Since this was written, a Discovery I have made in the Cœlestial Motions, seems to render a farther Account of the Use of the Cavity of the Earth, viz. To diminish the Specifick Gravity thereof, in respect of the Moon; for I think I can demonstrate that the Opposition of the Æther to the Motions of the Planets in long time becomes sensible; and consequently the greater Body must receive a less Opposition than the smaller, unless the Specifick Gravity of the smaller do proportionably exceed that of the greater, in which case only they can move together; so that the Cavity I assign in the Earth, may well serve to adjust its weight to that of the Moon, for otherwise the Earth would leave the Moon behind it, and she become another Primary Planet.
An Historical Account of the Trade Winds and Monsoons, observable in the Seas between and near the Tropicks, with an attempt to assign the Physical Cause of the said Winds, by Mr. Ed. Halley.
AN exact Relation of the constant and periodical Winds, observable in several Tracts of the Ocean, is a part of Natural History not less desireable and useful, than it is difficult to obtain, and its Phænomena hard to explicate: I am not ignorant that several Writers have undertaken this Subject, and although Varenius (Lib. 1. Chap. 21. Geo. Gen.) seems to have endeavour'd after the best information from Voyagers, yet cannot his Accounts be admitted for accurate, by those that shall attentively consider and compare them together, and some of them are most evident Mistakes; which, as near as I can, I shall attempt to rectify, having had the opportunity of conversing with Navigators, acquainted with all parts of India, and having liv'd a considerable time between the Tropicks, and there made my own Remarks.
The Substance of what I have collected is briefly as follows.
The Universal Ocean may most properly be divided into three Parts, viz. 1. The Atlantick and Æthiopick-Sea. 2. The Indian Ocean. 3. The Great South Sea, or the Pacifick Ocean; and though these Seas do all communicate by the South, yet as to our present purpose of the Trade Winds, they are sufficiently separated by the interposition of great Tracts of Land; the first lying between Africa and America, the second between Africa and the Indian Islands, and Hollandia Nova; and the last between the Phillipine Isles, China, Japan and Hollandia Nova on the West, and the Coast of America on the East. Now following this natural division of the Seas, so will we divide our History into three parts in the same order.
I. In the Atlantick and Æthiopick Seas between the Tropicks, there is a general Easterly Wind all the Year long, without any considerable Variation, excepting that it is subject to be deflected therefrom, some few Points of the Compass towards the North or South, according to the Position of the place. The Observations which have been made of these Deflections, are the following.
1. That near the Coast of Africa, as soon as you have pass'd the Canary Isles, you are sure to meet a fresh Gale of North East Wind, about the Latitude of 28 Degrees North, which seldom comes to the Eastwards of the East North-East, or passes the North North-East. This Wind accompanies those bound to the Southward, to the Latitude of ten North, and about a hundred Leagues from the Guinea Coast, where, till the fourth Degree of North Latitude, they fall into the Calms and Tornadoes; of which more hereafter.
2. That those bound to the Caribbee Isles, find, as they approach the American side, that the aforesaid North-East Wind becomes still more and more Easterly, so as sometimes to be East, sometimes East by South, but yet most commonly to the Northward of the East a Point or two, seldom more. 'Tis likewise observ'd, that the strength of these Winds does gradually decrease, as you sail to the Westwards.
3. That the limits of the Trade and variable Winds, in this Ocean, are farther extended on the American side than the African; for whereas you meet not with this certain Wind till after you have pass'd the Latitude of twenty eight Degrees on this side; on the American side it commonly holds to thirty, thirty one, or thirty two Degrees of Latitude; and this is verified likewise to the Southwards of the Æquinoctial, for near the Cape of Good-Hope the limits of the Trade Winds, are three or four Degrees nearer the Line, than on the Coast of Brazile.