Bamberg Broken Transit (early 20th C)

Usually referred to in the Greenwich literature as the Bamberg Broken Transit Telescope, telescopes of this type made by  Bamberg were also known as a Bamberg Transit Telescope (or Instrument), a Bamberg Reversing Portable Transit Instrument or as a Bamberg Elbow Transit Telescope.

The Royal Observatory Greenwich’s Instrument (serial number 8461) was borrowed from the Royal Observatory Edinburgh (ROE) which itself had acquired it secondhand from an unknown source in 1930. Borrowed from Edinburgh during the reporting year 1941/2 it was not used at Greenwich, but at Abinger, where the Time Department had been evacuated to at the outbreak of the Second World War.

The Bamberg Broken Transit at Abinger. Humphry Smith Photographic Archive

It remained unused until 1947, when following modification, it was brought into use initially alongside and then in place of one of the Small Reversible Transit Instruments for the determination of clock errors. It was used for this purpose until 1955 when it was withdrawn for repairs. It was never brought back into service at either Abinger of Herstmonceux. By 1983 it had been returned to the ROE. In 2009 it was acquired by National Museums Scotland (Museum Reference T.2009.108).

The history of the telescope prior to 1930 is currently unknown. So too is its precise technical specification. A similar telescope in Brazil (serial number 10783) is said to have been brought into use in 1907, suggesting that the ROE instrument was in excess of 23 years old at the time it was aquired.

Like all transit instruments, the Bamberg Broken Transit was only able to move in the plane of the meridian. Certain of the brighter stars, whose positions had been refined by repeated observation over a long period of time, were used as ‘clock stars’. By comparing their observed times of transit with their theoretical ones, the errors of the Observatory’s quartz and free pendulum clocks could be determined. Click here to read more about the astronomical basis of timekeeping.

 

Basic Construction

The telescope was designed to be reversible – in other words capable of being raised off the supporting Ys turned horizontally though 180º and placed back on the Ys again, observations being taken with the telescope in both orientations during each transit observed. Instead of the eyepiece being in line with the objective as in a typical transit telescope, it was at right angles to it at one end of the east-west horizontal axis about which the telescope rotated. A counterweigh on the opposite side of the east-west axis balanced the weight of the object glass and the tube supporting it. At the intersection of the two axes, was a cube containing a prism. This acted as a mirror and deflected the rays of light through 90º towards the eyepiece. This arrangement meant that the position of the eyepiece was independent of the altitude of the object being observed, making the instrument more comfortable to use than a normal transit instrument. A second advantage of the Broken Transit over the Greenwich Small Transits was that it was fitted with a level that hung permanently below the telescope rather than a striding level that had to be placed astride the pivots each time the level needed checking, with a risk of affecting its zero, and with the serious disadvantage that the level could only be placed on the pivot in certain positions, so that unequal wear resulted.

 

Images of the Instrument at Abinger

Peter Willmoth at the eyepiece of the telescope. He became an observer in 1951. Humphry Smith Photographic Archive

The telescope in the process of reversal. It was raised using the horizontal lever with the circular end attached to the centre of the base. Humphry Smith Photographic Archive

Detailed view of the counterweight and hanging level. The Maker's name stamp, (largely obscured by paint and unreadable), can just about be made out running across the centre of the cube above the counterweight

Detailed view of the instrumentation at the eye-end. The observer is Vinicio Barocas

Barocas about to enter the observing hut. Humphry Smith Photographic Archive

 

The history of the Bamberg at the Royal Observatory Edinburgh

The early history of the Bamberg Telescope is unknown. In 1930, it was ‘acquired’ from an undisclosed source by the Royal Observatory Edinburgh (ROE). Its acquisition and proposed use was described by the Astronomer Royal for Scotland in his 1931 annual report:  

An opportunity occurred of acquiring a Bamberg reversing portable transit, instrument which was in good order as regards all essentials. If provided with a recording micrometer it would furnish a valuable control of the determinations of time with the Transit Circle, since it is preferred by some experienced observers to the best transit circles, and the method of observation is widely varied. It is proposed to supply what is wanted and pursue the comparison of the two methods, as opportunity allows.

Preoccupation with other matters caused its installation to be delayed until 1933 when a suitable observing hut and pillar were constructed, the illuminating system updated and an impersonal micrometer fitted that had been made for it by the Observatory workshop. The following year various attempts were made to design a mechanical drive for the instrument. After much time and effort it was decided that this was impracticable and it was decided instead to improve the existing hand drive. This had been completed by 31 March 1935. It is not presently known how much use the instrument got at Edinburgh as there is no further mention of it in the annual reports in any of the years up to and including that for 1939.

