THE ATMOSPHERE. 97 



were there no counteracting influences, cause the north-east and 

 The effect of the south-cast tradc-wiuds to rush in with equal force. 

 tSde-wSJ! * But there is on the polar side of the north-east trade- 

 winds an immense area of arid plains for the heat of the solar ray 

 to beat down upon, also an area of immense precipitation. These 

 two sources of heat hold back the north-east trade-winds, as it 

 were, and, when the two are united, as they are in India, they are 

 sufficient not only to hold back the north-east trade-wind, but to 

 reverse it, causing the south-west monsoon to blow for half the 

 year instead of the north-east trade. 



263. We have, in this difference as to strength and stability 

 croS'^rtthe^ (^ ^^^^ between the north-east and south-east trade - 

 calm belts. wiuds, another link in the chain of faats tending to 

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

 greatest amount of evaporation takes place in the southern hemi- 

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

 BO 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 vapour 

 which affords this excess of precipitation brings the heat — the 

 dynamical power — from the southern hemisphere; this vapour 

 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 vapour is precipitated, thus assisting to make 

 the northern warmer than the southern hemisphere. In those 

 northern latitudes where the precipitation of vapour 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 north-east 

 trade, is tm^ned back and called in to supply the indraught towards 

 the north. Thus the north-east trade-winds being checked, the 

 south-east are called on to supply the largest portion of the air 

 that is required to feed the ascending columns in the equatorial 

 calm belt. 



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

 — ^rT^^r *™h^b ^^^ ^^ ^'^^ south side in the southern hemisphere, 

 pole in spirals. the prevailing direction of the winds is not towards 

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

 tropical region of each hemisphere the prevailing winds blow from 

 the equator towards the poles. These are the counter- trades 

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

 ]K)les produce in the amount of heat set free, according to Black's 



H 



