other important astronomical results deduced from these transits. They afford the means of fixing the positions of, the stars in right ascension, after the observations are properly reduced by the computer. But secondly, the transit-circle has also to perform the office of the old mural circle, by observing or fixing the positions of the stars in right ascension, after the observations are properly reduced by the computer. But secondly, the transit-circle has also to perform the office of the old mural circle, by observing or fixing the positions of the stars in declination. The graduated circle attached to the instrument is provided with a narrow band of silver let into the iron, of which the instrument is constructed, on which is engraved thin lines at very minute distances from each other. These lines or divisions are read by powerful microscopes, so that great accuracy can be attained in the observations.
The reader will bear with us for a minute, while we explain in a few words the meaning of right ascension and declination, which are the two elements determined by this instrument. Right ascension is the angular distance of any heavenly body, measured on the celestial equator from an imaginary point called the first point of Aries; and its angular distance north or south of the equator is called its north or south declination. Determinations of these elements by the transit-circle enable the astronomer to fix the positions of the planets and stars, in order that they may be arranged and catalogued for the benefit of others, whether astronomers or mariners.
This instrument is constructed with the intention that no fault should possibly arise from want of solidity. The telescope, which is twelve feet in length, and its axis six feet, is formed of iron, cast in about four pieces, the separate parts being afterwards firmly bolted together. The object-glass is eight inches in diameter, being sufficiently powerful for the observation of faint planetoids. The discovery of so many of these small bodies, now in number amounting to seventy-one has added considerably to the daily labour of the Observatory.
It must be noticed that as this instrument has only an up and down motion confined to the meridian, the observer must wait until the object to be observed is on the meridian. In this manner the sun, moon, and planets are daily observed, together with stars, to determine the correct Greenwich time, an element which enters largely into the daily business of the place. In addition to these, stars for special purposes are always on the list for observation.
The methods of observation with the transit-circle will be described when the duties of the Observatory at night are under consideration.
Before leaving the room, however, we must notice the transit-clock, which is the fundamental measurer of time at the establishment, being regulated daily by observation of stars, the transits of which have been observed over the wires of the instrument we have been just describing. The first instinct of the non-scientific visitor at the sight of this clock, is to pull out his watch and note how much it is fast or slow of true Greenwich time, which this principal clock ought most assuredly to show. But mark in a moment the hesitation plainly visible on his countenance. The clock and the watch differ by several hours! A word of explanation soon accounts for the apparent anomaly, and our visitor is himself again. This clock being regulated by the stars, is made to keep star time, known as sidereal time. The sidereal day being nearly four minutes shorter than a mean solar day, the clock which indicates sidereal time will gain nearly four minutes over ordinary clocks in one day; or in the course of a year twenty-four hours will be thus gained. No wonder, then, that our visitor found a difference between his watch and this clock. A very slight computation is sufficient to convert sidereal time into mean solar, or ordinary clock time. The transit-clock was constructed by Hardy, a clockmaker of repute in the last century. Improvements and alterations of various kinds have, however, been made in it during the last thirty years.
It is not proper to remain in this apartment for too great a length of time, though there is sufficient here to occupy our thoughts a little while longer, for other departments of the establishment must be visited, and our space forbids too much detail in our descriptions; we will accordingly again enter the courtyard, pass on the left two rooms, which, during the day, are always full of life and vigour, and in which a portion of the astronomical computations and all business correspondence are performed; then enter the passage which leads to the altazimuth.
A word or two must be said here about two old instruments which are suspended on the two sides of the western wall of the passage. These are Bradley's quadrant, on the wall in the passage, and Graham's quadrant, on the other side, in a room devoted to the care of manuscripts. No person can look at these remarkable instruments in comparison with those now in use, without again noticing the great improvement which has taken place in the present century, of every kind of astronomical instrument. The observations made by the use of Bradley's quadrant have, however, been fully reduced under the direction of the present Astronomer-Royal, and show by their results, though not equal to those of modern days, that the honour of the Observatory has been in no way tarnished by that very old-looking but venerable instrument, now suspended and admired as a valuable historical curiosity.
The altazimuth is placed in a dome erected on the site of what was formerly called the advanced building, where the mural arc of Flamsteed stood. We ascend by means of a staircase, which is partly carried round a three-rayed pier, which serves as a foundation for the instrument. The altazimuth was designed by the Astronomer-Royal for the observation of celestial objects in altitude and azimuth, by which their positions at a given moment can be accurately determined. The purpose, however, to which it is chiefly applied is the observation of the moon daily, if possible, in order to obtain observations of our satellite in larger numbers than are usually made with the transit-circle. The results of these observations are used for the improvement of the theory of the lunar orbit. From experience it is found that observations can be made from month to month in almost every part of the orbit, from which materials are obtained for a comparison of observed positions of the moon, with places calculated from tables formed from theoretical investigations. Similar observations are always made with the transit-circle, but, from the peculiar construction of that instrument, these are confined to the times of meridional transit. Now these transits cannot be observed on the meridian, even when the firmament is unclouded, for at least four days before, and four days after the new moon, in consequence of the proximity of that body to the sun. In addition to this, it frequently happens in this climate, that at the moment of the transit on the meridian when the moon's age is greater, the observation is rendered impracticable by a clouded sky. The Astronomer-Royal therefore contrived the altazimuth, constructed with unusual solidity, as the only instrument which could be satisfactorily adapted to extra-meridional observations, which would in their results bear comparison with those made on the meridian.
The altazimuth was erected in 1847. It is mounted in a tower, or dome, raised to such a height as to command the horizon, nearly in all directions, above the other buildings of the Observatory. The foundation of the instrument is, as before stated, a three-rayed pier of brickwork carried up from the ground to within a short distance of the floor of the room. On this pier is placed a cylindrical pillar, upon which the instrument is erected.
