CAUSE OF GENERAL TRADE WINDS HADLEY 7 



diminished, and in several successive circulations, may be supposed 

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



Thus I think the NE. winds on this side of the equator, and 

 the SE. on the other side, are fully accounted for. The same 

 principle as iiecessarily extends 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, 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 thereby 

 acquire density and gravity sufficient to make it approach its 

 surface again, which may be supposed to be by that time 'tis 

 arrived at those parts beyond 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 thereby become 

 a westerly wind, with strength proportionable to the difference of 

 velocity, which in several revolutions will be reduced to a certain 

 degree, as is said before, of the easterly 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 ap- 

 proaches to or recedes from the equator. I do not think it neces- 

 sary to apply these principles to solve the phenomena of the 

 variations of these winds at different times of the year, and differ- 

 ent parts of the earth ; and to do it would draw this paper into 

 greater length than I propose. 



From whatever has been said it follows : 



First, That without the assistance of the diurnal motion of the 

 earth, navigation, especially easterly and westerly, would be very 

 tedious, and to make the whole circuit of the earth perhaps imprac- 

 ticable. 



Secondly, That the NE. and SE. winds within the tropics must 

 be compensated by as much NW. and SW. in other parts, and 

 generally all winds from any one quarter must be compensated 

 by a contrary wind somewhere or other; otherwise some change 

 must be produced in the motion of the earth round its axis. 



