Page:Physical Geography of the Sea and its Meteorology.djvu/127

Rh thither by the winds, resides the chief source of the dynamical power which gives them motion. In some aspects vapour is to the winds what fuel is to the steam-engine: they carry it to the equatorial calm belt; there it rises, entangling the air, and carrying it up along with it as it goes. As it ascends it expands, as it expands it grows cool; and as it does this its vapour is condensed, the latent heat of which is thus liberated; this raises the temperature of the upper air, causing it to be rarefied and to ascend still higher. This increased rarefaction calls for increased velocity on the part of the inpouring trade-winds below.

262. The effect of the deserts upon the trade-winds.—Thus the vapours uniting with the direct solar ray would, were there no counteracting influences, cause the north-east and south-east trade-winds to rush in with equal force. 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. Indications of a crossing at the calm belts.—We have, in this difference as to strength and stability (§ 254) between the north-east and south-east trade-winds, another link in 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 hemisphere, 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 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 liberation 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 turned back and