It would be wearying the reader to enter into the detailed construction of the altazimuth, but it is right to mention that the parts are cast into as few pieces as possible; for example, those which compose the moveable part being no more than four, all bolted so firmly as to appear as almost of one piece of metal. Another peculiarity is, that the sockets which carry the microscopes for reading the divisions, on the engraved circles, are all cast in the same flow of metal as the checks of the instrument. All these arrangements tend to make the instrument of great solidity, improving considerably the accuracy of the observations by the attention originally paid to these details.
The observations made are transits over a series of vertical wires (cobwebs) for observations in azimuth, and over a series of horizontal wires for those in altitude. From these transits, the exact time of observation is obtained for the purpose of computing the tabular place for comparison with the place obtained from the readings of the circles, etc., of the instrument.
We hope the reader has been able to follow us in our explanations, for we are anxious to make him practically acquainted with a few of the peculiarities of the instruments; but we must now hasten down the narrow staircase into the passage, where the entrance to the principal computing rooms is to be found. We, however, offer our apologies for intruding into the more private portions of the establishment; but we have a carte blanche, and are therefore privileged. Well, these are the two rooms occupied by the Astronomer-Royal and the astronomical assistants, during that portion of the day allotted for computing and general business. We find the Astronomer-Royal in a separate room adjoining the assistants' computing room, the door between them being always open, seated at his table, which is covered with documents and letters, denoting an extensive correspondence, not only on astronomical subjects, but also on a great deal of miscellaneous business; while maps, models, or drawings, vouch for the varied nature of the vast scientific employment of his never-resting mind. But let us walk quietly into the next room, and admire the order which is evident at a glance. Let us look over the shoulders of some of the assistants; one computer, generally a junior, is entering the observations made on the preceding night; another is employed on an advanced stage of the computations, referring minute after minute to a rather ponderous-looking book, full of logarithmic tables; while a third is arranging in order for binding some of the manuscripts and letters of the preceding year. The principal assistants are generally occupied in the, examination or revision of what is performed by tho computers. These duties are never-ending: as soon as one set of computations is finished, another is ready to be commenced; indeed, the quantity of work involving a considerable amount of monotonous labour performed in this computing room is almost marvellous.
But computing is not the only day-duty of the assistant. Observations are also made, during the computing hours, of the Sun, Venus, and Mercury, when practicable, as well as large stars, most of which can be seen and observed at all times of tho day when they pass the meridian. The assistant on duty is always on the look-out in clear weather. The arrangement of the observing , duties of the assistants is made every Monday morning, under the personal direction of the Astronomer-Royal. A large paper is placed upon a board above the mantel, piece, containing the arrangement for each instrument for a week. Each observer's watch generally extends, for the transit-circle, from three o'clock in the morning till the same hour on the next; there are, however, occasional modifications of this rule. The duties are to observe the sun, moon, and planets, and certain stars, whose places are required. For the altazimuth the observer is expected to watch from moonrise to moonset, unless he is enabled to secure a successful observation before. With this instrument, the duty is exceedingly harassing in cloudy weather: it frequently necessitates the complete disturbance of the night's rest of the observer. While we are in the computing room, let us watch an assistant who is seen looking at a chronometer; he is noting how many minutes have to elapse before an important object, the transit of which he wishes to observe, passes the meridian; he soon quietly leaves the room, book in hand, remains absent a few minutes to make the observation, and then as quietly returns to resume his computing labours. In fact, what strikes a visitor most, on a casual inspection of this computing room, is the quiet, order, and regularity with which everything is done. Every one knows precisely what his duty is, and how to perform it well. Seven or eight assistants and computers are generally present.
A supplementary computing room has been very recently established in one of the rooms of the great equatorial building, in which most of the heavy computations are performed.
Considering the enormous amount of correspondence on questions affecting different sciences – for we have already said that the Astronomer-Royal is frequently occupied on important business unconnected with astronomy – and the yearly increase of folio volumes of computations, it is not to be wondered that considerable space is required for the reception of the accumulation of manuscripts from year to year. A new fire-proof record room has therefore been lately built, which contains the largest portion. Many or these manuscripts are of great interest and value. In this room are stored those of Flamsteed, Halley, Bradley, Maskelyne, Pond, and some collected by the present Astronomer-Royal. The remainder are preserved in iron-proof closets in other parts of the Observatory.
We have now visited in succession the transit-circle, the altazimuth, and have mingled with the busy staff in the principal computing room. We must next visit the three equatorials; but our space is exhausted, and accordingly we reserve our account for another article.
[From The Leisure Hour, Vol XI (London), No.525, 16 Jan 1862, pp.39–43. Note: the illustrations will be added when time allows]
ROYAL OBSERVATORY, GREENWICH.
A DAY AT THE OBSERVATORY – continued.
BY EDWIN DUNKIN ESQ., F.R.A.S.
THE equatorials must now occupy our attention. There are three specimens of this class of astronomical instrument at the Observatory; but it will not be necessary to devote any great length of time to them, as their use is of a more occasional nature than the two instruments of which we have already spoken.
Visiting them according to seniority, the first is constructed by Ramsden, placed in a dome near octagon room. This instrument was presented to the Observatory in the year 1811, by Sir G. Shuckburgh. It was first intended to be mounted as an altazimuth, but, from its general unsteadiness in that position, it was never used. Subsequently, it was erected where it now remains, and is occasionally used for observations of occultations of stars by the moon, eclipses of Jupiter's satellites, and a few other astronomical phenomena.
The next in order is the Sheepshanks equatorial, erected in 1838. It is rather more than eight feet in length, and its object-glass is nearly seven inches in diameter. This instrument is placed in a dome, east of the transit-circle room. The mounting is that usually known by the name of the Fraunhofer, or German mounting, the telescope being on one side of the axis, and counterpoised by weights on the other. Clockwork is attached to the instrument, to carry the telescope with a motion equal to the diurnal rotation of the earth on its axis.
The chief use made of this equatorial has been for the observation of comets, occupations, etc., and the measurement of the distances of double stars. The latter class of observations is made with a double-image micrometer, which is capable of measuring small distances with great accuracy. The shapes of Mars, Jupiter, and Saturn, have been well determined with this micrometer.
