82 THE PHYSICAL GEOGRAPHY OF THE SEA. 



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 would blow on the surface 

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

 current had carried off. 



151. These two winds would blow due north and south ; the 

 effects of heat at the equator, and cold at the poles, would 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 (§ 124), approach 

 the equator on both sides with easting in them, and each pole 

 with westing. 



152. The ckcumference 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 (§ 149) 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 (§ 144) by the current which is moving in the oppo- 

 site direction. 



153. 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 which increases its mo- 

 bility, 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, there- 

 fore, 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 grav- 

 ity alone, without any change in volume, this quality would also 



