THE ATMOSPHERE. 77 



ot the world that it actually has at this moment, what w^ould take 

 place, supposing the uniform temperature to be a mean between 

 that at the equator and that at the poles ? Why, this would take 

 place : a swelling up of the atmosphere about the equator by the 

 expansive force of inter-tropical heat, and a contraction of it about 

 the poles in consequence of the cold. These two forces, consid- 

 ering them under their most obvious effects, would disturb the 

 supposed atmospherical equilibrium by altering the level of the 

 great aerial ocean ; the expansive force of heat elevating it about 

 the equator, and the contracting powers of cold depressing it about 

 the poles. And forthwith two systems of winds would commence 

 to blow, viz., one in the upper regions from the equator toward 

 the poles, and as this warm and expanded air should flow toward 

 either pole, seeking its level, a wind w^ould blow on the surface 

 from either pole to restore the air to the equator which the upper 

 current had carried off. 



These two winds would blow due north and south ; the effects 

 of heat at the equator, and cold at the poles, wo^ld cause them so 

 to do. Now suppose the earth to commence its diurnal rotation ; 

 then, instead of having these winds north and south winds, they 

 will, for reasons already explained (^ 97), approach the equator 

 on both sides with easting in them, and each pole with westing. 



111. The circumference of the earth measured on the parallel 

 of 60° is only half what it is when measured on the equator. 

 Therefore, supposing velocity to be the same, only half the vol- 

 ume of atmosphere (^ 109) that sets off from the equator as an 

 upper current toward the poles can cross the parallel of 60° north 

 or south. The other moiety has been gradually drawn in and 

 carried back (^ 108) by the current which is moving in the oppo- 

 site direction. 



Such, and such only, would be the extent of the power of the 

 sun to create a polar and equatorial flow of air, were its power 

 confined simply to a change of level. But the atmosphere has 

 been invested with another property w^hich increases its mobility, 

 and gives the heat of the sun still more power to put it in motion, 

 and it is this : as heat changes the atmospherical level, it changes 

 also the specific gravity of the air acted upon. If, therefore, the 

 level of the great aerial ocean were undisturbed by the sun's rays, 

 and if the air were adapted to a change of specific gravity alone, 



