|Author:||George Forbes, Leader of the British party sent to observe the 1874 Transit of Venus from Hawaii and later Professor of Natural Philosophy at Anderson’s University, Glasgow
|Title:||The Royal Observatory, Greenwich
|About:||Published in 1872 in two episodes in Good Words (Strachan & Co, London), this extensive account appears to be based on a visit Forbes made to the Observatory that year. The timing of his visit means that it is the only account to describe either Airy’s Water Telescope or the temporary observing huts that were being prepared for the 1874 Transit of Venus expeditions.
|Epsiode 1: pp.792–96|
|Episode 2: pp.855–58|
|Click here to read episode 1 as originally published
|Click here to read episode 1 as originally published|
THE ROYAL OBSERVATORY, GREENWICH.
IN the year 1675, and on the 2nd of June, Charles II. signed a warrant which declared his resolution to build an Observatory in Greenwich Park, “in order to the finding out of the longitude for perfecting navigation and astronomy.” The best way of determining the longitude by means of astronomy is to take advantage of the moon's motion. The moon goes round the heavens once in every four weeks. Suppose that an astronomer observes the position of the moon when she is in the south; her position with respect to the fixed stars will change from day to day. At the opposite side of the world an astronomer will not see her in the south until twelve hours later; she will by that time have changed her place among the stars considerably. If other astronomers observe her position from other countries, when she is due south of them, her position will be different, and it is easy to see that the longitude can be determined by noticing how far she has changed her position from the time she is south of one station until the time she is south of another whose longitude is known. The advantage of having an Observatory employed in observing the moon is twofold. It insures a regular series of observations with which travellers can compare their observations at different parts of the earth; it also furnishes data from which the place of the moon can be predicted at any future time with tolerable accuracy.
In accordance with the royal wish, Sir Christopher Wren was called upon to design an Observatory. It may be interesting to antiquarians to know that the cost of the building was exactly five hundred and twenty pounds nine shillings and a penny. This consisted of the handsome building that forms so conspicuous an object from the north side of the park. Great additions have been made since then, but they are chiefly at the back; the octagonal tower with a dome at each side, and the terrace in front, still stand in their original condition. The view from this terrace, or from the top of the tower, is very extensive, and the winding Thames is constantly busy with the ships of all countries. The trees of the park form a pleasant foreground, with the pretty spire of the Roman Catholic chapel rising above them. Twice in the year does the sun sink behind the dome of St. Paul's, and the smoke of London, instead of spoiling the sunsets, often adds to their beauty.
But we must make the most of our time, and enter the precincts of the Observatory. First, let us go and see the Transit Circle. This is the most important of all the instruments, and is seldom at rest for long by day or by night. Round the walls of the room are hung many instruments now no longer used. Here is the Zenith Sector, with which Bradley made his great discovery of aberration and nutation. There are the transit instrument and mural circle of Troughton, noble instruments in their day, but now superseded by a more perfect instrument. They did good work in their youth; and, like men, they have handed down their virtues to their descendants, together with all the advantages derived from experience. From the parent instruments has risen a child inheriting the good points of both, and they may well look down from those walls with pride on their progeny. In the middle of the room stands the noble instrument devised by Sir George Biddell Airy, the present Astronomer Royal. There are two massive piers of stone lying east and west. They are about eight feet high, and between them swings a telescope, twelve feet long, supported by a horizontal axis resting on the piers. From this construction the telescope can only be directed to stars which lie north or south of the Observatory, or between those points and the Zenith (which is the point of the heavens exactly overhead). But all stars pass across this north and south line (the meridian as it is called) in the course of twenty-four hours, on account of the earth's rotation. On the north and south sides of the room there are shutters which open quite up to the roof, and another shutter opens in the roof, right across from the north to the south wall. This allows a clear view in every direction in which the telescope can be pointed.
