Asironomical Societj/. S69 



knowing that the laws which governed refraction were so little under- 

 stood* as to render all observations in which that element was mate- 

 rially involved, liable to errors greater probably than the quantities 

 he was in search of, invented the zenith sector. It was erected at 

 Gresham College, and consisted of a telescope 36 feet long, a divided 

 lire, and a plumb-line. The star selected for observation, and with 

 reference to which, indeed, his instrument was entirely constructed, 

 was one which passed within two or three minutes of the zenith of 

 Gresham College, was visible in the day-time throughout the year, 

 and was y Draconis : by observing its zenith distance when the earth 

 was in opposite points of her orbit, he found (as he erroneously con- 

 cluded) a sensible parallax, amounting to about 20 seconds, and, 

 consequently, determined that the Copernican system was the true 

 onef. 



In the mean time, the eclipses of Jupiter's satellites, — thanks to the 

 facilities of predicting them afforded by Cassini's tables, — had been 

 assiduously observed; and in the year 1G75, the discordances found 

 between the predicted and the observed eclipses enabled the cele- 

 brated Roemer to demonstrate that light was not instantaneously pro- 

 pagated, and that the discordances between the tables and the obser- 

 vations might be considered as the measure of its velocity. 



The year of Roemer's discovery was further marked by iinother epoch 

 in astronomical history, namely, the foundation of the Royal Obser- 

 vatory. Flamsteed, with his mural quadrant, detected a change of 

 place in the pole-star, amounting to 35, 40, or 45 seconds, attributed 

 it to parallax, and regarded it as confirmatory of Hook's discovery. 

 Indeed, the observations of Hook, as well as of those who preceded 

 him, although nominally in search of parallax, had for their object little 

 else than tlie confirmation or verification of the Copernican system ; 

 and this arrived at, there seems to have been but little disposition to 

 repeat them. 



Hence it was that, the brilliant discoveries of Newton having placed 

 the accuracy of the Copernican system beyond all possibility of doubt, 

 the investigation of parallax was not resumed till the latter end of 

 November, in the year i72"», at which time Molyneux erected his 24- 

 feet zenith sector, by Graham, in his observatory at Ke\v + . " On the 

 3d of December, y Draconis was, for the first time, observed as it 

 passed near the zenith, and its situation carefully taken with the in- 

 .strument J and again, on the 5th, 1 1th, and 1 2th, when, no material 

 change in the star's place having been detected, further observations 

 seemed needless, since it was a time of the year when no sensible 

 alteration of parallax could soon be expected." Bradley, however, 

 being on a visit to his friend Molyneux, was " tempted by curiosity 

 to repeat the observation on the 17th, and perceived the star pass a 

 little more southerly than when it had been observed before :" sus- 



• An Attempt to prove the Motion of the Earth, from Observations made 

 by Robert Hook. F.R.S. pp. 10 & 1 1. 



t Idem, p. i!5. 



X Philosopliicn! Transactions, vol. xxxv. p. 61^9. 

 N. S. Vol. 7. No. 11. Mm/ 1830. 3 13 pecting 



