VOL. LXXXIII.J PHILOSOPHICAL TRANSACTIONS. 301 



After Tycho 1 meet with no instrument of this sort till the time of Christopher 

 Scheiner, about the year 1620, who made use of a small telescope, moving on a 

 polar axis, with an arc of 47° of declination, to observe the sun's disc commodi- 

 ously, and examine his spots ; an account of which will be found in his Rosa 

 Ursina, folio, Bracciani, l630, p. 347. But this instrument can hardly be con- 

 sidered as an astronomical one, being merely a contrivance to follow the sun with 

 a telescope, by means of one motion only, similar in its object with the heliostate, 

 described by Dr. Desaguliers, in his Mathematical Elements of Natural Philo- 

 sophy, lib. 5, c. 2. 



Again, Flamsteed's sector, which he has described in the prolegomena to the 

 3d volume of his Historia Coelestis, p. 103, though mounted on a polar axis, and 

 very ingeniously contrived for the purpose it was intended for, viz. to measure 

 the angular distances between the stars, having no divided circle at right angles 

 to the polar axis, to take right ascensions, cannot come into the class of equa- 

 torial instruments. Nor need I here mention Mr. Molyneux's telescopic dial, 

 (Sciothericum telescopicum, in 1686) though depending on the principle of a 

 polar axis, which, like a ring dial, or equinoxial dial, was little more than a play- 

 thing for an amateur in astronomy. 



But about the year 1730 or 1735, when the practice of astronomy had assumed 

 a new face in this kingdom, under the skill of Dr. Halley and of Dr. Bradley, 

 Mr. Graham invented his sector, for taking differences of right ascension and de- 

 clination out of the meridian ; and this may be considered as bearing a considera- 

 ble affinity to the equatorial instrument in principle, and differing from it only in 

 the extent of its powers. Of this instrument, which is well-known to every 

 practised astronomer, a complete account will be found in Smith's Optics, v. 2, 

 § 885, and in Mr. Vince's Astronomy. I approach now to the period when the 

 modern equatorial instrument, properly so called, took its origin. 



Mr. James Short, a person of very considerable eminence for his skill in the 

 theory and practice of optics, and particularly for the unexampled excellence to 

 which he had carried catoptric telescopes, in which, I believe, he has never yet 

 been exceeded : Mr. Short, I say, probably finding himself capable of making 

 telescopes, of very moderate dimensions, fit for many astronomical purposes, and 

 able to exhibit several of the heavenly bodies by day-light, provided they were 

 furnished with a convenient apparatus and movement for that purpose, applied a 

 2 feet reflecting telescope, for the first time, to a combination of circles, repre- 

 senting the horizon, the meridian, the equator, and moveable horary circle, or 

 circle of declination, each divided into degrees, and every 3d minute, furnished 

 with levels, &c. for adjustment to the place of observation. This machine was 

 invented in or before the year 1749, and is described in the Philos. Trans, for 

 that year. But as this instrument was furnished with no counterpoises in any 



