190 AIR AND LIFE. 



MOVEMENTS OF THE ATMOSPHERE. 



We must now consider another side of the general topic of the phys- 

 ics of air. I refer to the movements which unceasingly occur in the 

 vast ocean of gas which surrounds our planet. They are familiar to 

 all. It is these that swell the sails of vessels and carry them across the 

 oceans, that give the impulse to the old-fashioned windmill, that lift 

 the waves and send them rolling from continent to continent; these, 

 also, that, with cyclones and tornadoes, uproot trees, blow down bouses, 

 destroy crops, snap the giants of the forests like mere twigs, raise 

 clouds of dust, and spread ruin and death on every side. Breeze or 

 tempest, it always is air in motion, and in this case as well as* in others 

 air is both beneficent and maleficent. Concerning the cause of this 

 motion, be it gentle or be it violent, it is enough to remind the reader 

 that the main if not exclusive cause is in difference of calefaction, and 

 that the wind blows from cool areas to warm ones. 



What part can these movements of the atmosphere play in the life of 

 our planet 1 ? What is their influence? A superficial glance is enough 

 to show that this is manifold. 



In the first place, they help to intermingle the constituents of the 

 atmosphere. To be sure, the general constitution of air is the 

 same everywhere, no considerable difference existing. But we have 

 referred more than once to the numerous local causes of alteration. 

 Consider, for instance, a large industrial town or a volcano. Both 

 exhale an enormous amount of obnoxious gases which are poured 

 into the atmosphere — carbon dioxide, carbon monoxide, and a hundred 

 other substances, toxic or inert; at all events undesirable for breath- 

 ing. A few figures have been given above concerning the amount 

 of such gases produced by mankind, by combustion, etc., and we all 

 know that in cities the composition of air is less pure than in the 

 country; that Manchester, Birmingham, Chicago, Pittsburgh, etc., are 

 less healthy than their surroundings. If there were no winds, most 

 certainly things would be much worse than they are, and, the very fact 

 that city air is inferior to country air, substantiates this assertion. 

 Without winds all these gases would accumulate about the place 

 where they originated. Of course some diffusion would take place, 

 but the process would be a slow one, and a much too great proportion 

 of unhealthy gases would at all times be found in the air of such places, 

 which would thus be more insalubrious than they now are. Without 

 winds locally vitiated air would remain such, just as is the case with 

 the atmosphere in a closed room where men or animals are assembled; 

 wind is the cleanser of the atmosphere, the great purifier, which mixes 

 and purifies it, which chases it over lands and seas, over fields and 

 forests, from pole to equator, and from equator to pole, thus dissipating 

 in the whole mass those elements which, for one reason or another, are 

 produced in greater abundance at some points; it maintains the purity 



