312 On IVhids, 



the weftern, to which the fun approaches nearer, is fnor« 

 heated than the eaftern, from which the fun recedes; there- 

 fore in liiis, and all the fucceeding inftants, the eaftern, being 

 niore cooled, will prefs on the wcftern, and thus an eaftera 

 wind will be eflablidied. 



It is true, that, in the northern hemifphere, thq northern 

 air alfo preffes upon the more heated fpaces ; but as this alfo 

 follows the fun's palh to the weflward, it becomes alfo eaft- 

 erly, preferving oulv a few points of its primitive dlrertion. 

 D'Alembert adds alio the folar attraftion, which, according 

 to him, elevates the air in the points over which the fun is 

 vertical, and confequenth' produces a dilatation advancing 

 from eaft to weft. But M. De la Place, not denying this 

 caufe, confidcrs it too weak to produce fmgly any conliderd- 

 ble eflea*. 



About the year 1735 Mr. Hadley publifhed a very different 

 account of the origin of the trade winds (Phil. Tranf. Abrid. 

 viii, p. 5C0); which, however, has been rejcfted by the moft 

 diftinguifhcd aftrononiers that have fince attended to this ob- 

 jeft, "as d'Alembert, fur la Caufe des Vents, art. 376 and 

 385; Gentil Voy.; Bergman Erde Befchreib. ii. p. 91. 



According to Mr. Hadley, the air, being rarefied towards 

 the equator, is confcqnentlv invaded in the northern hemi- 

 fphere by the northern, and in the fouthern hemifphere by 

 the fouthern colder air. 



.' But as the parallels of latitude enlarge as they approach 

 the equator, and as the equatorial fpace is nearly in the pro- 

 portion of 1000 to 917, the difference of their circumference 

 is nearly, 2083 miles ; confequcntly, the furface of the globe 

 at the equator moves fo much fafter than under the tropics ; 

 and hence the northern or fouthern air, moving from the 

 tropics towards the equator, muft polfefs lefs velocity than 

 the parts it arrives at, and confequentlv appear to move in a 

 direftion contrary to that of the earth's motion ; which being 

 from weft to eaft, the air arriving fooner at. the weftern parts, 

 will appear to m(?ve from eaft to weft; and this relative mo- 

 tion being combined with that towards the equator, a north- 

 caft wind will be produced on the north fide, and a fouth-eaft 

 wind on the fbutli fide of the equator. 'I'hefe as they ap- 

 proach the e(|uator fl-.ould become ftronger and more eafterlv, 

 and appear due eaft in the equator ilfelf, by reafon of the 

 concourfe of both currents from the north and from the 

 'fouth. There the vcl<Scity of each ftiould l)e at the rate of 

 20^'3 miles in the fpace of one natural day, or above r33 



• Mlii\ Paris 1776. 



miles 



