In the year 1685 Halley had been appointed clerk to the Royal Society, and was the principal person employed in the publication of its Transactions.
In 1686, he published a paper, entitled 4n Historical Recount of the Trade Winds and Monsoons, observable in the Seas between and near the Tropics, with an attempt to assign their physical cause ; and in 1687, appeared his Es tinzate of the Quantity of Vapour raised out of the Seu by the warmth of the Sun, which gave so much satisfaction to the Royal Society, that they requested hint to pursue the sub ject. lIe accordingly published in 1691, his paper On the Circulation of the Watery Vapours of the Sea, and the Origin of Strings ; in which he first pointed out that line provi sion of nature, by which a constant circulation of water is kept up between the atmosphere and the ocean. Halley published in 1687, his paper On the Numbers and Limits of the Roots of Cubic and Iliquadratic Equations. in 1688, he published a correct epnemcris !Ur that year ; and in 1691, appeared his paper On the Time and Place of Julius Ciesar's Descent upon Great Britain, in which he considers it as demonstrated that Caesar landed in the Downs, a little to the northward of Dover cliffs, on the 26th of August, 55 before Christ. In consequence of having had the good
fortune to observe the transit of Mercury when he was at St Helena, Halley directed his attention particularly to this class of phenomena ; and in 1691, he published a paper On the visible Conjunctions of the Inferior Planets with the Sun, in which he has calculated all the transits of Mercu ry from A. D. 1605 to 1799, and all those of Venus from 918 to 2004. He was fully aware at this time of the great advantages which astronomy would derive from the obser vation of these phenomena ; and he remarks, that " the principal use of these conjunctions is accurately to deter mine the distance of the sun from the earth, or his paral lax, which astronomers have, by several methods, attempt ed in vain, while the smallness of the angles sought do easily elude the nicest instruments ; but in observing the ingress of Venus into, and egress from, the sun, the space of time between the moments of the internal contacts may be obtained to a second of time, that is, to of a second, or 4'" of the observed arch, by means of an ordinary tele scope and clock that goes accurately for 6 or 8 hours." Halley had, however, been anticipated in this ingenious re mark by our countryman James Gregory. See our life of