But by far the largest and most noble instrument in the Observatory is the great equatorial, recently erected at great cost. The object-glass, which is 12¾ inches in diameter, was purchased of Messrs. Merz, of Munich, for about £1200, including expenses. The iron castings for the supports of the telescope, and the engineer's work generally, were made by Messrs. Ransomes, of Ipswich, and the general optician's work by Mr. Simms, of London.
The appearance of this beautiful instrument never fails to astonish the visitor who is favoured with the privilege of a, personal inspection of its peculiarities and improvements over other telescopes similarly mounted; it is, however, totally impossible to convey to the mind of the reader, in what manner these improvements differ from other instruments, without entering into the mechanical detail of all its parts; he must therefore take for granted, on the evidence of those best able to judge, that, as a specimen of astronomical engineering, it is considered to be a triumph of mechanical constructive art, which no other country but Britain could have produced. And we must add, that, taking, into account its great stability, in conjunction with its unique construction. it is one of the most important astronomical instruments to be found in any country.
This equatorial is provided with all the necessary adjuncts for making astronomical extra -meridional observations in every branch of the science. It is furnished with microscopes for reading the graduated circles, eye-pieces of different powers and construction, and other appliances too numerous to mention. If the position of a planet or star be wanted; if the magnitude or diameter of Jupiter. Saturn, or any object having a disc, be required; or, if the rapid changes noticed during an eclipse of the sun are to be measured, or, indeed, the observation of any other phenomenon, this instrument, in the hands of a skilful observer, will give results which no one can doubt.
But there is one contrivance, in connection with this equatorial, which is very striking to the uninitiated. It is necessary, when an object; is under scrutiny, that it should remain visible for a considerable time in the same position, in order that the observer might be able to give all his attention to the object of his inquiries. Now, everybody who has, for amusement or otherwise, watched a celestial object in a telescope fitted with a moderately high magnifying power, could not fail to perceive that it is continually passing out of view. The earth is performing its daily revolution, giving an apparent motion to the stars. The astronomer, however, has adapted to most equatorials of moderate size a clock movement, which is generally regulated by balls suspended to the ends of a horizontal arm which is carried by a vertical spindle, the motive power being a weight. In this great equatorial the moving power is water, supplied direct from the waterworks. The force of water is sufficient for working a reaction machine (Barker's mill), revolving four times in a second. This, acting through two worm-screws, drives the massive instrument. The regulation is effected by a beautiful contrivance, called Sieman’s chronometric governor; the pendulum, which is mercurial, has a uniform conical motion. The success of this apparatus may be understood, when we state that on one occasion, the instrument was left with Jupiter visible near the central wire of the telescope, and, on the return of the observer, after an interval of more than an hour, the planet was still there, so wonderfully accurate is the adjustment.
We have now visited the principal instruments which sustain the character of our national Observatory in the eyes of the scientific world; much, however, that remains to be described, though also interesting in the highest degree to the savant, is intimately connected with the active business of life. Before entering upon this connection with the outer world, one small but important instrument claims a passing word. This is the reflex zenith tube, an instrument designed for the observation of one special star in Draco, which passes across the zenith at Greenwich. We are afraid the subject would be of too technical a nature to warrant a proper explanation of its theory and use; we must therefore pass it over with this slight notice.
Perhaps in no department of the Observatory is greater activity apparent, during the morning, than in that relating to the business of chronometers and the dissemination of correct time to various parts of England by means of the electric telegraph.
Once more, then, let us enter the passage leading to the altazimuth, and proceed up a narrow staircase near the entrance. We are now in the chronometer room; and what a wonderful instance of the proof of our maritime power is this apartment! On entering, the visitor is startled by a universal buzz, which sounds almost like the hum of the beehive; sometimes more than two hundred chronometers, all delicate specimens of ingenious workmanship, are stored here at one time. Everybody has heard the ticking of his watch, when placed on the looking-glass at night; fancy, then, the indescribable sensation of listening to so many as two hundred chronometers, all registering the time independently of each other, the ticking being considerably louder than ordinary pocket watches. In this room are kept in store, and rated, all chronometers belonging to the Admiralty which are not required for the immediate use of her Majesty's navy. Many of these before us are, however, placed here by the makers, on a competitive trial for purchase, the chronometers keeping the best time, with the least change of rate, being bought by the Admiralty, after they have undergone a severe trial in different temperatures. A large closet, heated by gas, is in the corner of the room, for the reception of the chronometers when under trial for high temperatures.
Let us follow the assistant, who is now busily engaged. He opens the lid of each box, winds the chronometer, then proceeds to the next in order, and so on till the whole are wound up. It is then the duty of another assistant to follow over the same ground, to prevent any accidental passing over. When the winding is completed, one assistant takes his seat at a table, writes down in a book the numbers called out by the other, who is comparing each chronometer with one of the sympathetic galvanic clocks, which always shows correct time. The rapidity with which this is done is perfectly astonishing, especially when it is considered that the comparison is made to tenth parts of a second. No one, without considerable practice, can hope to rival the skill of the observers in this department. It is of no use, therefore, lingering too long in this room; but we leave it, gratified in no small degree at the thought that our noble ships can obtain an instrument brought by modern improvements to such a high state of excellence, and then preserved and rated so accurately for their use, that they may be able to traverse the ocean from shore to shore in safety.
The business of the chronometric department is very extensive; in addition to the duties already mentioned, the management of the repairs of defective chronometers, as well as the issuing and receiving them to and from the ships in commission, is under the superintendence of the Astronomer- Royal. The Observatory, therefore, performs a national duty, which can well be appreciated by all who take an interest in the welfare of the navy.