The object of this instrument is to determine with great exactness the places of the sun, moon, planets, and stars when they cross the meridian in their daily course. Catalogues of stars are then formed. The calculated places of the moving members of our system are compared with the observed places; and the errors enable us to improve the tables by means of which we predict the places of the sun, moon, and planets. The manner in which the place of a heavenly body is determined has been chosen for convenience; and will best be explained by an earthly illustration. In this country we usually measure the distance of one place from another by miles. But in the mountainous parts of Switzerland you will find that if you ask the distance to some place, a native will tell you that it is so many hours. He means that it will take you that time to walk the distance. If all people walked at the same pace, and if any one person never altered his pace, this would be a perfectly accurate way of measuring distance. Now all the stars do move uniformly, and they all take the same time to go round the heavens. So that with them we may safely measure distance by time. But if a traveller is walking along a road he cannot describe the position of all the objects he passes simply by saying at what time he passed them. He must also say what distance they were from the road, and whether they were on his right or his left. So with the stars. We note the time when one of them passes the meridian, and the telescope being pointed at the star at that time, the position of the telescope tells us what distance it is from the pole of the heavens, and it is easy to see on which side the pole it lies. To measure the angular distance from the pole a large circle, six feet in diameter, is attached to the telescope, and revolves with it. This circle is divided into degrees and smaller divisions. It is close to the western pier. This pier is pierced by six holes, through which pass six microscopes, each about two feet long. These are pointed at different parts of the divided circle, and being fixed while the circle revolves, any one of them is sufficient to determine the position of the telescope at any moment. But to prevent errors from reading the microscopes, and from imperfections in the divisions of the circle, all the six are used, and a mean is taken as the true position.
It is known by every one that a telescope consists of two lenses, one near the eye called the eye-lens, and the other at the farther end called the object-lens. The rays from a star fall on the object-lens, and are caused by it to converge to a point called the focus. This point is examined by the eye-lens, which is really a magnifying glass. The image, at the focus, of any star lies on the line which passes through the star and the centre of the object-lens. So that when a star moves, its image at the focus changes its place. Suppose we have an upright wire in the focus of the telescope, so placed that when a star is on the meridian its image is on the wire. If a clock is made to tick loudly at our side, when we see a star advancing towards the wire we can look at the clock to see what second the hand is pointing at, say five, and then go on counting the ticks of the clock, six, seven, eight, and so on. The star is not likely to be on the wire exactly at a tick of the clock. Suppose at the twenty-second second the star has not reached the wire, and at the twenty-third it has passed. We can estimate the fraction of a second very accurately by noticing the position that the star held with respect to the wire at the twenty-second and at the twenty-third second. Having determined the second, and fraction of a second, we can look at the clock-face and see the minute and hour.' This method is often employed, but five or seven wires are used, and the mean value is taken. It is called the Eye-and-Ear method, because the ear judges of the seconds, and the eye of the fractions of a second.
The wires would be invisible at night if the whole of the field of view were not illuminated. This is done by making the axis of the telescope (which rests on the piers) hollow. A light sent along this tube is reflected down the tube of the telescope towards the eye, so that the wires appear dark on a bright ground. The illumination comes from a gas-lamp, whose light is also concentrated by means of lenses upon those parts of the divided circle which are read by the microscopes. To do this the pier had to be pierced. This is a most ingenious and effective arrangement.
The Eye-and-Ear method is very accurate, but it has given place to a still more accurate one. A cylinder of metal has a sheet of, paper stretched on it. The cylinder is made to revolve perfectly uniformly by means of a clock with a conical pendulum. At the end of every second one of the wheels of a most accurate clock causes two springs to come in contact. This sends a current of electricity through some wires, and this current causes a pricker to press down on the paper which is stretched on the cylinder, and a mark is made. The person who is observing with the transit instrument has also the power, by simply tapping a spring fastened to the telescope, of causing an electric current to pass which will make a prick on the paper. The pricks made by the clock are about an inch apart, and the time when a star crossed one of the wires can easily be read off. An arrangement is made by which the first second of each minute makes no mark on the paper. This prevents confusion. The axis of the cylinder is a screw, so that while the cylinder turns round it also advances, and the dots made by the clock lie in a spiral round the paper.
It is a feature of modern astronomy that nothing made by man is considered perfect. Every instrument has some errors; these are found out, and their values determined, and corrections applied. There are three important errors in the transit circle. If the wires in the focus of the telescope be not in the right place, the star will seem to pass too soon or too late. If the axis of the telescope be not exactly horizontal, stars near the zenith will seem to pass too early or too late. Lastly, if the axis do not lie exactly east and west, stars near the horizon will seem to pass too soon or too late. Some idea may be obtained of the exactness of the instrument from the following example:– A few years ago the axis was found to be not level enough; the corrections were inconveniently large. To remedy this, a few tons of stone were placed on the pier that had been found to be the highest; it was left then for some time, but not the slightest effect was produced. The metal which supported the axis on the lowest pier was now lifted off. A piece of paper, 1/250 of an inch thick, was laid down, and the metal replaced. The error was now almost totally corrected. This error, called the level error, is determined so accurately that the temperature of the season is found to affect it The piers are made of different materials, so that one expands more than the other with heat, and thus affects the level error. So minute is the correction, that in summer one pier is highest, in winter the other.