 

Transfer to and use at Abinger

In the early days of the Second Word War, an ad-hoc emergency time-service was set up away from London at Abinger (on the same site as the Observatory’s Magnetic Observatory which had moved there in 1923). Initially, it was only a skeletal service, but by the middle of 1940, a complete set of equipment had been installed that was able to take over all the functions of the time service at Greenwich. The clock errors were initially determined using one of the small reversible transit telescopes transported there from Greenwich.

The telephone lines from both Greenwich and Abinger passed through the Control Room at the G.P.O. London. If this room had been put out of action by enemy action, time-signals could not have been distributed from either Greenwich or Abinger. Arrangements were therefore made for sending signals via Birmingham. In the meantime, arrangements were made with the Astronomer Royal for Scotland by which time-signals (with the exception of the Rugby rhythmic signals) could be sent from the Royal Observa­tory, Edinburgh, should it prove necessary.

This parallel backup service was set up in 1941 and supervised by Leonard Symms (one of the Junior Assistants at Greenwich). For security reasons, Edinburgh was referred to in the Reports of the Astronomer Royal to the Board of Visitors, as Station B and Abinger as Station A. The transmissions from Greenwich were terminated towards the end of November 1940. After the War, the Edinburgh Statiion was shut down. At the end of January 1946, the equipment was dismantled and returned to Greenwich and Abinger. A secondary time service was subsequently set up at Greenwich, the main Station remaining at Abinger until 1957. 

The text in the remainder of this section has been extracted directly from the Reports of the Astronomer Royal. Given how the various Stations were intertwined, some (but not all) references to the Small Transit Instruments have also been included.

1942

The Report here presented refers to the period from 1941 May 1 to 1942 April 30 and exhibits the state of the Observatory on the last-named day.

The following instruments are on loan to the Royal Observatory:-

.... A small transit instrument, of the broken type, with stand, by Bamberg, from the Royal Observatory, Edinburgh. ...

 

1943

The Report here presented refers to the period from 1942 May 1 to 1943 April 30 and exhibits the state of the Observatory on the last named day.

At station A a new observing hut has been erected to accommodate the Bamberg transit instrument loaned by the Astronomer Royal for Scotland. It is proposed to equip this instrument with an electrically-driven micrometer but pressure of urgent work has delayed the completion of this scheme.

 

1947

The Report here presented refers to the period from 1946 May 1 to 1947 April 30 and exhibits the state of the Observatory on the last named day.

The Bamberg Broken Transit Instrument, on loan from the Royal Observatory, Edinburgh, has been completely overhauled, preparatory to being brought into regular use for time determinations. In the past difficulty has been experienced from wear of the pivots of the small transit instruments; the effects of pivot errors on the observed transit times cannot be allowed for and the determinations of level error with a striding level are impaired. The pivots have therefore been heavily plated with chromium and then ground and lapped in the workshop to a high degree of precision. An impersonal micrometer, formerly belonging to the Cooke transit, damaged by enemy action, has been fitted, while a second altitude circle and reading microscope have been added. The system of field illumination, designed by Dr. Atkinson, has also been fitted.

 

1948

The Report here presented refers to the period from 1947 may 1 to 1948 April 30 and exhibits the state of the Observatory on the last named day.

A new sliding shutter has been fitted to the roof of the Bamberg transit hut. ... The Bamberg broken transit, which had been very thoroughly reconditioned in the Observatory workshop, was installed at Abinger in July. The pivots had been chromium-plated and then lapped by hand with extensive Mikrotast measurements. Their final figure departed from the circular by a very few hundredths of a micron only. The actual operational figure, including the effects of oil and all errors of reading, is such that a whole night’s levels frequently fall well within an overall range of a second of arc, and so far as can be judged by appearance the chromium is standing up well to wear. The final scatter of a night’s observations is about two thirds of that previously obtained with Small Transit D.

The following observations for clock error have been obtained during the periods stated:-


Abinger, Small Transit D (to Jan. 20) 73

Abinger, Bamberg (from July 13) 188

Greenwich, Small Transit B (year) 270

Total 531
 
1949

The Report here presented refers to the period from 1948 May 1 to 1949 April 30 and exhibits the state of the Observatory on the last named day.