The great discoveries and appliances in galvanic science have added very important labours to the establishment. It is not our intention to enter into the history of the progress of the application of galvanism to practical purposes, further than to show its connection with the duties which the Observatory has imposed upon itself for the benefit of the country. Who, in the most distant corner of England, has not heard, at one time or other, a controversy between the advocates of the time indicated by the old church clock, and those modern innovators, who desire that all the different local times should be made to agree with the railway time, which is the same all over the country? Now, " railway time" is nothing more or less than Greenwich time, which is sent by signal to the different railroad stations directly or indirectly from the Observatory. The uniform time of starting trains depends, therefore, in some measure, on the accuracy of the observations made in this establishment.
Every visitor to Greenwich Park, or indeed all who have sailed up or down the Thames, cannot have failed to notice a black ball on the east turret of the principal building of the Observatory. At five minutes to one this ball is raised half-mast, and at about two and a half minutes to one, to the top. Precisely at one the ball is dropped. This public exhibition of correct time has been of the utmost importance to the mariner, who is able to compare the going of his chronometer with Observatory time, without the trouble and expense of coming to Greenwich. How necessary it is to the sailor, before leaving the docks, to know the error of his time-piece, on which he must so constantly depend while steering his course far away from land! The captain of a vessel is always anxious to start with the power of obtaining Greenwich time at any hour of the day or night; he therefore, while stationed in the docks, ought to compare his chronometer at the same hour on several days in succession, by which means he determines how much it gains or loses in a day. A good instrument will generally have the same rate for a long period. On sailing, the officer knows the error of his chronometer; daily he adds the rate, and is thus able to fix his longitude from day to day by his sextant observations, which gives him his local time, the difference of which, from his chronometric time, being the longitude from Greenwich.
This principle of exhibiting the correct time, by the dropping of a ball at a stated hour, has been carried out in various places throughout the country. At Deal, however, there is a ball dropped at one P.M., by a direct galvanic current from the Observatory, the ball being erected for the benefit of the shipping lying in the Downs. After the ball has dropped, a return current is transmitted from Deal a few seconds after one, to inform the assistant at Greenwich that the time of drop was correct. The value of this ball is very great, as it gives the master of each ship the opportunity of checking his chronometer at the last moment, before sailing on a long voyage.
Several other time-balls are dropped indirectly by the Greenwich galvanic current; but the Observatory is responsible at present only for that of Deal, which is placed by the Admiralty under the control of the Astronomer-Royal.
Clock signals are also sent automatically: every two seconds to the London Bridge Railway Station, regulating a large clock which is kept by this means identical with the parent or motor clock at Greenwich. Hourly signals are also sent to this station for distribution to the different towns in Kent and Surrey, and to the Lothbury office of the Electric and International Telegraph Company, for the transmission of correct time to other parts of England. The observer of a clock-star with the transit-circle, accordingly plays no unimportant part in the internal economy of the nation, for, in this active age, many events may depend upon the accuracy of his observation.
None more so, perhaps, than in the business of the Post Office. It may be asked at once, What can the Observatory have to do with the Post Office? Surely there can be little connection between astronomy and sorting letters! But wait a moment. It will be shown that the two departments are intimately connected, for they carry on a quiet daily conversation, which, in its results, is of considerable importance. Those of our readers who may have stood in the great hall of the Post Office at six o'clock in the evening, and noticed the immense business going on till the last stroke of six, must also have seen that at that moment the boxes are suddenly shut: in an instant, in fact, the scene is changed. What would not some unfortunate individual have given if the last stroke of six had been delayed only a few seconds longer! but here punctuality is the law. Now, this punctuality is regulated by galvanic signals transmitted daily from the Royal Observatory. Perhaps the space will not be wasted if we explain briefly the process adopted, after many experiments, to bring the regulation of the Post Office clocks into good working order. At that establishment, there are four principal clocks, three at the chief office, in St. Martin's-le-Grand, and one at the branch office, in Lombard Street. At a stated time before noon, a galvanic signal is sent by each of these clocks to the Observatory, giving notice to the observer by striking a bell, which informs him how many seconds it is in error. Exactly at noon, by a mechanical contrivance attached to the clocks, a signal from Greenwich sets them in accordance with the motor clock; and again, at a stated time after noon, signals are received at Greenwich to assure that the correction in each case has been satisfactorily performed. These duties are in this manner carried on day after day, connecting visibly, and in a tangible form, the quietness of the astronomer with the active business of life. Each of the four clocks in question regulates a group of dependent clocks by local galvanic currents, in a manner nearly similar to that by which the Greenwich current at noon regulates these four principal clocks; and by these means, more than thirty are kept very nearly to accurate time.
For the convenience of the Observatory itself, galvanic sympathetic clocks, acted upon by the motor clock, are placed in several of the apartments, and an additional one of larger size, already mentioned, near the entrance gate. The latter is made much use of by the public, for the regulation of their watches. The majority, however, consider it is some mysterious clock placed there to puzzle the visitors, many of whom frequently devote a considerable time to understand the meaning of ten minutes past twenty-two, or half-past fifteen. There is generally some one sharper than the rest, who is able to explain this "curious" clock. Perhaps it is well to remark, that astronomers always begin the day at noon, and count the hours consecutively to the next noon.
The chief assistants, who are specially responsible in carrying out the instructions of the Astronomer-Royal, in the astronomical department of the Observatory, are Mr. Stone, first assistant; Mr. Dunkin, principal superintendent of astronomical instruments; and Mr Ellis, who has the general charge of the clocks, chronometers, and galvanic arrangements.
We have now conducted the visitor through the astronomical observatory, offering brief explanations as we proceeded, of the peculiarities of the different instruments; it remains only, therefore, to take a walk to the magnetical and meteorological observatory, and our visit will be completed.
Proceeding, then, by the great equatorial, along a narrow path, and passing on the right the private garden of the Astronomer-Royal, we come upon a cross-like building, built of wood, pieces of bamboo instead of iron nails being used in its construction. A pole eighty feet high is erected near the entrance, to the upper part of which is attached a wire stretched across the grounds to the highest part of the Observatory, both ends being properly insulated. This wire is intended to collect the atmospheric electricity, which is conveyed to proper instruments inside the building, where it is examined and analysed.
