Transcript of Airy’s 1847 Address to the Board of Visitors regarding the possible acquisition of a Transit Circle












AT the last Annual Visitation of the Royal Observatory, I stated to the Visitors (in my Annual Report) my opinion, that the Meridional Instruments of the Royal Observatory, though satisfactory in other respects, are not furnished with sufficiently large Object-Glasses; and that the effects of this deficiency, in the present state of Astronomy, are so serious, that I think it imperatively necessary for us to take steps for obtaining Meridional Instruments with larger Object-Glasses. The Visitors entertained this proposal so far as to desire that I would make inquiries and report to them the result. Before making an Official Report or Proposal, I beg leave unofficially to state the information which I have received, and to describe the Instrument which I am disposed to recommend.

The clear aperture of the Object-Glass of our Transit Instrument is very nearly five inches – that of our Mural Circle is very nearly four inches. I think that it is useless to propose to change these Instrument for others, carrying Object-Glasses of six inches aperture  – the advantage of the increased light not compensating for the trouble and expense of making the change. I have, therefore, in all my thoughts on this subject, contemplated the construction of Meridional Instruments with Object-Glasses of eight inches aperture; and having satisfied myself that there is no mechanical difficulty which will, in the smallest degree, interfere with the accuracy of the results to be obtained by instruments of that size, I have made some inquiries regarding the possibility of procuring the Object-Glasses.

I cannot learn that there is, at the present time, actually on sale an Object-Glass of eight inches aperture. But Messrs. MERZ & SON, of Munich, undertake to make an Object-Glass of eight French inches aperture, and I38 French inches focal length (or 12 feet English nearly), for 4,920 florins (between £410 and £420). One such Object-Glass has been procured from them, under my direction, for a Provincial Institution; and though I have not had very good opportunities for examining it, I have reason, as far as I have seen, to be satisfied with it.

To the Visitors, who are acquainted with the construction of our Mural Circle, it is almost unnecessary to remark that an Object-Glass of this size and weight cannot be carried by an Instrument constructed in any way like the Mural Circle; but I see no difficulty whatsoever in mounting it with the form and general arrangement of a Transit Instrument.

My proposal, therefore, is this: To construct a Transit-Circle, with a Telescope of 12 feet focal length, and eight inched aperture. This Transit-Circle to consist of a Transit Instrument, carrying a graduated Circle of five or six feet diameter on one of the conical arms of its axis. On the other arm, a fixed clamping Circle of the same dimensions may be carried (a construction which I greatly prefer to that of the Clamp by a small Ring, usually adopted by the German artists). The transit-pivots to be furnished with Microscopes for the examination of their movements; the inside of the Telescope to be furnished with stops for strength, and also with stops to protect the wire-plate from the sun’s rays: and such other details of construction to be adopted as have been found conducive to the firmness and accuracy of the best modern Instruments. The graduations of the Circle to be read by five Microscopes, diverging conically through a stone pier, and other Microscopes to be permanently fitted for the examination of the graduations.

I propose to use this Instrument without reversion. For determining the Error of Collimation as respects transit-observations, I propose to have two large Telescopes fixed to stone piers, one on the north, and the other on the south side: a proper apparatus must be provided for raising the Instrument only so far, as to permit these auxiliary Telescopes each to view the other. For the Zero Point in Altitude, I propose to use observations by reflection in mercury, in the same manner as in the actual system of the Royal Observatory.

I propose to have no level; referring to the zenith, in transit-observations, by observing transits by reflexion, or observing the wires by reflexion.

For the horizontal wire of the Instrument, I propose to use a double wire (a construction adopted in many German instruments), carried by a Micrometer.

A single observer, so far as I can see, would find no difficulty in observing the Right Ascension and the Polar Distance, with the utmost accuracy, at the same transit. This remark applies to all objects, except the Sun, and the Moon on those occasions when both the upper and the lower Limb are to be observed; at these times, the assistance of another person to read the Microscopes would be required.

The double observation of a great number of objects would undoubtedly be laborious. But, having regard to the local circumstances of the Observatory, it would be a great point gained to reduce the number of persons required for Observation, as it would then be possible to enforce a regularity of attendance, not contingent on the state of the weather, which at present it is difficult to maintain.

I have omitted some small details of construction, which, though sufficiently arranged in my own thoughts, are not so important as now to require special mention. I have also omitted some points of greater importance, which cannot be arranged without serious discussion with an Instrument-maker. Among these is the general question, whether the Instrument should be constructed of Iron or Brass.

I trust that I may shortly receive from the Visitors such remarks upon my proposal, as will enable me either to lay it aside for the present, or at once to solicit from the Government the powers which will be necessary to enable me to proceed with it.


Royal Observatory, Greenwich,

20th December, 1847.





Transcribed from a printed copy deposited with the minutes of the Board of Visitors (ADM190/4)