86 THE PHYSICAL GEOGRAPHY OF THE SEA. 



racliets in the atmospherical machinery. We have seen (§ 230) 

 some of their uses : let us examine them more in detail. Now, 

 as the tendency of air in motion is (§ 120) to move in arcs of 

 great circles, and as all great circles that can be drawn about the 

 earth must cross each other in two points, it is evident that the 

 particles of the atmosphere which are borne along as wind must 

 have their paths all in diverging or converging lines, and that con- 

 sequently each wind must either be, like the trade-winds (§ 222), 

 drawing down and sucking in air from above, or, like the coun- 

 ter trades (§ 226), crowding out and forcing it off into the upper 

 currents. 



235. This tendency to move in great circles is checked by the 

 Tendency of air forccs of diumal rotatiou, or by the pressure of the 

 To^iweln th^piane ^iud whcu it blows toward a common centre, as in 

 of a great circle. ^ cyclouc. lu uo casc is it entirely overcome in its 

 tendency, but in all, it is diverted from the great circle path and 

 forced to take up its line of march either in spirals about a point 

 on the surface of the earth, or in loxodromics about its axis. In 

 either case the pushing up or pulling down of the combing, curd- 

 ling air from layer to layer is going on. 



236. Thus the laws of motion, the force of gravity, and the 

 The results upon its figurc of the earth all unite in requiring every wind 



circulation of this ■% , i ^ • ,^ , n • n ,^ n 



tendency. that blows either to lorce air up irom the surlace 



into the regions above, or to draw it down to the earth from the 

 cr^^stal vaults of the upper sky. Add to these the storm-king — 

 traversing the air, he thrusts in the whirlwind or sends forth the 

 cyclone, the tornado, and the hurricane to stir up and agitate, to 

 mix and mingle the whole in one homogeneous mass. By this 

 perpetual stirring up, this continual agitation, motion, mixing, and 

 circulation, the airy covering of the globe is kept in that state 

 which the well-being of the organic world requires. Every breath 

 we draw, every fire we kindle, every blade of grass that grows or 

 decays, every blaze that shines and burns adds something that is 

 noxious, or takes something that is healthful away from the sur- 

 rounding air. Diligent, therefore, in their of&ces must the agents 

 be which have been appointed to maintain the chemical status of 

 the atmosphere, to preserve its proportions, to adjust its ingredi- 

 ents, and to keep them in that state of admixture best calculated 

 to fit it for its purposes. 