Our determination of a star's distance from the pole is affected not so much by instrumental errors as by what may be called natural errors. One of these is refraction. The light which comes from a star is bent down out of its course by the earth's atmosphere. In consequence of this, a star seems to be higher than it really is. The nearer a star is to the horizon the greater is this effect. The rays from the lower edge of the sun are sometimes so much more bent out of their course than those from the upper edge, that the sun seems to be flattened. This effect is greatest when a star is on the horizon; it then seems to be higher than it really is by a quantity nearly equal to the sun's diameter. Tables have been calculated to show what corrections ought to be applied.
The place of the moon is affected by the position of an observer on the earth. In high northern latitudes, she seems to be farther south than she would, if viewed from the centre of the earth. The stars are too far off to be affected by this error, but the moon's place is always corrected for it. Even after all these corrections have been applied, there are other ones which we have not yet mentioned. It appears then, that the act of observing is but a small part of the labour of determining a star's place, inasmuch as only a minute is taken up on this, while the reductions take much longer.
But we have spent a dreadfully long time in the Transit Room; and though this is the instrument that most deserves our attention, yet we must hurry away, or we shall not have time even to catch a glimpse of the other instruments. We have spoken of the clock which marks the time by electricity; it is worth while going a hundred yards to the south to see where it is put. In a dark underground room, called the magnetic basement, which we shall have occasion to visit again, stands the clock. It is placed here because the temperature is nearly constant. The dial has twenty-four hours. It keeps neither local time nor railway time, but what is called sidereal time. Twenty-four of its hours are a little less than a day and night, and in this time each star has made a complete circuit of the heavens; so that the time when a star passes the meridian, as told by this clock, and the distance of the star from the pole are the two elements that determine its place (the Right Ascension and Polar Distance, as they are called). An astronomical clock may be in error, and no one will object so long as they know the correction that has to be applied. But its rate ought to be as nearly constant as possible. Its rate is the quantity which it loses in a day. The mechanism of this clock is very fine. It has an escapement invented by the present Astronomer-Royal. Temperature hardly affects the rate of the clock. The pressure of the air, which offers a varying resistance to the pendulum, is almost the only cause of variation of the rate. These variations are extremely minute, but it is almost possible to tell what is the reading of the barometer by observing the rate of the clock.
Now let us come back to the astronomical department, and leaving the Transit Room we come to the Zenith Telescope. This instrument is used solely for observing a certain bright star which passes the zenith, called Gamma Draconis. The object-lens is placed as level as possible. The rays from a zenith star falling on it would converge to a point below; but they are intercepted by a trough of mercury which reflects them up again, so that they converge to a point a little above the object-lens. Here they are reflected to the side into the eye-lens. To make a complete observation the eye-lens is at first on the north of the object-lens. The object-lens and eye-lens are then turned round to the opposite position, and the star is again observed. If the star were exactly in the zenith it would not seem to change its place. Gamma Draconis is very nearly in the zenith, and the small quantity that has to be determined is thus found with the utmost accuracy. The reason why this instrument is used is that one star observed under the most favourable circumstances possible is worth a hundred observed with less accuracy. For determining with great exactness the amount of certain constant errors which affect all observations, this instrument is unsurpassed.
Passing now, with noiseless tread, through the Computing Room, we reach the altitude and azimuth instrument. This might be described as a small transit circle, which is placed on a turn-table so as to be able to point at any part of the heavens. The distance that the instrument has to be turned round from the north and south position is measured by a divided circle. The angle read off is called the azimuth of the star. The other divided circle, placed as in the transit circle, shows the altitude of the star. If the heavens were fixed, these two angles would be sufficient to tell a star's place. But the stars all move, so we must know the time as well. Consequently the observer, with this instrument, has also the power of making marks on the revolving cylinder to mark the time when a star is on a wire in the focus of the telescope. The great advantage of this instrument is that it insures a more regular series of observations of the moon than could be obtained with the Transit Circle alone; for she is often hidden by clouds when on the meridian, though she may be visible at some other time of the night. Sir George Airy first saw the great importance of this point; and since the instrument was set up the Greenwich observations of the moon have been one of the most valuable contributions to astronomy.