Astronomical observations for the determination of time have been made throughout the year at Abinger using the Bamberg transit, and at Greenwich using Transit B. The number of observers has been limited in order to ensure that each is adequately represented in the results obtained over any period of a few weeks. Whenever possible two groups of observations are taken at each station on the same evening, which are separately reduced. At Abinger, a total of 320 observations were secured on 174 nights. At Greenwich, both observers had to discontinue observing in 1948 April for medical reasons; several temporary observers used the small transit instrument B during the summer, but their observations were not used for the determination of clock performance. By September, conditions had improved and, since October, the Greenwich small transit results have been used again. Of the total of 143 observations on 106 nights, 91 observations secured on 67 nights were used in the operation of the time service.

 

1950

The Report here presented refers to the period from 1949 May I to 1950 April 30 and exhibits the state of the Observatory on the last named day.

The Meridian Group will include [at Herstmonceux], in addition to these two major items [pavilions for the Cooke Reversible Transit Circle and the {{A1047], pavilions for the P.Z.T. and the Bamberg Small Transit, and also a small building comprising control-room for the P..Z.T., an observers' room, and accessory services. The planning of these items is in an advanced stage.

Work on Small Transit C was completed during the year. On final re-assembly, it was found that the micrometer which had been used with this telescope was of such dimensions that a practicable focus-setting was quite out of reach, although it had been focussed and used successfully at Abinger before recondi­tioning; the explanation was found to be that the objective had been used backwards, in which position it gives over half an inch longer working distance and surprisingly good images. In order to use the lens correctly with this micrometer, it has been necessary to shorten the telescope-tube, and this has now been done. The instrument has been fitted with the “full-aperture” illumination system, designed by Dr. Atkinson, which has been found satisfactory in the Bamberg Broken Transit. A rheostat-control  has been fitted whose position and direction of operation always appear the same to the observer, whether the instrument is reversed or not and whether it is pointed north or south.

Throughout the year the Time Service has been based upon astro­nomical observations made at Abinger, using the Bamberg Broken Transit instrument, and at Greenwich, using Small Transit B. At Abinger, 250 observations were secured on 144 nights, and at Greenwich, 187 on 134 nights, making a total of 437 observations during the twelve months covered by this report. Owing to various staff changes, some experienced observers are no longer available, and new observers are being trained to take their places. Transit C, whose over-haul, has now been completed, will be installed at Greenwich, in place of Transit B, which will be transferred to Abinger, where it will be used for additional time determinations and for training.

 

1951

The Report here presented refers to the period from 1950 May 1 to 1951 April 30 and exhibits the state of the Observatory on the last named day.

The positions which had been provisionally selected [at Hersmonceux] for the P.Z.T, and the Bamberg were altered, in April, to facilitate a re-location of the P.Z.T. control-room and observers' building, in conformity with the views of the Architect.

Throughout the year the Time Service has been based upon astronomical observa­tions made at Abinger, using the Bamberg Broken Transit instrument, and at Greenwich, using Small Transit B.

At Greenwich 320 observations were secured on 166 nights, and at Abinger, 249 on 123 nights, making a total of 569 observations during the twelve months covered by this report. Transit C, referred to in last year’s report, has been set up in the Small Transit Pavilion at Greenwich, and is undergoing final adjustments. It has not yet therefore been possible to release Transit B for return to Abinger, which station has been without a second instrument since January 1948. The levels used with the Bamberg and Small Transit instruments were recalibrated by an optical method in the laboratory at Abinger.

 

1952

The Report here presented refers to the period from 1951 May 1 to 1952 April 30 and exhibits the state of the Observatory on the last named day.

[At Herstmonceux] contracts for the building to house the Photographic Zenith Tube, the building to hold the control console for this instrument, and the pavilion for the Bamberg Broken Transit Instrument are due to be placed. The foundations and piers for the instruments and the foundations for the buildings themselves are to be constructed departmentally.

Throughout the year the Time Service has been based upon astronomical observa­tions made at Greenwich using Small Transit B and at Abinger using the Bamberg Broken Transit instrument.

At Greenwich 201 observations were secured on 160 nights and at Abinger 308 on 135 nights, making a total of 509 observations during the twelve months covered by this report. Transit C was included in the equipment sent to the Sudan for the observation of the eclipse of February 25, so that the position of one of the observing stations could be accurately determined. It was dispatched on December 4 and has not yet been received back. Routine re-calibration of the Bamberg level has been carried out twice during the year and no change in scale value has been detected.

