VOL. LXXXIII.] PHILOSOPHICAL TBANSACTIONS. 307 



sit instrument, and passing in review before such a microscope, or a pair of such 

 microscopes, would answer the purpose. I did not then know that a microscope 

 of that kind had been applied by the late Due de Chaulnes, to his dividing engine, 

 for determining the divisions; described minutely by him, and published in 1768; 

 a copy of which is in our library. Neither did I then know of the same idea 

 having been the foundation of Roemer's method of reading off the divisions on 

 his circulus meridionalis ; an account of which was published by Horrebow, in 

 the beginning of this century ; where a reticule of 10 squares was made, by trials 

 of its distance from the limb of the instrument, to coincide with a division of 10' 

 on that limb. With them I was not acquainted, till after my instrument was 

 already in some forwardness. Whether Mr. Ramsden took the first hint from 

 either of them, and improved on it, I cannot say. He has brought it into use 

 among us ; I certainly derived it from him ; and to him I acknowledge myself in- 

 debted for it. 



This method of reading off has indeed been applied already with great success 

 to different instruments ; but I do not know that it has ever yet been adapted to 

 the transit. Circles of various kinds have been constructed with great accuracy, 

 yet all have been formed with another view ; and their turning freely in azimuth, 

 seemed to render them less fit for the purpose which I wanted ; i. e. a circle, 

 firmly fixed, and turning truly in the plane of the meridian by means of a trans- 

 verse axis ; with all the adjustments of a transit at the end of the axis itself; and 

 at the same time with the opposite readings, and all the adjustments of the circles 

 now in use. 



On this idea the following instrument was constructed. My first design was, 

 not to have given orders for one myself, but merely to communicate the thought 

 to those who might improve on it. Accordingly I mentioned it first to Mr. 

 Ramsden, in 1788 : but the multiplicity of his engagements, and the fertility of 

 his own imagination, rendered him disinclined to listen to a scheme for one on 

 another plan. The same was the case with Mr. Troughton, I mentioned it 

 likewise to several of my acquaintance : but no one was set about. After 3 years 

 waiting, and becoming more and more convinced of the advantages of such an 

 instrument to astronomy ; and Mr. Gary being recommended to me, as fully 

 qualified for the purpose ; though I am growing too old to expect to make many 

 more observations, I gave orders for one of a size and form which I thought 

 most convenient to myself. 



The instrument stands on 3 feet, adjustable by screws. The bottom plate 

 (of 21i inches diameter) turns in azimuth ; not on a long axis, but on a centre; 

 and rides on a bell-metal circle, truly turned, and to which the bottom plate it- 

 self is ground. In this way it moves very smooth by hand ; but it is capable of 

 being turned by a winch, with tooth and pinion. The intent of its turning thus, 



RR2 