It is a curious mistake many people make to suppose that the great advantage of telescopes, when applied to astronomy, is that they magnify the heavenly bodies. This was what they were chiefly used for at first; but ever since Gascoigne found out, in the seventeenth century, that wires could be put in the focus of the object lens, so as to determine the exact position of a star, this has been the chief use of telescopes in astronomy. All the instruments that we have examined up to this point have telescopes mainly for this purpose, and though by their aid we are enabled to see objects that would otherwise be invisible, yet this is of secondary importance.
Now let us turn in another direction, and visit a telescope used chiefly with a view to magnify objects. There are many such in the Observatory, but there is one that deserves especial attention. From the park you may see a large cylindrical tower that looks as if it was a water-cistern or else a gasometer. It is neither one nor the other, but contains the magnificent telescope which is the largest instrument in the Observatory. It is thirteen feet long, and the object-glass is twelve inches in diameter. There are two things wanted in such a telescope, magnifying power and plenty of light. The larger the object-lens is, the more light will be admitted. The longer the telescope is, the greater is the magnifying power. But it is a matter of great difficulty to make a glass so large as twelve inches so as to give a good image of the object we are looking at. Much larger ones have been made, but they need such a steady atmosphere to give a clear view, that there is a general feeling that in this climate twelve inches is as large as is convenient for general use. The great object is to mount a telescope very steadily, so that no tremblings shall cause the image of a star to dance about. With a large telescope the image of a star runs across the field of view in a very short time. This is very annoying, so a method is used to avoid it. The telescope is moved constantly in the direction that the stars move. This is done quite uniformly by clock-work. The image of the star now seems to remain always in the same position. The mounting of the instrument is so firm that you might do gymnastics on its supports and an observer would see not the slightest trembling in the image of a star. One of its chief uses is in making observations when a new comet appears in the heavens, or when we wish to examine the eclipses of Jupiter's satellites or the occultation of stars by the moon. Comets are generally so faint that a powerful telescope is required to see them. We wish, however, to determine the places of these wanderers on every possible occasion. This can be done by measuring the distance of the comet from some star which is in the field of view of the telescope at the same time. Here we find one great use of star catalogues. For we know the positions of the stars, and we can measure the distances of the comet from the stars, and thus we can determine the position of the comet. Occasionally again there are eclipses of the sun which are well worth studying with a powerful telescope; and the continual appearance of new phenomena in the heavens makes it essential that every observatory should have a powerful telescope.
We shall not stop longer to speak of the wonderful sights that may be seen with this noble instrument, but it may be well to mention that a large spectroscope is going to be attached to it. By means of this instrument we can tell what the sun and stars are made of, and by a beautiful device that was discovered in 1868, we can see parts of the sun which, without its aid, would be overwhelmed by the solar light, and would thus become invisible.
There are many other things in the Observatory which we shall visit together on another day. All the magnetical and meteorological instruments must be examined carefully, and the preparations for observing the transit of Venus across the face of the sun, by means of which the distance of the earth from the sun is to be measured. These are all most interesting subjects, but they must wait for another month.
HAVING visited most of the astronomical instruments, we may now ascend to the top of the tower, which is so conspicuous an object from the park. But before doing so we shall enter a small room at the base of the tower, where we shall find the clock that is kept constantly regulated to Greenwich time. The amount of work that this clock does is something appalling. It causes a current of electricity to pass through some wires every second. This serves as the motive force for several clocks, and regulates a large number of others scattered over England. At one o'clock each day, a current is sent which fires guns at Newcastle and South Shields. At the same instant a time-ball is let fall at Deal, similar to the one which simultaneously falls at the Observatory. Nothing could give us a better example of the manner in which electricity annihilates time. The importance of having these signals sent over the country is twofold. In these days of railways and telegraphs, it is more important than ever to have correct time for our ordinary purposes, and it is evident that this can best be accomplished by having the time transmitted from the place where it is known with the greatest accuracy. But sailors have another reason for desiring accurate time. Chronometers can be made so perfectly that the longitude of a ship can be determined with fair accuracy by that means, if no very long time has elapsed since a comparison was made with an accurate clock.
