TOL. XI.] PHILOSOPHICAL TRANSACTIONS. 327 



astronomers by the name of the parallax of the orbit ; this inequality, which is 

 easily discovered by observations, I make the solid basis of the following 



chiefly applied himself to mathematics and astronomy, in which he soon distinguished himself in a 

 remarkable manner, being only in his ipth year when he produced the above paper in the Philoso- 

 phical Transactions, on the aphelia and eccentricity of the planets. He made a great number of ac- 

 curate observations in astronomy; and tlie forming in that way an entire new catalogue of all the 

 stars was a favourite object ; but finding that project already occupied by Hevelius and Flamsteed, he 

 formed tlie design of completing their scheme, by the addition of the stars about the soutli pole, 

 which could not be seen by those astronomers in the latitude of Dantzic or Greenwich. For this 

 purpose, he left the university before he had taken any degree, and sailed for the island of St. 

 Helena in 1676, when he was only 20 years of age. Here, with great diligence he soon completed 

 his catalogue of those stars, with which he returned to England the latter end of l678, when the 

 Royal Society immediately elected him one of their members, and the king (Charles 2d) gave him a 

 mandamus to the university of Oxford, for the degree of A. M. In l679 he went to Dantzic, at the 

 request of tlie Royal Society to endeavour to adjust a dispute between M. Hevelius and Mr. Hook, 

 concerning the preference between plain and telescopic sights in astronomical instruments j from 

 whence he returned in about two months. 



In 168O he set out on a tour tlirough France and Italy, to establish a friendly communication 

 among the astronomers of Europe. In Paris he completed his observations on the great comet of that 

 year, which he had before seen in England. He returned to England in 168I, and married a lady, 

 with whom he lived happily for 55 years after. In l6S3 he published his " Theory of the Variation 

 of the Magnetical Compass 3" in which he supposes the whole globe of the earth to be one great 

 magnet, having four magnetical poles or points of attraction, &c. The same year also he entered on 

 a new method of finding the longitude, by an accurate observation of the moon's motion. In the 

 beginning of ifiS-i, contemplating Kepler's laws of the periods and distances of the planets, he con- 

 cluded that the centripetal force must decrease in proportion to the square of the distance reciprocally. 

 He found himself however unable to make it out in any geometrical way ; and therefore, after ap- 

 plying in vain for assistance to Mr. Hook and Sir Christopher Wren, he repaired to Cambridge to Mr. 

 Newton, who fully supplied him with what he so ardently sought. But Halley having now found 

 an immense treasure in Newton, could not rest till he had prevailed with the owner to enrich the 

 public with it j and to this interview the world is in some measure indebted for the immortal Prin- 

 cipia of Newton. That great work was published in 16863 and Halley, who had the whole care of 

 tlie impression, prefixed to it a discourse of his own, giving a general account of the astronomical 

 part of the workj and also an elegant copy of verses in Latin. In l687 he undertook to explain the 

 reason why the Mediterranean Sea never rises higher, though there is no visible discharge of the 

 prodigious quantity of water that runs into it from nine large rivers, besides many small ones, and 

 tlie constant setting in of the current at the mouth of the Straits ; which he accounted for by the 

 great quantity of waters raised from its surface by evaporation, which he showed by a calculation was 

 fiiUy adequate to the purpose. Halley's active and elevated mind next ranged through various other 

 fields of science ; hence resulted his tracts on tlie constiuction of solid problems, or equations of the 

 3d and 4th powers, with a new method for the number and the limits of their roots ; exact tables of 

 the conjunctions of Venus and Mercury, with their use in discovering the parallax and distance of the 

 sun ; new tables for showing the values of annuities on lives, calculated from bills of mortality j the 

 universal theorem for finding the foci of optic glasses. But it would be endless to enumerate all his 

 valuable discoveries, then communicated to the Royal Society, and published in the Philosophical 

 Transactions, of which for many years his pieces were the chief ornament and support, in all the 