The building contains three principal instruments, disposed so as to influence as little as possible one another's motions. One is a suspended magnet, intended to measure the magnetic declination, or variation of the needle; another, twisted, by means of the lines which suspend it, away from the magnetic meridian, so that it hangs nearly east and west, measuring the variation in the horizontal force of magnetism; while a third, which is similar to a beam of nicely-poised scales, without the scales attached, measures the variation in the vertical force. The deviations of these magnets were formerly observed every two hours, night and day; but for many years, thanks to the improvements in photography, the changes in the positions of the magnets have been automatically registered in a more effective manner, superseding entirely the system of day and night watches, practised for so many years.
It is impossible for us to enter into any detail of the processes employed in these self-registrations; it must suffice, therefore, to state that the deviations of the three magnets, the oscillations of the barometer, and the rise and fall of the thermometer, are daily recorded by a most interesting photographical manipulation. The daily sheets, when properly marked and timed, are preserved for reference, the principal deviations of the magnets being extracted for publication in the annual volume of "Greenwich Observations.”
Besides the three principal magnetical instruments, a unifilar magnetometer has lately been mounted. This consists of an apparatus for deflection of a magnet, and another for vibration, corresponding to the two parts of the process by which the absolute horizontal force of magnetism is determined; the experiments of deflection consist in observing the angular deflection of a suspended magnet, produced by the influence of a second magnet, which is placed on a support at one or more distances from the suspended magnet; the experiments of vibration consist in suspending the magnet which was used as the deflecting magnet in the former experiment, and then observing its time of vibration.
Observations for the determination of the magnetic dip, or inclination of the needle, at Greenwich, are also regularly made.
In addition to the magnetical observations, the observers in this department note day by day, at stated hours, the height of the barometer, and of thermometers of various kinds placed in different positions. For example, the temperatures of the air in the sun and shade, of the dew-point, of the ground at depths varying from one inch to twenty-four feet, are daily recorded. Rain-gauges at different altitudes are measured; and the electrical instruments are watched at frequent intervals. The wind is registered automatically by three anemometers, Osler's being the principal. A large vane is carried by a hollow tube, which, at its lower end, near a small table in the western turret on the top of the Observatory, carries a toothed wheel. This wheel gives motion to a rack-work, furnished with a pencil, which is moved backwards and forwards as the wind changes the direction of the vane. The small table is moved by clock-work, so that, as it is carried forward, a line is made on the paper by the pencil. The sheet of paper is divided into equal spaces, equivalent to one hour's motion; the direction of the wind at any hour of the day or night is therefore very easily found. The instrument also registers the force of the wind, and the amount of rain. The other anemometers are chiefly to register the velocity of the wind.
Abstracts of the meteorological observations are sent weekly to the Registrar-General, and published in the weekly report of births and deaths.
The observations in the magnetical and meteorological observatory are made under the personal superintendence of Mr. Glaisher, so well known in the scientific world a careful analyser of meteorological data.
The numerous duties which we have endeavoured to explain, and of which only an imperfect outline has been given, could not be performed without a considerable staff. With the Astronomer-Royal are associated seven permanent assistants, one assistant for special service at the magnetical department, and eleven supernumerary computers. All of these gentlemen perform their allotted work with fidelity and zeal, assisting to the utmost in upholding the scientific character of the establishment. That it ranks high in the estimation of men of science is evident from the eagerness with which they accept the invitation on the first Saturday in June of each year, to visit this active centre of astronomical progress, to admire and profit by the inspection of the noble instruments, which will bear comparison with those possessed by any other Observatory. At this annual gathering a report on the progress of the Observatory during the preceding year is read by the Astronomer-Royal before a select board of visitors.
From what has been said, it will be perceived that the assistant of the Observatory is not occupied in the most romantic portion of observing astronomy. His attention is not generally directed to the gazing at planets or nebulæ, or to the watching the appearances of the spots on the sun, or the mountains in the moon, which charm the amateur astronomer so much. But it is to the regular observation of the sun, moon, and planets, etc, when they pass the meridian night and day; to observe the position of the moon, day after day, with the altazimuth: observations which require not only every attention to the state of the instruments, but also imply such a mass of computations afterwards as none but professional astronomers could for a moment undertake. The interesting class of observations is not, however, entirely neglected, as several beautiful drawings of Mars, Jupiter, Saturn, and Donati's comet, have been made within the last few years.
Our Greenwich astronomer, in addition to his regular duties, is also supposed to devote some attention to astrology, if we consider the frequent applications made to him for some elucidation of the future events of life. This idea has evidently descended from a former period, when the astronomer rose very little higher than the astrologer in the estimation of the public. In those days the astronomical student had little hesitation in occasionally practising the black art, either on his own account or for the benefit of some anxious inquirer after the secrets of futurity, and, therefore, the astronomer and astrologer were frequently found in the same individual. Even the celebrated Flamsteed amused himself, on his accession to Greenwich, by drawing the horoscope of the Observatory – an accomplishment which few of the existing astronomers would acknowledge. The belief in nativity casting is considered in this educational acre to be well-nigh exploded; but still there are many persons – and those not always the most humble in station – who believe that the Greenwich astronomer is not continually contemplating the starry heavens in vain. This is very evident from the fact of individuals calling frequently at the Observatory gate, requesting information about their future destiny; letters even have been received inclosing Post-Office orders, requesting a nativity cast in return. The writer of this article on one occasion received a visit from a well-dressed young female, in great distress at the absence of a relative in the Pacific, who had not been heard of for several years. She left in tears because the stars were not able to tell her whether he was alive! As a final example, however, of the march of intellect in the nineteenth century, we conclude by inserting a copy of a letter received a short time ago. "I have been informed that there are persons at this Observatory who will, by my inclosing a remittance and the time of my birth, give me to understand who is to be my wife. An early answer, stating all relative particulars, will greatly oblige."