Details of the observations made at Abinger with the Bamberg Transit Instrument were transferred to punched cards for analysis by Hollerith equipment. As the equipment at the Nautical Almanac Office was in full use and not available for this work, a member of the Observatory staff carried out the machine operations with the Hollerith equipment of the Mathematics Division of the National Physical Laboratory, through the courtesy of the Superintendent of the Division. This Hollerith analysis has been supplemented by various investigations by conventional methods undertaken at both Greenwich and Abinger and the results are now being analysed and collated.

 

1953

The Report here presented refers to the period from 1952 May 1 to 1953 April 30 and exhibits the state of the Observatory on the last named day.

[At Herstmonceux] contracts have been let for the manufacture and erection of the buildings to house the Cooke R.T.C., the Photographic Zenith Tube, and the Bamberg Small Transit, The foundations for each of these buildings, as well as the piers for the telescopes, will be done Departmentally. The expected completion dates for these buildings will be much delayed, but it is still hoped that the buildings will all be erected by the end of the present year.

Throughout the year the Time Service has been based upon astronomical obser­vations made at Greenwich using Small Transit B and at Abinger using the Bamberg Broken Transit instrument.

At Greenwich 188 observations were secured on 149 nights and at Abinger 232 on 131 nights, making a total of 420 observations during the twelve months covered by this report. At Greenwich, the adjustable bubble, which had been used on Small Transit C, was mounted on the tube of Transit B on June 19. The bubble from the old striding level has been mounted on instrument C. Parallel tests between the two instruments are being made. It is intended to withdraw Transit B and to give it a thorough reconditioning, including fitting it with precision-worked chromium-plated pivots, as soon as it can safely be spared. At Abinger, observations are being made in the early morning as well as in the evening, as a control over periodic errors in the system of right ascensions.

The time determinations made at Greenwich and Abinger are reduced to the standard Greenwich meridian by the application of the appropriate adopted longitudes. It has been found that there are significant variations between the time systems defined by the two instruments. From an examination of the residuals of observations from the finally-adopted time, “observer corrections” are derived which take into account the “personal equation” of the observer concerned as well as the mean difference between his observing station and the adopted mean of the two stations, called the “station correction”. The determined “observer corrections” are, not strictly constant; for current use the adopted values are based on an examination of a series of quarterly means.

As in previous years, the astronomical observations have been corrected for the effects of polar motion, for which the current latitude variation data, communicated regularly by the U.S. Naval Observatory, are used. These provisional estimates are subsequently amended, when the polar motion data from the International Latitude Service become available. The observations, after correction for polar variation, are referred to the mean of five or six selected quartz clocks and analysed to derive values of the seasonal fluctuation in the Earth’s rotation. The values have proved to be in substantial agreement with those obtained in 1950–51, which were smaller than those derived in earlier analyses, though a slight tendency for the amplitude to show a small and persistent increase has been noticed in the last few quarters.

 

1954

The Report here presented refers to the period from 1953 May 1 to 1954 April 30 and exhibits the state of the Observatory on the last named day.

[At Herstmonceux] the foundations for the Photographic Zenith-Tube Pavilion are under construction and the manufacture of the pavilion is well in hand at the works of the contractors. The plans for the pavilion to house the Bamberg Small Transit are nearing completion.

At Abinger the Bamberg broken transit instrument was in use throughout the year, 204 observations being obtained on 121 nights. At Greenwich, Transit B was withdrawn from service on September 16, on the completion of the parallel tests mentioned in the previous report. Transit C was in use in the Courtyard dome until July 18; it was then transferred to the Altazimuth dome, but was returned to the Courtyard dome on January 1. Transit B was transferred from the Altazimuth dome to the Courtyard dome on July 18. Transit B was adopted as the standard Greenwich instrument until June 30, and Transit C from July 1. The number of observations at Greenwich used for controlling the clocks was 196, obtained on 152 nights. In addition, for the parallel comparison between Transits Band C there were 69 obser­vations on 69 nights, between 1953 March 25 and September 15.

The pivots of Transit B have been chromium plated, and the instrument is being re-conditioned in the workshop at Greenwich. The figure of the pivots of the Bamberg instrument, which had been in continuous operation for some six and a half years, was examined. Wear had occurred at the parts of the pivot which are in contact with the supporting Y’s and the pads of the hanging level, but the departure from circularity nowhere exceeded 1 micron and has no significant effect on the determined time.

