irct. ii.] DISSERTATION SECOND. 63 



made an addition to the catalogue of the stars, of 360 from 

 the southern hemisphere, but returned with great acquisi- 

 tions both of nautical and meteorological knowledge. His 

 observations on evaporation were the foundation of two 

 valuable papers on the origin of fountains ; in which, for the 

 first time, the sufficiency of the vapour taken up into the at- 

 mosphere, to maintain the perennial flow of springs and ri- 

 vers, was established by undeniable evidence. The diffi- 

 culty which men found in conceiving how a precarious and 

 accidental supply like that of the rains, can sufficiently pro- 

 vide for a great and regular expenditure like that of the 

 rivers, had given rise to those various opinions concerning 

 the origin of fountains, which had hitherto divided the scien- 

 tific world. A long residence on the summit of an insulat- 

 ed rock, in the midst of a vast ocean, visited twice every 

 year by the vertical sun, would have afforded to an observer, 

 less quick-sighted than Halley, an opportunity of seeing the 

 work of evaporation carried on with such rapidity and co- 

 piousness as to be a subject of exact measurement. From 

 this extreme case, he could infer the medium quantity, at 

 least by approximation ; and he proved that, in the Medi- 

 terranean, the humidity daily raised up by evaporation is 

 three times as great as that which is discharged by all the 

 rivers that flow into it. The origin of fountains was no 

 longer questioned, and of the multitude of opinions on that 

 subject, which had hitherto perplexed philosophers, all but 

 one entirely disappeared. 1 



Beside the voyage to St. Helena, Halley made two 

 others ; the British government having been enlightened, 

 and liberal enough to despise professional ciiquelle, where 

 the interests of science were at stake, and to entrust to a 

 Doctor of Laws the command of a ship of war, in which 



1 Philosophical Transaction.?. 1GS7. Vol. XVI. p. Sfifc. 