The normal clock in the Observatory has, of course, some errors. These can be determined by comparison with the astronomical clock, which is regulated by means of the stars. To do this, two clocks are kept in the computing room, one regulated to astronomical, the other to Greenwich time. These can be compared; and the difference between true astronomical and true Greenwich time can be found from tables which are calculated for every day and for every hour in the day. But the astronomical clock gains a second on Greenwich time in the course of every six minutes; so that great care is necessary. Now comes the question, how is the normal clock to be regulated? We cannot stop the clock or advance it so many seconds, because this would not advance or retard, by the same number of seconds, the clocks which are moved by its means. Electricity comes to our aid once more. A magnet is attached to the end of the pendulum, and beneath this is a coil of wire, so arranged that when a current of electricity passes through it in one direction, its influence on the magnet makes the pendulum move slower; and if the current passes in the contrary direction, the pendulum moves quicker. The superintendent of the time department knows, that by sending a current through the wire for ten seconds, he alters the time of the clock by one second; so that, sitting at his desk in the computing room, he is enabled to regulate the normal clock to within a tenth of a second of the true time.
Now let us mount to the top of the octagonal tower. After having devoted a short time to admiring the extensive prospect, we turn our attention to the two turrets. Above one of them rises a pole, on which slides the large time-ball that falls every day at one o'clock. Above the other turret we see a weather-cock. It is connected with a pencil inside the turret, which moves up and down over a sheet of paper as the wind changes its direction. The sheet of paper is moved forward uniformly by clockwork, and thus we are able to tell what was the direction of the wind at any time of the day. A metal plate is also attached by a spring to the weather-cock, so as to present its face constantly to the wind. The pressure of the wind acting against the spring, moves this plate, and by a simple mechanical arrangement, another pencil traces on the sheet of paper a line which denotes the pressure of the wind at any hour.
At another part of the tower is an instrument for measuring the velocity of the wind. It is called an anemometer. It consists of a cross made of metal, all the arms of the cross being of equal length. This is supported horizontally on a vertical axis, which can easily turn round. At the end of each arm of the cross is a hemispherical cup of metal. These are so arranged, that in whatever direction the wind blows it has the hollow of the cup on the right-hand side to blow against, and the back of the cup on the left-hand side. The wind acts far more powerfully on the hollows than on the backs of these cups. So that the whole instrument is turned round the axis by the force of the wind. This motion is proportional to the velocity of the wind, and the motion of the instrument is communicated to a pencil, which also traces a curve on a moving sheet of paper, and thus the velocity of the wind at any time can be read off.
While we are on the tower we may take notice of a wire which passes from here to the top of a pole beside the magnetic part of the Observatory, a distance of about a hundred yards. Its use is to collect the electricity of the atmosphere. At the top of this pole a small lamp burns, protected by a copper case. This is to keep the wire dry, so as to prevent the electricity from escaping. The wire passes down the pole, and thence into the magnetic computing room through a window. On a ledge inside the window are various instruments which can be connected with this wire, and the amount of electricity in the air can in this manner be roughly determined. But the results are not satisfactory. The subject of atmospheric electricity is one about which we know little, but which promises to give us valuable aid in prognosticating the weather when we shall have amassed a sufficient number of observations to establish a theory. But the position of the Observatory is unfavourable for these observations. It is found that anything pointed has a great influence in drawing away electricity. Now the place is surrounded by trees, every leaf of which is covered with points, each one helping to draw away the electricity from the wire that is intended to collect it. Moreover, the instruments necessary for measuring small quantities of electricity are very difficult to construct, and the principles of their construction are not known by many makers of instruments. No such delicate electrometer has been set up at Greenwich, and, consequently, the observations are of less value.
