VOL. XXXIX.] PHILOSOPHICAL TKANSA.CTIONS. 2t 



the diurnal motion of the earth in those parts; which being combined with the 

 motion towards the equator, a n. e. wind will be produced on this side of the 

 equator, and a s. e. on the other side. These, as the air comes nearer the 

 equator, will become stronger, and more and more easterly, and be due east at 

 the equator itself, according to experience, by reason of the concourse of both 

 currents from the n. and s. where its velocity will be at the rate of 2083 miles 

 in the space of one rotation of the earth or natural day, and above 1 mile and -^ 

 in a minute of time ; which is greater than the velocity of the wind is sup- 

 posed to be in the greatest storm, which according to Dr. Derham's observa- 

 tions, is not above 1 mile in a minute. But it is to be considered, that before 

 the air from the tropics can arrive at the equator, it must have gained some 

 motion eastward from the surface of the earth or sea ; by which its relative mo- 

 tion will be diminished ; and in several successive circulations, may be supposed 

 to be reduced to the strength it is found to be of. 



Thus it appears the n. e. winds on this side of the equator, and the s. e. on 

 the other side, are fully accounted for. The same principle as necessarily ex 

 tends to the production of the west trade-winds without the tropics ; the air 

 rarefied by the heat of the sun, about the equatorial parts, being removed, to 

 make room for the air from the cooler parts, must rise upwards from the earth ; 

 and as it is a fluid, it will then spread itself abroad over the other air, and so its 

 motion in the upper regions must be to the n. and s. from the equator. Being 

 got up at a distance from the surface of the earth, it will soon lose great part 

 of its heat, and thus acquire density and gravity sufficient to make it approach 

 its surface again, which may be supposed to be when arrived at those parts be- 

 yond the tropics where the westerly winds are found. Being supposed at first to 

 have the velocity of the surface of the earth at the equator, it will have a 

 greater velocity than the parts it now arrives at ; and thus become a westerly 

 wind, with strength proportionable to the difference of velocity, which in 

 several rotations will be reduced to a certain degree, as before said of the easter- 

 ly winds, at the equator: and thus the air will continue to circulate, and gain 

 and lose velocity by turns from the surface of the earth or sea, as it approaches 

 to, or recedes from, the equator. To solve the phasnomena of the variations of 

 these winds at different times of the year, and- different parts of the earth, would 

 too far extend this paper. From what has been said it follows : 



1. That without the assistance of the diurnal motion of the earth, naviga- 

 tion, especially easterly and westerly, would be very tedious, and to make the 

 whole circuit of the earth perhaps impracticable. 2. That the n. e. and s. e. 

 winds within the tropics, must be compensated by as much n. w. and s. w. in 

 other parts : and generally all winds from any one quarter, must be compen- 