[From The Leisure Hour, Vol XI (London), No. 526, 23 Jan 1862, pp.55–60. Note: the illustrations will be added when time allows]
THE ROYAL OBSERVATORY, GREENWICH.
A NIGHT AT THE OBSERVATORY.
BY EDWIN DUNKIN, ESQ., F.R.A.S.
THE gates of Greenwich Park are closed to the public. The merry faces which were visible in every nook and corner only a few hours ago, have disappeared. Even the deer are nowhere to be seen. The clocks of the neighbouring churches have struck the hour of midnight; indeed, the whole town is at rest. And yet, a solitary individual may be noticed quietly unlocking one of the iron gates, and then as quietly disappearing amongst the dark avenues of trees which lead to the Royal Observatory. This is the astronomer, who has already been engaged in his duties with the transit-circle from sunset, returning, after visiting his residence for refreshment, to complete his allotted work. We should like to follow him; for, though we have found that a visit during the day is highly interesting and instructive, yet a look at a few of the principal celestial objects would be gratifying our curiosity in no small degree. But we are told that a visit to the Observatory at night is strictly prohibited. According to the rules, no person, whatever his rank, can claim admission; and it is proper to say that the Admiralty without scruple, refuses every application of the kind made to them. We can fully understand the necessity for this exclusion of visitors at night; for the whole staff is absent, excepting those who are required to make the observations with the different instruments.
But though no visitor is admitted after dark, we will exercise our official privilege of entrance, and endeavour to describe, in a popular manner, the duties performed in the Observatory at night, trusting to our acquaintance with the internal management of the establishment for much of the information which will form the subject of this article. We have already shown the vigorous activity of the place in our visit by day, and we shall be sorely disappointed if the reader be not convinced that the same activity is evident at night, though necessarily confined to fewer individuals.
Entering the gate by a latch-key soon after sunset, we find the observer is already at work with the transit-circle. As we enter the room, tap, tap, tap, tap, salutes our ears. A clock-star is passing the meridian at this moment, and the tapping is caused by the observer pressing with his finger an ivory key, which, by making a contact between two insulated brass rings, allows a galvanic current to pass through the wires, registering the observation on a recording apparatus. We shall ask no excuse for attempting to -explain briefly this method of chronographic registration, as its adoption at the Observatory has considerably simplified the observation of the transits of stars.
Before describing the recording apparatus, it is necessary, however, to state that the original method of observing transits consists in counting the beats of a clock, and estimating the fraction of a second when the object passed one of the wires in the telescope. The clock-time is then entered into the observer's book, the operation being repeated as many times as the number of wires. The new method of observation, or the chronographic method, is much more simple when the registering apparatus is in good working order.
The chronographic recording apparatus is placed in the ground floor of the north dome. The clock, which is of peculiar construction, the motion being governed by the conical rotation of a pendulum, gives a sensibly uniform motion to a revolving brass barrel, which is in connection with it. The barrel is covered with woollen cloth, upon which a sheet of paper is folded, the ends of the paper being gummed together. A spindle which is attached to the clock turns two long screws, causing a travelling frame to traverse the whole length of the barrel. This frame carries two levers, each armed at one end with a pricking point, mounted in such a way that, when the opposite end of the lever is pulled away from the barrel, the pricking end is pushed against it, and makes a permanent puncture on the paper. Two galvanic magnets are fixed on the travelling frame, so as to attract the lever ends opposite to the pricking points. All that is required, therefore, to cause those points to make punctures upon the paper, is to send galvanic currents through the magnets.
One of the prickers is devoted to the registration of the seconds of the clock in the transit-circle room, the communication being made by wires connecting the clock with the travelling frame. The other pricker is used for the registration of the times of observation when a star passes behind a wire in the telescope, a similar communication with wires being made. For the proper generation of the galvanic force, a voltaic battery is included in the circuit of each course of wires.
On this recording instrument, therefore, nearly all transits observed with the transit-circle, altazimuth, or great equatorial, are permanently registered. They are extracted from the sheets, and converted into figures on the following morning-an operation requiring care, but which, in skilled readers, is not considered a very difficult proceeding.
We have entered into more detail than usual on the description of this beautifully constructed apparatus, chiefly because its application is not generally adopted in other Observatories in England.
Our attention must now again be directed to the observer in the transit-circle room. He has finished the observation which first attracted us on our entrance. He is preparing for another observation, for his duties during the night are extensive. On examining his prescribed list of objects, which has been carefully prepared during the day by one of the computers, we find that the moon, and certain stars near the moon, are the first important objects for observation; these are followed by several of those very minute planetoids which revolve around the sun in orbits situated between those of Mars and Jupiter. Some of these specks in the solar system are so difficult to observe, even with the largest telescopes, that the observer is frequently obliged to darken the room as much as possible, several minutes before the observation, in order that his eye should be free from the dazzling effects of gaslight. After all these precautions are made, the planetoids are often so faint that with the most powerful telescopes they cannot be seen. The next object on the list is Jupiter, shortly followed Saturn. Again some planetoids extend it to the last moment of the time included in the evening watch, which reaches, on this occasion, from sunset till some time after two o'clock in the morning. In winter, the evening watch frequently consists of ten or eleven hours of nearly continuous observing.
