The Greenwich mural circle was placed on its stone pier in the month of May 1812, and the observations for publication commenced the 11 th of June. In the last volume of the Transactions for the present year, (1813), the ast•onomer-royal has communicated his observations of the summer and winter solstice of 1812, and likewise a catalogue of some of the principal fixed stars, deduced from nearly 2000 observations. From what little is said of the construction of the instrument, it may be collect• ed, that no use whatever is made either of level or plumb line, but that the position of the polar point on the in strument is found by assiduous observation of the pole star ; and that the position of this point on the instru ment is inferred, by comparing- the observed places with the real places of a certain number of standard stars, pre viously determined by the instrument itself. By this method the ast•onomer-royal observes, that the position of his intrument may always be known to within a tenth of a second, a degree of precision which no level or plumb-line can ever be expected to obtain. We shall have occasion to insert this valuable catalogue, and to re commend the adoption of a method somewhat similar, when we shall have to explain the different uses of Bor da's repeating circle, which we have hitherto omitted to notice, certainly not from its inferior importance, but because we have confined ourselves chiefly to the instru ments of our own country. And as we propose to give a very minute account of the construction of this instru ment, we shall only at present remark, that in France it seems to have superseded the use of every other astrono mical instrument, for measuring either the zenith distan ces of heavenly bodies, or the angular distances of ter restrial ones.
This brief introduction embraces all the principal as tronomical circles which we know of in fixed observa tories. It is, however, our intention to give, under the article OBSERVATORY, a more detailed account of some of them, particularly Mr Groombridge's transit circle, and the mural circle now in use at Greenwich. For we mean here to confine our descriptiong, &c. to the three distinct classes of portable circles, namely, the Re flecting Circle, the Altitude and Azimuth Circle, and the 1?elleating Circle; and beginning with the reflecting cir cle, we shall endeavour to trace its progress through the principal improvements which it has undergone, from its first invention down to the present time.
On the Reflecting Circle.
TUE instrument which bears the appellation of reflec ting circle, is of the nature of Iladley's quadrant, and is built upon the same foundation; and, for the present, we shall consider our readers as acquainted with that well known instrument, a full account of the principles, properties, and use of which, shall be reserved, in order to be introduced under its appropriate title.
Although there are few purposes in practical astrono my and surveying, to which this kind of instrument may not be advantageously applied, yet it was originally con structed for, and has been chiefly used in 'making lunar observations, for finding the longitude at sea. The late Professor Mayer of Gottingen, the ingenious and labo rious astt onomer who first furnished tables for that pur pose, seems to have judged well of what kind of instru ment would ultimately be used, to effect the best prac tical solution of the lunar problem. Agreeable to this idea we find, that when he transmitted to England his solar and lunar tables, they' were accompanied by draw ings, and a description of the first reflecting circle, and Were published by the British Board of Longitude in the year 1770.
As it is not our intention to extend this article to an unnecessary length, we shall, without farther introduc tion, proceed to a description of Mayer's instrument.
Fig. 1. of Plate CXLIV. is a plan of this circle. Its chief part is a cross, surrounded by a circular limb all in one piece. Immediately above this is an index A, to which is affixed the telescope and horizon glass. Still higher, another index 13 carries the central glass. Both A and B move freely round upon a pin fixed in the cen tre of the circle, quite independent of each other, and both of them apply close to the border of the circle, for the purpose of reading off its divisions. From the form of the exterior ends of the indices, one would imagine that the limb was intended to be divided by diagonal lines, but as the vernier was known at that time, it is probable, that no instruments were ever constructed in this respect like the figure. At the time the circle was proposed by Mayer, the art of dividing instruments was in a very rude state, and to render bad dividing of little or no consequence, was the aim of this particular con struction.
It will be presently seen, that, with this circle, as many observations as we please may be successively made from point to point upon the limb, without reading off any more of them than the accumulated angle, having previ ously indeed noted the point at which we commenced. This is a very different thing from taking many observa tions with a sextant or quadrant, when the angles that compose a set, being all read off nearly upon the same part of the arc, will most probably be all 'affected with nearly the same error. It is also very different from ma king many unconnected observations with a circle, where the beginning and end arc read off in each ; for, in this ease, every angle will be charged with two errors ; whereas, when taken in the manner which the nature of this instrument requires, no more than two errors are charged upon the amount of observations, however often these may have been repeated.
To spew more particularly how this is performed, it will be necessary to return to our Figure. There the different parts are represented, as when the index and horizon glasses are parallel to each other ; in which po sition, an object will be seen in the telescope through the transparent part of the horizon-glass, in exact contact with its own doubly reflected image ; but by moving the index B towards the telescope, the reflected image will pass along to the left, and being followed by the instru ment, may be made to coincide with any object seen di rectly on that hand; and now the index B would mark the distance between the reflected and direct objects, but it is totally unnecessary, and would even counteract the advantages of the instrument, to pay any attention to this point upon the circle. Instead of this, let the index A be carried forward until a new point of parallelism is found, by making an object coincide with its own reflec ted image as at first. This being done, take the angle over again, by moving the index B towards the tele scope, and following the motion of the reflected image to the left. In this manner proceed, by alternate obser vations for parallelism of the glasses, and angular dis tance of the objects, until you have made a sufficient set. By this beautiful contrivance, a small instrument is, as far as dividing may be concerned, rendered at least equal to a large one without it ; but as the telescope, by which the contacts are made, is necessarily of small power, large instruments in this respect seem to have the advantage; yet, as many observations are required to make a set, the error arising from imperfect contact must be considera bly diminished on the well known principle of taking means.