ASTRONOMICAL CIRCLES having now entirely superse ded the use of the quadrant, and become the principal instruments of almost every observatory, we feel our selves called upon to devote a considerable portion of our work to the explanation of the principles of their con struction, and of the most eligible methods of using them to advantage.
The astronomical quadrant is so simple in its construc tion, and one instrument of this class differs so little from another, that the description of one will easily render in telligible the construction of all the rest. It is not so, how ever, with the astronomical circles. So very various have been the views and conceptions of the different al tists who have been employed in these instruments, and perhaps even of the same artist at different times, that, except in circles of small dimensions, we believe that two have ne ver yet been made alike. Hence we find no small diffi culty in selecting such examples as may be reasonably expected to continue of universal and permanent utility.
The circle of Piazzi, at Palermo, constructed by Mr Ramsden, was the first large instrument of modern con struction, and has become justly celebrated in the history of practical astronomy ; but it owes its celebrity certainly more to the industi y and sagacity of the observer into whose hands it has fortunately fallen, than to the accura cy or soundness of the principles on which it is construct ed ; fur though it contains numberless examples of beau tiful invention and contrivance, yet upon the whole it is an instrument that no judicious artist would ever wish to make again : We shall not therefore detain the reader with any particular description of it, but shall only ob serve, that it is an altitude and azimuth instrument. The diameter of the vertical circle is five feet. The plumb line by which it is adjusted, referring to two fixed points in the limb of the circle, cannot he examined at the mo ment of observation : From this cause, and from the ef fect of expansion, to which, from its faulty construction, it is much exposed, a single observation cannot be relied on, nearer than to five seconds, though a mean of several will probably be much within that limit. In the year 1793, Mr Wollaston published a description of a circle made for him by Mr Carey, and which he very properly terms a Transit Circle. This instrument had an accidental de fect, (the axis having been made rather too slender,) but with this exception the instrument was a very good one.
The beauty of the workmanship of Carey's instruments cannot be surpassed ; and the whole of this instrument was better imagined, than the large one above noticed of Ramsden. It was not, however, intended to turn freely in azimuth, but, on the contrary, principally designed as a transit instrument, which intention appears to have been accomplished with the greatest precision. For this prac tical success of combining two instruments in one, astro nomers are certainly much indebted to Mr Wollaston. Piazzi, in enumerating the advantages of the circle, men tions this combination ; but certainly his circle would be a very imperfect transit instrument, or more properly speaking, it would be absolutely useless in this capacity. Carey has constructed several of these instruments, which perhaps would be all improved by a very slight alteration in the mode of applying the plumb-line. His method is nearly similar to Ramsden's above noticed, and subject to the same objection, as will be better understood when we come to treat of the more detailed construction of parti cular instruments. It is in this circumstance (among many others) that Troughton's instruments differ from Carey's and Ramsden's. The principle of these two methods is thus shortly described by Mr Pond, in the Philosophical Transactions for 1806.
In Mr Ramsd..n's method, two points are taken on the limb of the circle ; and when these are brought into a given position, by means of a plumb-line passing over them, the microscope or index is made to coincide with the zero point of the divisions. By this method the er ror in collimation remains constant ; and if the adjust ment is by any accident deraired, it can easily be recti fied, and there will be no absolute necessity for frequent ly reversing the instrument ; so that this method seems well adapted for large instruments, particularly if placed on stone piers. But it is liable to this defect, that the adjustment cannot be examined at the moment of obser vation ; and if any change should take place in the gene ral uisposition of the frame-work, the observation will be erroneous, without the means of detection. It was pro bably to avoid this inconvenience, that Mr Troughton in most of his instruments, particularly if they were intend ed to move freely in azimuth, has preferred the other method.