Let us now follow the observer through one of his observations, which will be sufficient to explain the general method of observation with the transit-circle. An object to be observed is within a few minutes of passing the meridian. The shutters of the roof are already open. The observer looks at his list, which contains the time of passage, as well as the approximate position of the object in the heavens. He turns the instrument, by means of small projecting handles, to the proper place, seats himself in a very comfortable chair with a reclining back capable of being adjusted at pleasure to any inclination. The observing chair rolls backward or forward at the will of the observer, whilst he applies his eye to the telescope, his head being supported by the adjustable back of the chair. When the object is high, the observer is low; if it be a star near the zenith, the observer lies almost on his back; and in this very agreeable posture, he might surely be excused, at the end of a fatiguing watch, for indulging in a nap. Sometimes, indeed, when his fatigue is greater than usual, the temptation is very strong, but, like the soldier sentinel, the astronomer is seldom neglectful of his duty. Be this as it may, taking an observation is no sleepy matter; the impatience for the entrance of the star or planet into the field of view, is like that of the sportsman whose dog has just made a full point, and, who awaits the rising of the game. After the object has entered, it is seen to approach rapidly towards the vertical wires of the telescope; in a few seconds it is behind the first wire; the ivory key is then pressed down, and a puncture is instantaneously recorded on the chronographic apparatus. He repeats the tapping at the nine wires; nine punctures will therefore be recorded. With the other hand he turns a delicately constructed screw, which carries a horizontal wire, until the wire is exactly on the star, which appears bisected, the instrument at the time being fixed. The observer then leaves the telescope, after having read the figures indicated by the screw head, technically called a micrometer head, and proceeds to bisect the graduated divisions on the circle by certain cross wires in six large microscopes, the eye-pieces of which are on the back of the western stone pier. These microscopes are simply inclined perforations through the pier, the object-glasses being on the eastern side. The observer has now only to read the microscopes, the barometer, and thermometer, when the observation is complete. It is the duty of the computer to analyse these readings, and deduce from them the proper results.
The practical object of these astronomical observations is to determine, at a given time, the accurate positions in the heavens of the planets and principal stars, in order that the observed places of the planets may be compared with those computed from existing authorities, to discover the amount of error of the planetary tables employed in the calculation of the “Nautical Almanack.” These errors form the fundamental means of correcting the old tables, which were calculated from data found from inferior instruments to those now in use, and enable the theoretical astronomer to construct others of greater accuracy than those previously in existence. The nation has had to rely on the labours of the Greenwich astronomers for all the raw material necessary for the construction of nearly every edition of nautical tables for predicting the places of the moon and principal planets; the most recent being the celebrated lunar tables of Professor Hansen, of Gotha, which will ever remain a lasting monument of the care and skill of the Greenwich observer, combined with the mathematical genius of one of the greatest of living philosophers.
The advantage of correct lunar tables to the seaman cannot be overstated, as the position of a ship at sea may be considerably out of its reckoning, if the places given in the " Nautical Almanack” are not to be depended on. In future years, beginning with 1862, the places of the moon in the Almanack will be computed by Hansen's tables; the sailor will therefore be able to rely still more on his book companion, and thus guide his vessel with an almost unerring certainty.
It will not be proper to interrupt the observer with the transit-circle any longer; we therefore leave him, to look in for a moment or two on the altazimuth observer, for he too is here and at work, as was very evident by a card placed on the chronographic apparatus, stating "In use for the altazimuth." We will accordingly mount the narrow staircase leading to this instrument. Tap, tap, again is heard before we enter. Yes, the observer is, what is sometimes facetiously termed, " tapping off a transit." All is activity here. The moon is being observed in azimuth, the observation in altitude having been previously made. The observation in both of these elements is made by transits across six vertical wires for an azimuthal observation, and over six horizontal wires for that in altitude. Let us follow, the, assistant, who has just observed a transit; watch how he is carefully creeping up and down the instrument, reading the microscopes and levels attached to various parts. One cannot help being struck with the skill of the observer; the massive instrument, though about a ton in weight, seems moveable at his will, and obeying him without much apparent exertion on his part. Watch again how rapidly he reverses the moveable cheeks, redirects the telescope to the moon, and before you can fancy the instrument to be in its proper position, tap, tap, again reaches your ear – another observation is being made.
We must not remain in this department, for we have a strong desire to have a peep through the great equatorial, in order that our curiosity may be at length gratified by the telescopic appearance of a few of the celestial wanders which, to the unassisted eye, are never seen. We could see from the altazimuth done that some one of the busy staff was penetrating into the mysteries of some distant world, as revealed by this stupendous instrument; our imaginations, therefore immediately became excited at the prospect of enjoy an astronomical feast once in one's life. Let us not delay it for a moment. Still, we must inquire why the smaller domes, containing the remaining two equatorials, are lighted up, and the roofs open? We are informed that in each an observer is noting the time of occultation, or disappearance of a star behind the moon, an observation of considerable importance in the determination of longitude. Occasionally during the evening the same telescopes are also used for observing the times of disappearance and reappearance of the satellites of Jupiter, into or out of the planets shadow. Other phenomena are frequently observed.
As we entered the outer door of the great equatorial building, the working of some kind of machinery attracted our attention. This is the clock-work which gives motion to the instrument; it is in action, because the observer is making a pictorial delineation of the planet Jupiter, which makes it necessary that the instrument should move corresponding to the diurnal rotation of the earth, to allow the object to appear motionless. We found the observing astronomer with his eye to the telescope, lying in an inclined position on the massive chair, which he is enabled to move at pleasure in any direction, by several ingenious mechanical contrivances. The observer is holding in his hand a representation of Jupiter, which he has just finished, whilst he is giving the planet a few parting glances to confirm the delineations, on which all the peculiar cloudlike belts are faithfully given. We are assured that many extraordinary appearances, never before noticed, have engaged the attention of astronomers during the last few years.
Of all the bodies of the solar system, the moon, perhaps, alone excepted, Jupiter presents to the telescopic observer the most magnificent spectacle, although his distance from the sun is so great, being more than five times that of the earth. He appears accompanied by his four moons, the apparent positions of which are continually changing, caused by their revolution round the body of Jupiter.
