403 NOTES OF A NATURALIST. 



stream goes to supply the rainfall of Western Europe, or to form 

 snow in the arctic regions, acts as a vehicle to transfer heat 

 from the tropics to the temperate and frigid zones. But it is 

 more than probable that a large part of the vapour in question 

 is carried back to the torrid zone, and that some of it is even 

 restored to the southern hemisphere. The south-eastern branch 

 of the Gulf-stream flows, at least partially, into the area of the 

 north-east trade-winds. These winds reach the lower region as 

 cold and very drj' winds. As they advance towards the equator, 

 and are gradually warmed, their capacity for aqueous vapour 

 constantly increases, and there can be no doubt that in both 

 hemispheres the trade-winds bear with them a large share of 

 the vapour which goes to supply the heavy rainfall of the 

 tropics. 



In the Pacific region we have direct evidence to this effect, in 

 the fact that in Hawaii, and elsewhere, the side of the islands 

 exposed to the trade-winds is that of heavy rainfall, and is 

 generally covered with forest. No sufficient data exist for 

 estimating the amount of vapour thus carried back to the 

 tropics from high latitudes on both sides of the equator, nor the 

 amount of heat set free by its condensation ; but we may form 

 some conception of its probable amount by considering that at 

 the moderate estimate of a mean annual rainfall of seventy-two 

 inches for the portion of the globe between the tropics, this 

 amounts to a yearly fall of 88,737 cubic miles, and that we can 

 scarcely reckon the share of this great volume of water supplied 

 by evaporation from the same part of the globe at more than 

 one-half. Still less is it possible to calculate the amount of 

 vapour annually transferred from the northern to the southern 

 hemisphere, which goes to neutralize the apparent effect of the 

 diversion of portions of the equatorial waters to the north side 

 of the line. In the Atlantic basin it is probable that the larger 

 part of the rainfall in the region including and surrounding the 

 Gulf of Mexico and the Caribbean Sea is supplied by vapour 

 carried from the temperate zone by the north-east trade-winds. 

 There is some reason to believe that a portion of the rainfall 

 of the great basin of the Amazons, south of the line, is also 

 supplied from the same source. Several travellers report that 

 during the rainy season the prevailing winds are from the west 

 and north-west, the latter being especially predominant at 



