ON THE PRODUCTION OP RAIN. 151 



entering on this n3w phase of enquiry. And in view of these, it 

 seems expedient to begin at the beginning; and by anticipating, 

 prevent the questionings which might have no end if a different 

 course were followed. 



Sect. I. — On the Production of Rain. 



In consequence of the comparative heat in the tropics the air there 

 expands, and colder air of greater density flowing in over the surface 

 of the earth or sea, the expanded air is borne upwards from below, 

 till, cooled at a higher elevation, it begins to descend, to the north 

 and to the south of the equatorial region ; but, excepting in excep- 

 tional circumstances, it reaches not the earth again until it has been 

 cooled down to the temperature of the air at that lower level. This 

 process going on continuously, there are produced two sets of cun-ents 

 in the atmosphere : one from the Northern Artie regions, sweeping 

 along, over earth and sea, towards the equator, occasioning the north- 

 east Trade Winds, and the north winds and east winds so dreaded by 

 the suflferers from rheumatism in the northern hemisphere ; and 

 another from the Southern Antartic regions, sweeping along in like 

 manner, on or near the surface of the globe, towards the equator, 

 occasioning the south-east Trade's and the famous South-easter of the 

 Cape of Good Hope. But though these two sets of currents inter- 

 mingle, it may be, at the equator, they can scarcely be said to combine; 

 foi', ascending there in a conjoined m^ss, each portion curves over 

 almost in its entirety towards the region whence it comes, but with a 

 tendency from west to east, instead of that from east to west with 

 which it came. This is accounted for thus : the earth rotates from 

 west to east, cai-rying with it not only the water of the sea but the 

 air of the atmosphere ; being a sphere, or spheroid, each spot in the 

 polar regions is carried through a more limited space in the twenty- 

 four hours than is any spot nearer to the equator ; its motion per 

 hour, minute, or second, is less rapid than is that of any spot over 

 which the air may pass in its course to the equator ; and at this spot 

 the wind, moving onward, without diminishing its velocity, may be 

 dragged forward a little ; but at last it will be left behind ; and thus 

 an apparent current from some easterly point to some westerly point 

 is given to the current flowing from the pole towards the equator. 

 And the returning currents from the equator, carrying with them the 

 velocity from east to west, there acquired when they impinge upon 

 the surface of the globe, having a greater velocity in that direction 