A brief time only has elapsed before we are occupying the observer's comfortable seat, and partaking of the intellectual feast for which we have had so many anxious desires. The reader will therefore be so good as to bear with us while we describe the remarkable appearance of this planet, as viewed with this equatorial. It appears to exhibit a considerable disc, the ground of which is of a light yellowish colour; but towards the poles it seems to melt into a leaden-coloured grey. Upon the ground are seen brownish-grey streaks, resembling in their form and arrangement the streaks of clouds which are often observed in the sky on a fine calm evening after sunset. Some observers have noticed that these streaks are occasionally tinged with red. Their general direction is parallel to the equator of the planet. Some are more conspicuous than others. Two are generally strikingly observable, being extended north and south of the planet's equator, separated by a bright yellow zone, being a part of the general ground of the planet. Occasionally these streaks will undergo great changes from day to day, which seem to confirm their nature as being analogous to the clouds of our atmosphere. At other times, bright and dark spots will appear, and remain for several days; by these spots, astronomers have been able to observe the time of rotation of the planet on its axis, from which has been proved that a Jovian day consists of rather less than ten hours.
After obtaining the above information concerning the largest planet belonging to our system, our attention is suddenly directed to the young crescent moon, to which the observer has, with marvellous rapidity, pointed his telescope. Again we are seated in the observing-chair, and our eye eagerly applied to the small aperture in the eye-piece. If we were astonished at the view of Jupiter, how much more so are we at the telescopic appearance of the crescent moon. Though the illuminated portion is only a crescent, the entire disc is faintly visible, caused by the reflection of earth-light. The surface is seen in the illuminated part rugged and serrated, whilst brilliantly shining points are seen in the dark part at some distance from it, and dark shadows of considerable length appear to break into the illuminated surface. The bright points seen within the dark hemisphere are the peaks of lofty mountains tinged with the sun's light. They are similar to what all travellers in mountainous countries have noticed; after the sun has set, and darkness commenced in the valleys at the foot of the chain of` mountains, the sun still continues to illuminate the peaks above.
We were, however, much struck with the numerous apparently extinct craters, some of which are of enormous size. For example, Tycho, the principal crater on the moon's surface, is forty-seven miles in diameter. Its external ridges rise about three miles above the inclosed plain. Within the inclosure there is a central mound nearly a mile in height, besides a few lesser hills. On applying a high magnifying power to some of these craters, the observer is able to see the different layers of rocks composing them, standing out with almost the appearance of a stereoscopic projection. From several, radiating streaks of light and shade are noticed, especially when the moon is full, at which time they are always seen to the greatest advantage.
Another remarkable object is in a convenient position in the heavens for scrutiny; we must, therefore, give up looking at lunar craters, for we are recommended to take a glance at Saturn. What a magnificent object! A ball, of a yellowish colour, with streaks as in the case of Jupiter, though fainter in intensity. Around the ball are several rings, two, if not three, being illuminated like the planet; and another, a dusky, or semi-transparent ring. Four of his moons, out of the eight which accompany him, are visible, the remaining four being too faint for regular observation.
Saturn appears in the firmament as a star of the first magnitude, and shines with a faint reddish light. His disc, like Jupiter, is visibly oval; a proof that each of these planets must revolve on its axis, to produce that form. The time of rotation of Saturn is between ten and eleven hours.
We are beginning to consider it necessary to offer some apology for detaining the observer so long; but he has suggested to us a few other astronomical wonders, well worthy of inspection. He therefore turns his instrument to an object of a very different class to what we have been noticing. It is a faint telescopic comet, which, we are told, is rather a brighter one than usual. Well, we must confess that, for several minutes, we could distinguish nothing, and began to think our vision was affected by our lesson in star-gazing; but at last the faint comet became visible. We could not, however, help contrasting this minute object with that splendid specimen of comets which astonished us all in the autumn of 1868, known as Donati's comet. This celebrated celestial visitor was nightly operated on by the astronomers of all countries, and several very beautiful delineations of it were made. Our illustrations will give the reader some conception of its beauty. The form of the nucleus, or head, underwent considerable changes from hour to hour; and even whilst under the scrutinizing eye of the observer, jets of light were frequently seen to pour forth.
The great comet of 1861 demands a passing notice. This beautiful object, which so suddenly appeared near the end of June last, was first observed in the month of May, in Australia and other portions of the southern hemisphere. Its position when at its greatest brilliancy being near the well-known constellation of Ursa Major, gave to that portion of the heavens an appearance, which possibly will never be effaced from the memory of the spectator. Its telescopic view was similar in many respects to that of Donati in 1858, though the changes of the nucleus from hour to hour were less marked. This comet was still the subject of observation among astronomers so lately as December last, but it was of the last degree of faintness, and scarcely observable. (See " Leisure Hour," No. 504.)
We are still entreated to remain yet longer, to see one or two stellar objects. The first which engages our attention is a multiple star. To the naked eye this appears only as a single star, but, on looking through a telescope of moderate power, the observer is able to see several hundred congregated. All of these are at so great a distance as to be beyond the power of the astronomer to calculate. We are shown another object, which is called a binary star, because it has been observed that this single star to the naked eye, but double in the telescope, appears to have a system peculiar to itself, one star being noticed to revolve around the other. There are several of these binary stars scattered about the firmament, but that situated in the constellation of the Virgin [Virgo] has most probably received greater attention than others. The stars composing this celebrated binary star appear quite close to each other, and of about the same magnitude. They have been under observation nearly one hundred and fifty years, and in that period have completely revolved around each other.
We are not willing to leave this fine instrument; but time flies rapidly in such agreeable employment, and warns us that the hour of the night is advancing. But before we leave the Observatory, we must look into the magnetical department. Here all is quietness. Nothing is heard but the ticking of the clocks. The instruments are, however, doing their work silently and surely, thanks to the modern science of photography, which has taken out of the hands of the observer, all his former nightly official duties. Perhaps the time may come when the astronomer may be also relieved of some of his most laborious work by the help of the same science; but, to all present appearance, it seems scarcely possible.
Returning rapidly to the astronomical department, we re-enter the transit-circle room. The observer is where we left him, still observing the planets as they pass the meridian. He is, however, about to return home for a short time for refreshment; we avail ourselves of his company through the very dark and gloomy avenues of the Park, and return to our abode with thoughts full of wonder and satisfaction, at having spent a few hours during the night at the Royal Observatory.