The Magnetic Observatory is built in the form of a cross, the arms of which were constructed in the year 1838, so as to point to the four cardinal points of the compass. Every one knows that the compass needle points nearly to the north. But it is not so generally known that the greater part of terrestrial magnetism acts in a downward direction. If a bar of steel be exactly balanced by a support of any kind at its middle point, and if it be then magnetized, and replaced on its support, it will not now rest horizontally as before, but, if the support allows it, will point nearly downwards. There are, then, two components of terrestrial magnetism; one a horizontal component, tending to place the needle in a north and south direction; the other a vertical component, tending to place the needle vertically. Now these forces are found to vary both in quantity and in direction. A magnetic needle which is weighted at one side, so as to make it rest horizontally, will point in a direction which varies not only from year to year, but from day to day, and even from hour to hour. This is well shown by the Declination Magnet. The declination of a magnet is the angle at which the compass needle is inclined to the meridian. In the south arm of the Observatory a magnet two feet long is suspended by a skein of silk. The direction in which it points is at a considerable angle with the wall to which it was originally parallel. The declination at any fixed time is read off in an ingenious manner. At the south end of the magnet a pair of fine wires are mounted, in the form of a cross. These are in the focus of a lens placed at the north end of the magnet. By this arrangement the wires are seen as if they were at a great distance from the observer, who looks through the lens. They are in a condition to be observed with a telescope. They are observed through the telescope of a theodolite, which you may see a few feet north of the magnet. The telescope is turned round till the wires on the magnet are exactly in the centre of the field of view of the telescope. The distance of the telescope in this position from the meridian can easily be found, for there is a small shutter in the roof, through which we can observe the pole star. There is a great deal to be seen, so we must now go down a flight of steps, into subterranean regions. Here we find a dark room, feebly lit up by gas. It is here that photographic records are taken of the magnetic elements. But it is not merely for the sake of darkness that this underground room is used. The chief advantage is that the temperature can be kept almost constant. In some magnetic instruments changes of temperature introduce very great changes in our measures. It is here, also, that the astronomical clock is kept, which is connected by electricity with the astronomical department.
When your eyes have got used to the darkness you will be able to see magnets of different kinds suspended in different parts of the room. You will also see cylinders with paper stretched upon them, which are supported on horizontal or vertical axes, and which are capable of being turned slowly round by clockwork. The paper which is stretched on these cylinders is all prepared for receiving photographic impressions. Upon each magnet is placed a small mirror at the point of support of the magnet; the light from a gas-lamp is reflected by this mirror on to one of the sheets of paper. If the magnet make the slightest movement the mirror will be moved too, and the tell-tale spot of light will move along the paper, and the photographic impression will show the amount of the change, and we shall also know at what time the change took place.
One of the magnets is suspended in the same manner as the one up-stairs. The upper one gives the absolute declination of the magnet at fixed times, the lower one gives the variations at intermediate times. Another magnet is supported in a horizontal position by two strings about an inch apart. If the magnet be turned round in its supports from the north and south direction, terrestrial magnetism will try to bring it back, but the torsion of the strings will try to keep it in that position, and owing to the conflict between these two forces the magnet will take an intermediate position. We wish by means of this instrument to measure variations of intensity in the horizontal component of terrestrial magnetism. This is done by twisting the magnet in its support until it takes up a position at right angles to the magnetic north and south line (or the magnetic meridian, as it is called); any slight increase in the magnetic force will help to overcome the torsion of the strings, and a decrease will allow the torsion to have a greater influence. These changes in the intensity of the horizontal force are registered by photography.
To measure the variations of vertical force another magnet is supported at its centre by a fine knife-edge which points north and south, so that the magnet points east and west If left to itself the magnet would be driven by the vertical part of terrestrial magnetism to a vertical position; but on one arm of the magnet is placed a weight which can be moved along to any distance from the centre, so that there is here a conflict between gravity and magnetism. The weight is so adjusted that the magnet is horizontal. Any slight increase in the vertical component of the earth's magnetism will help to overcome gravity, and a decrease will allow gravity to exert a greater influence. These changes are recorded by photography on one of the revolving cylinders.
In another part of the magnetic basement, as this subterraneous chamber is called, we find two instruments for measuring the strength of currents of electricity in the earth. It nearly always happens that places which are separated by a considerable distance are in different electrical conditions, so much so that if a wire be carried above ground from one place to the other, and the two ends be sunk, one at each station, a current of electricity will pass through the wire without the aid of any battery. In some cases it has been possible to make use of these earth-currents in working a telegraph. Two wires are carried each between a pair of stations about three miles distant. Each wire is made to pass through one of the galvanometers which we see in the magnetic basement. A galvanometer is an instrument for measuring currents of electricity. A magnet is suspended freely, and the current of electricity moves it out of the magnetic meridian. A mirror is attached to the magnet, and a beam of light is reflected by it on to a revolving cylinder with photographic paper on it in the same way as the motions of the other magnets are registered. The two earth-currents that are measured run in a direction from N.E. to S.W., and from N.W. to S.E. respectively.
