302 PHILOSOPHICAL TKANSACriONS. [aNNO 1793. 



part, and the length of the telescope, 1 feet, was found considerably too great 

 for circles whose diameter was not more than 6 inches, it became unsteady, and 

 unfit for any other purpose than that of finding and following a celestial object, 

 and, on account of its high price also, was, as far as I believe, but little made 

 use of. 



However, after some years had elapsed, the idea of an equatorial telescope was 

 again renewed by 3 several artists in this kingdom, Messrs. Ramsden, Nairne, 

 and DoUond, with many and very material improvements, such as to carry the 

 portable equatorial almost to perfection. Of this instrument Mr. Ramsden had 

 made 3 or 4, as early, I believe, as the year 17 70 or 1773 ; viz. one for the 

 late Earl of Bute, one for Mr. M'Kenzie, another for Sir Joseph Banks, and 

 lastly, one for myself; with which I made a great many astronomical and geo- 

 metrical observations in France and Italy, in the years 1774 and 1775, some of 

 which may be seen in a Memoir on the Heights of some of the Alps, printed in 

 the Philos. Trans, for 1777- Of this machine a plate and description in French 

 was printed in the year 1773, and reprinted in English in 1779- An ample ac- 

 count of this equatorial will be found in Mr. Vince's Treatise on Practical Astro- 

 nomy, p. 132. In 1771 Mr. Nairne published an account of his equatorial tele- 

 scope, in the Philos. Trans, for that year; and in 1772 or 1773, Messrs. P. and 

 J. Dollond printed an account of theirs. All these instruments were furnished 

 with counterpoises, and, in general principles, were at least similar, if not the 

 same. The preference that I was inclined to give at that time to my own instru- 

 ment, made by Mr. Ramsden, was owing to the peculiar advantage of a swing- 

 ing level, to the unexampled accuracy of its divisions, and its great portability. 

 If, in what I have just now said of the last 3 instruments, I should have com- 

 mitted any error with respect to the priority of their improvements, I must leave 

 that point to be settled by the artists themselves, and shall hasten to the descrip- 

 tion of the instrument I set out with. But first one wordvvith respect to an in- 

 strument that has been in frequent use on the continent, called, very absurdly, 

 a parallactic machine. 



The first notice that I find of it, is in the History of the Academy of Sciences 

 at Paris, for 1721, p. 18, in a memoir of Mr. Cassini, with a description and 

 plate of it ; also in the History of the same Academy for 1746, p. 121, where it 

 is said to have been proposed by Mr. Passement, but without any description of 

 it ; it will however be found described, with a plate of it, in the Dictionaire de 

 Mathematique par Mr. Saverien, 2 vols, quarto, 1753; and this account has been 

 copied into Owen's Dictionary of Arts and Sciences, in 4 vols, octavo. It ap- 

 pears to have been a frame of wood supporting a polar axis, with an equatorial 

 and declination circle, of only a few inches in diameter ; and was in fact no more 

 tli;m a very b:id st:md to a refracting telescope of 8 or 10 icet long, giving it a 