The application of observer corrections to the time observations was discon­tinued on July 1, with the exception of a significant personal correction retained for one observer (AC). It was found that the personal equations of the observers, with this one exception, are much smaller than the fluctuating difference between the two stations, Abinger and Greenwich.

The series of morning observations undertaken at Abinger was discontinued on May 1; no significant differences of a systematic nature were detected between the morning and evening observations. A similar series was commenced at Greenwich on January 31, after Transit C had been finally mounted in the Courtyard dome.

The determinations of time have been corrected for the effects of polar motion, based on current latitude observations communicated regularly by the D.S. Naval Observatory. After this correction, they have been compared with several selected quartz clocks and analysed to derive the apparent annual fluctuation in the rotation of the Earth. The fluctuation is in close agreement with those observed in the past two or three years.

During the summer of 1953, the time determinations indicated an increasing systematic departure of the standard clocks from the performance predicted on the basis of the observations of the previous 6 to 12 months. Clock inter-comparisons gave no reason to suspect that the clocks had suffered any change, and the divergence was attributed to a change in the rate of rotation of the Earth. The magnitude and form of the change could not be precisely evaluated because of the considerable scatter in the time determinations and because of some uncertainty about the rate of frequency drift of some of the clocks. The change was represented at first as a simple decrease of 1.3 milliseconds in the length of the day, occurring in 1953 March, but subsequent evidence suggests that it was more complex, and may have occurred as early as 1952 December.

 

1955

The Report here presented refers to the period from 1954 May 1 to 1955 March 31 and exhibits the state of the Observatory on the last named day.

[At Herstmonceux] the small pavilion to house the Bamberg Small Transit and the pavilion for the Photographic Zenith Tube have been erected.

At Abinger the Bamberg broken transit instrument was in use for the whole of the eleven-month period covered by this Report, 180 observations being obtained on 103 nights. The contact length and the level scale value received the usual routine check in October, and the values adopted were confirmed. At Greenwich during the same period Transit C was employed in the Courtyard Dome. Altogether, 234 observations were obtained: 103 of these were made on 79 mornings, and 131 on 127 evenings. The observations at Abinger and Greenwich, totalling 414, were combined for the purposes of time determination.

It was noticed in May that the effective length of the micrometer contact of Transit C differed from the value adopted from previous measurements. There was no independent evidence to indicate precisely when the change had occurred. As the instrument had been in operational service for less than a year, it was decided to recompute the observations on the assumption that the new value of the contact length had been applicable throughout. This micrometer was replaced on June 11 by the Cooke micrometer, which had previously been used on Transit B and which had been reconditioned and the drive altered so as to double the speed of manual turning. The contact length was redetermined in June and November and was found to have the same value as used throughout its 25 years of service. Since the introduction of this micrometer the mean error of the time determinations at Greenwich has been appreciably reduced.

The determinations of time have been corrected for the effects of polar motion, based on current latitude observations communicated regularly by the U.S. Naval Observatory. Various instrumental changes have occurred, but overlapping series of observations have preserved the continuity of the polar motion adopted. After this correction, the Greenwich and Abinger time observations have been compared with several selected quartz clocks and analysed to derive the apparent annual fluctuation in the rotation of the Earth. There has been no significant change in the amplitude and phase of the observed fluctuation.

The two solar-sidereal transformers, mentioned in last year’s Report, have been delivered and tested, and one is now in operation with a 20-1 frequency divider supplying 50 cycles per sidereal second to drive a synchronous clock dial in the Bamberg hut. This is an experimental installation for providing experience in the operation of the time distribution system that is being planned for Herstmonceux.

 

1956

The Report here presented refers to the period from 1955 April 1 to 1956 March 31 and exhibits the state of the Observatory on the last named day.

At Greenwich, astronomical time determinations were made throughout the year with Transit C in the Courtyard Dome, 99 observations being obtained on 96 mornings and 160 on 157 evenings. The reduction in the mean error of the observations, referred to in last year's report, has been maintained. At Abinger, the Bamberg broken transit instrument was removed from service on 4 November for replacement of the wiring which had become faulty on account of condensation. The opportunity was taken to give the instrument a mechanical overhaul. When observations were resumed, there appeared to be a discrepancy of some 100 milliseconds. In spite of various efforts to locate the cause, it was not until 17 February that backlash between the lead-screw and the wire carriage was detected by means of a displacement indicator gauge. On further dismantling the micrometer it was found that the return spring was oversize for its housing. A new spring was made and inserted, but was apparently of insufficient strength. A third spring was made and inserted on 23 March. Subsequent observations indicate that the discrepancy persists, and the investigation continues. In all, 232 observations were taken on 138 nights, of which only 146 on 89 nights were used for time determination. The combined total for the two stations of observations used for time determination was thus 405.

