156 PHILOSOPHICAL TRANSACTIONS. [_ANNO 1781. 



the arm f, screwed on the upper part of c, which arm serves also to confine the 

 wheel d to its place on c. The other pivot is held by the arm g fastened to f. 



Fig. g, is a plan of the brass work. The wheel d, which is in the form of a 

 ring, is laid on the upper part of p or c, and held by 2 small arms f and h, 

 screwed down to e with the screws i, i. 



Fig. 10 is a plan of the brass work ; d, d, is the wheel placed on the bed or 

 socket of the rim of the cylinder c, c, and is held down by the two pieces f, h, 

 which are screwed on c, c. The piece f projects over the centre of the index 

 plate to receive the upper pivot of the pinion ra, n, the fixed wire fastened to 

 c, c. o, p, the moveable wire fastened to the annular wheel d, d. The index 

 plate a is divided into 60 parts, each sub-divided into 2, and milled on the edge. 

 When the finger is drawn over the milled edge of the index plate from q to- 

 wards r, the angle mso, will open, and if drawn from r towards q, it will shut 

 again. The case c, c, must have a sharp corner t, which serves as a hand to 

 point out the division on the index plate. 



XXXIII. Concerning the Longitude of Cambridge in New Engkmd. By Mr. 



Joseph Hit lard. p. 502. 

 The difference of meridians between Greenwich and Cambridge has been 

 generally reckoned 4 h 44 m . This was what the late Dr. Winthrop made use of; 

 but I do not find that he determined it by actual observations, made by him at 

 Cambridge, compared with corresponding ones, made at the Royal Observatory 

 at Greenwich. It appears, that in 176Q, at the time of the transit of Venus, 

 the doctor was not quite certain of the longitude of Cambridge. He mentioned 

 4 h 44 m as near the truth ; but for better fixing it, he gave several of his obser- 

 vations of the eclipses of Jupiter's satellites to be compared with those made at 

 Greenwich ; but there were too few corresponding ones to determine the point 

 with precision ; and as modern astronomers do not place absolute dependence on 

 the difference of meridians deduced from the eclipses of Jupiter's satellites, 

 unless there has been a series of observations, both of immersions and emer- 

 sions, I have wished to find some observations of solar eclipses and occultations 

 of fixed stars by the moon, made at Cambridge, of which corresponding ones 

 were made at Greenwich. I have met with no observations of occultations made 

 by Dr. Winthrop ; but a solar eclipse was observed by him and several other 

 gentlemen, at his house, August 5, 176Q, at which I was present and assisting, 

 being then a resident graduate at Harvard College : this eclipse I find was ob- 

 served at Greenwich, where the beginning of the eclipse was seen at 5 h 2Q m 56 s 

 p. m. and the end at 7 h ll m 27 s p. m. apparent time. At Dr. Winthrop's house 

 at Cambridge, lat. 42° 25' n. the beginning of this eclipse was observed at 

 ll h 39 m 23 s A. M. and the end at 2 h 45 m 9 s P. m. apparent time. Allowing for 



