6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 I 



motion of the air will be communicated to the superficial parts, 

 and by little and little produce a revolution of the whole the same 

 way, except there be the same quantity of motion given the air 

 in a contrary direction in other parts at the same time, which 

 is hard to suppose. But if the globe of the earth had before a 

 revolution towards the east, this by the same means must be con- 

 tinually retarded. And if this motion of the air be supposed to 

 arise from any action of the parts of it on one another, the con- 

 sequence will be the same. For this reason it seems necessary to 

 show how these phenomena of the Trade Winds may be caused, 

 without the production of any real general motion of the air 

 westwards. This will readily be done by taking in the considera- 

 tion of the diurnal motion of the earth. For, let us suppose the 

 air in every part to keep an equal pace with the earth in its 

 diurnal motion; in which case there will be no relative motion of 

 the surface of the earth and air, and consequently no wind, then 

 by the action of the sun on the parts about the equator, and the 

 rarefaction of the air proceeding therefrom, let the air be drawn 

 down thither from the N. and S. parts. The parallels are each. 

 of them bigger than the other, as they approach to the equator 

 and the equator is bigger than the tropics, nearly in the propor- 

 tion of 1000 to 917, and consequently their difference in circuit 

 about 2083 miles, and the surface of the earth at the equator 

 moves so much faster than the surface of the earth with its air 

 at the tropics. From which it follows, that the air, as it moves 

 from the tropics towards the equator, having a less velocity than 

 the parts of the earth it arrives at, will have a relative motion 

 contrary to that of the diurnal motion of the earth in those parts, 

 which being combined with the motion towards the equator, a 

 NE. wind will be produced on this side of the equator and a vSE. 

 on the other. These, as the air comes nearer to the equator, will 

 become stronger, 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 revolution of the earth or 

 natural day, and above one mile and one-third in a minute of 

 time; which is greater than the velocity of the wind is supposed 

 to be in the greatest storm, which according to Doctor Derham's 

 observations, is not above one 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, wherebv its relative motion will be 



