§ 257, 258. THE ATMOSPHERE. 95 



Indications of a (S 247) between tlie northeast and southeast trade- 

 crossing at the calm . ^ , ^.^.^ . 



belts. wnids, another imk m the chain of facts tending to 



show that there is a crossing of the winds at the calm belts. The 

 greatest amount of evaporation takes place in the southern hemi- 

 sphere, which is known by the simple circumstance that there is 

 so much more sea-surface there. The greatest quantity of rain 

 falls in the northern hemisphere, as both the rain-gauge and the 

 rivers show. So likewise does the thermometer; for the vapor 

 which affords this excess of precipitation brings the heat — the 

 dynamical power — from the southern hemisphere; this vapor 

 transports the heat in the upper regions from the equatorial 

 cloud-ring to the calms of Cancer, on the polar side of which it 

 is liberated as the vapor is precipitated, thus assisting to make 

 the northern warmer than the southern hemisphere. In those 

 northern latitudes where the precipitation of vapor and libera- 

 tion of heat take place, aerial rarefaction is produced, and the air 

 in the calm belt of Cancer, which is about to blow northeast trade, 

 is turned back and called in to supply the indraught toward the 

 north. Thus, the northeast trade-winds being checked, the south- 

 east are called on to supply the largest portion of the air that is re- 

 quired to feed the ascending columns in the equatorial calm belt. 



257. On the north side of the trade-wind belt in the northern, 

 The counter trades and ou the south sidc ill the southern hemisphere, 



— they approach the , .,. ,. . _, .,. , 



pole in spirals. thc prevailing direction 01 the winds is not toward 

 the equator, but exactly in the opposite direction. In the extra- 

 tropical region of each hemisphere the prevailing winds blow 

 from the equator toward the poles. These are the counter-trades 

 (§ 204). The precipitation and congelation that go on about the 

 poles produce in the amount of heat set free, according to Black's 

 law (§ 253), a rarefaction in the upper regions, and an ascent of 

 air about the poles similar to that about the equator, with this dif- 

 ference, however : the place of ascent over the equator is a line, or 

 band, or belt; about the poles it is a disc. The air rushing in from 

 all sides will give rise to a wind, which, being operated upon by 

 the forces of diurnal rotation as it flows north, for example, will 

 approach the north pole by a series of spirals from the southwest. 



258. If we draw a circle about this pole on a common terres- 

 trial globe, and intersect it by spirals to represent the direction 

 of the wind, we shall see that the wind enters all parts of this cir- 