Very interesting results are likely to arise from these observations. It is found that occasionally magnetic storms arise, and it has been found that, at the same time, there are great disturbances in the electric currents. But the daily variations of the earth currents seem to follow a different law from those of the earth's magnetism.
When we leave the magnetic observatory we find, to the south of the building, a plot of grass, where many of the meteorological instruments, and thermometers for observing underground temperature, are kept. But these do not differ from other instruments sufficiently to warrant us in taking up our time by examining them carefully. On the south side of this grass-plot are several rooms called the magnetic offices. In one of these the direction of the magnet in a vertical direction is measured. This element of terrestrial magnetism is called the Dip. We have already seen how the declination of the magnet is determined at any time. At present a magnet points about twenty degrees to the west of true north. To measure the dip a magnet has pivots at its centre by which it rests on a support. It is placed so that it has free motion only in the magnetic meridian. It is easy to see that, by this arrangement, the magnet will be drawn down out of the horizontal position until it points in that direction in which the magnetic force is acting. The position that it takes up is observed by two microscopes. whose positions are read oft' on a divided circle. The details of the instrument are complicated, and it is sufficient here to point out the general principle. By means of all these observations on the different elements of terrestrial magnetism, we may hope eventually to arrive at better conclusions on that subject than we are able to do at present.
When we pass to the south of the magnetic offices, we come to a space which is covered with wooden huts. What is their use? They are portable observatories; and they have been built to send out with expeditions which are to go to all parts of the world to observe the passage of the planet Venus across the face of the sun in the year 1874, and again in 1881. The object of this is to determine the distance of the earth from the sun. This is a very important element in astronomy, and we only know it very roughly at present. People found a few years ago that they were three millions of miles wrong in their calculations; but what is a million of miles to an astronomer, or a million of years to a geologist? By the help of these expeditions, however, we may hope for much more accurate results. The way in which it is done is this: when Venus approaches the edge of the sun it will appear to be touching its edge, when looked at from that side of the earth where the sun is just going to set; but from the other side, where the sun has just risen, it will not by this time have reached the edge. If, then, we can observe, at two or more stations suitably chosen, the exact time at which Venus seems to touch the edge of the sun, astronomers can calculate what distance of the earth from the sun and from Venus would cause this difference in time. To determine the time exactly it is necessary to know the longitude of the place of observation exactly. This is a very laborious operation, but it will be known with tolerable precision after about three months' observations. The observation is of great value, and the Astronomer-Royal has taken every precaution to ensure satisfactory results.
In one of these huts, older and more weather-stained than the rest, some valuable observations have been made, which show Sir George Airy's determination in ridding astronomy of every possible source of error The position in which a star appears to be is affected by an error called " the aberration of light," whose value is known, and for which a correction is applied. This error depends on the velocity of light, as compared with the velocity of the earth. Just as in a steady downpour of rain you hold your umbrella upright if you are standing still, but a little forwards if you are walking fast, for the direction in which the rain comes depends on the pace you are walking at, and on the rate at which the raindrops fall; so with the rays of light from a star – the telescope has to be inclined a little forwards in the direction the earth is moving, to catch these rays. Now, the velocity of light through glass is less than through air, and through air it is a little less than through a vacuum. It seems likely, then, that the direction in which a star is seen will be affected by the atmosphere and the lenses of our telescopes. To test this the Astronomer-Royal filled the tube of a telescope with water. This ought to exaggerate the effect. The telescope was pointed to a star, and the position of the star carefully found at times when aberration has its greatest effect in opposite directions. Not the slightest effect of the supposed kind could be discovered. This result is most remarkable and most interesting. Professor Respighi, of Rome, whose labours on the twinkling of the stars we had occasion to speak of last July, had already arrived at the same conclusion; but the Greenwich observations placed the matter beyond doubt.
We have now visited, in a hurried manner, all the chief instruments of the Observatory, and on leaving the building we may pass the knot of people outside (who are always trying to make out the time on the clock with twenty-four hours) with a consciousness of superior knowledge. The importance of this establishment to navigation is incalculable, in more ways than are at first sight evident; and the noble science of astronomy owes more to this than to any other Observatory in the world. Long may it retain its supremacy; and long may it benefit from that superintendence which has gone so far to raise it to its present position.