No further references to the Bamberg Instrument were made in the later Reports. Although as late as 1955, there were still plans to install the Bamberg Instrument at Herstmonceux this doesn't appear to have happened, Small Transit Instrument C having been placed in the pavilion instead. Details of this were given in the Report for 1958 which covered the period 1 April 1957 to 31 March 1958:

Observations made with the Small Reversible Transit C were used until October 16 [1957] in the establishment of the provisional time system employed for the adjustment of the clocks controlling the radio time signal transmissions. It was moved from the Courtyard Dome at Greenwich following an observation on April 23 [1957] at 03.9 U.T. and transferred to Herstmonceux, where the first observation was obtained on April 23 at 20.0 U.T.

From October 16 [1957] the P.Z.T. observations were used exclusively in current work, and retrospectively from July for the establishment of the definitive time system given in the Time Service Bulletin.

George Wilkins statement in his Personal History of the Observatory that the Bamberg Instrument was taken first to Greenwich and they used at Herstmonceux from April to October 1957 appears to be incorrect.

Although the telescope was returned to Edinburgh it is not known when this happened. What is known, is that it was certainly in Edinburgh by 1983, by which time it had been stripped of its wiring. A photograph (with an incorrect caption) was published in Hermann Brück’s 1983 book The Story of Astronomy in Edinburgh (p.85),  In 2009, the telescope was acquired by National Museums Scotland (Museum Reference T.2009.108), who have a picture of it on their website.

 

Description of similar instruments

Carl Bamberg (1847–92) served an apprenticeship with Carl Zeiss at Jena and studied at the Universities of Jena and Berlin before establishing a Werkstätten für Präzisions-Mechanik und Optik in Friedenau, a suburb of Berlin, in 1871. After Bamberg’s death, the firm was managed by his widow, and then by his son. The firm displayed a Broken Transit made for the Kgl. Geodätische Institut at Potsdam in the German Educational Exhibition at the World Fair in St. Louis in 1904. In 1921 Bamberg merged with the Centralwerkstatte Dessau, forming a new company known as Askania Werke.

No description of the Bamberg Instrument is known to have been published by the Royal Observatory. That below is taken from a catalogue of Scientific Instruments that accompanied the German exhibition at the World Fair in 1904.

The instrument, having a focal length of 0·65 m. and a clear aperture of 68 mm., is fitted with a registering micro­meter with motion of the eyepiece which may be replaced by a micrometer capable of rotation through 90º (for Horrebow-Talcott observations). A revolution of the micrometer-screw corresponds to 80”. The three eyepieces supplied with the instrument have magnifying powers of 44, 65 and 86. The field is illuminated by means of an electric incandescent lamp or by an oil-lamp; the light is thrown into the interior of the telescope through the hollow rotation axis.

The eyepiece end of the horizontal axis is fitted with two juxtaposed levels for Horrebow-Talcott observations which can be finely adjusted in relation to each other, and whose carriage can be firmly clamped to the axis. The finding circle of 136 mm. diameter is graduated to 1/6º and may be read to 1’. It is fitted with an alidade level.

The axis can be easily reversed within a few seconds and without danger of disturbing the instrument, the sus­pended level being left in place. The relief of the axis­-bearings is effected by the pressure of a central spring. All the clamps of this instrument are flange-clamps preventing any strain on the axis. This instrument rests on a stand with adjustment in azimuth.

Later instruments of this type made by Askania are all believed to have had a speherical rather than a cubical connection at the junction of the two axes. A catalogue of their equipment published in about 1930 gave the following information:

  • They had been made for several decades and were in use in all parts of the world
  • They were currently available with objectives of 50, 70, 90 or 100 mm diameter and 450, 644.5, 859 or 1031 mm focal length
  • They were free from any tension and equipped with axles of specially hardened steel
  • They had a semi-automatic reversing system
  • The sensitivity of the level for the axle and the Horrebow-Talcotte level was 1”.

In 1914, the United states Coast and Geodesic Survey purchased two Bamberg Broken Transits for survey work. A description of the instruments was published in their Manual of Geodetic Astronomy (Special Publications Issue 237 (Washington, 1947)). Figure 1 and Figure 2 from this publicaton indicate the purpose of many of the parts of the Greenwich Instrument.