29 f) HOW RAIN IS FORMED. 



iiig- currents are dry and bring fine weather. And tins holds good 

 whatever may be the immediate cause of these movements. We may 

 now proceed to consider tliese greater examples to which I have already 

 referred. 



In the great workshop of nature, in so far at least as concerns our 

 earth, with but few exceptions, all movement and all change, even the 

 movements and energies of living things, proceed either directly or 

 indirectly from the action of the sun. Nowhere is this action more 

 direct and more strikingly manifested than in the movements of the 

 atmosphere. Were the sun extinguished, and to become, as perhaps 

 it may become long ages hence, a solid cold sphere, such as Byron im- 

 agined, " wandering darkling in eternal space," a few days would suffice 

 to convert our mobile and ever-varying atmosphere into a stagnant 

 l)all, devoid of vapor, resting quiescent on a lifeless earth, held bound 

 in a more than Arctic frost. From such a consummation, despite the 

 sui)posed decaying energy of our sun, we may however entertain a 

 reasonable hope that we are yet far distant. 



Bearing in mind the all-embracing importance of the sun, let us see 

 how the great movements of tlie atmosphere are determined by the way 

 in which the earth presents its surface to the solar rays. 



Since the quantity of solar heat received on each part of the earth's 

 surface depends on the directness or obliquity of his rays — in other 

 words, on the height to which the sun ascends in the heavens at noon — 

 being greatest where he is directly overhead, as in summer in the trop- 

 ics, it follows that the hottest zone of the earth is that in the immediate 

 neighborhood of the equator, and the coldest those around the poles. 



Did time allow, and were the necessary appliances at hand, it would 

 be easy to show you that both as a matter of experiment, and also as a 

 deduction from physical laws, there must be under such circumstances 

 a flow of air from the colder to the warmer region in the lower atmos- 

 phere, and a return current above. And to a certain extent we have 

 these constant winds prevailing for about 30° on either side of the 

 equator in the trade-winds, which blow towards the equator in the lower 

 atmosphere, and the anti-trades blowing in the opposite direction at a 

 great height above the earth's surface. 



In the neighborhood of the equator there is a zone extending right 

 round the earth in which the barometer is lower than either to the north 

 or the south. It is due to the greater heat of the sun, and it is towards 

 this that the trade-winds blow. It shifts to some extent with the seasons, 

 being more northerly in the summer of the northern hemisphere, and 

 more southerly in that of the southern hemisphere; and its average 

 ])Osition is rather to the north of the equator, owing to the fact that 

 there is more land in the northern than in the southern hemisphere, and 

 that land is more heated by the sun than the ocean. 



This simple wind system of the trades and anti-trades does not extend 

 right round the earth, nor beyond 80° or 40Oof latitude in either hemi- 



