Dr. Croll's Theory of Glacial and Warm Periods. 79 



bourhood of the equator, which may be looked upon as a 

 terrestrial furnace, and their least, at the two poles, which 

 may be considered as two refrigerators. 



It follows that about the equator the air is not only 

 being heated and expanded, but since heat is the great 

 evaporator it is also being charged with moisture more 

 rapidly than it is elsewhere, and that if the earth were 

 homogeneous, and entirely covered with water, there would 

 be a progressive lightening of the air going on from its 

 densest and least humid portions at the poles to its most 

 expanded, and most humid at the equator. That is to say, 

 there would be a continuously diminishing atmospheric 

 pressure as we neared the equator from the poles. The 

 result would be that a column of damp and expanded, and 

 therefore light air would rise up continuously round the 

 equator. In order to restore equilibrium, the void thus 

 created would be filled by an indraft from the colder 

 regions on either side, the ascending column would presently 

 spread out in both directions, north and south, and thus 

 a circulation would begin causing near the ground a 

 more or less north wind in the northern hemisphere, 

 and a south wind in the southern to blow towards the 

 equatorial region, while above these two winds would 

 be two other currents flowing in the opposite direction. 

 If the earth were a cylinder and not a sphere, and there 

 were no cold of space to invite radiation and cool the 

 currents of air as they rise from the ground, and if its surface 

 were homogeneous, and covered all over either with water 

 or dry land, there would probably be a perfect and con- 

 tinuous circulation of winds between the equator and the 

 poles — the postulated foci of least and greatest barometrical 

 pressure. The three factors just named, however, introduce 

 a very different condition of things, and limit the area over 

 which these winds can blow very materially. 



In the first place let us turn to the shape of the earth. 



