90 PHYSICAL GEOGRAPHY OP THE SEA, AND ITS METEOROLOGY. 



while that from the northern side, moving in the opposite 

 direction, is, like the trade-winds (§ 223), bringing down air 

 from above. 



234. The upper currents— their numbers and offices. — By the 

 motion of the clouds upper currents of wind are discerned in 

 the sky. They are arranged in layers or strata one above the 

 other. The clouds of each stratum are carried by its winds in a 

 direction and with a velocity peculiar to their stratum. How 

 many of these superimposed currents of wind there may be 

 between the top and bottom of the atmosphere we know not. As 

 high up as the cloud-region several are often seen at the same 

 time. They are pinions and ratchets in the atmospherical ma- 

 chin eiy. 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 consequently each wind 

 must either be, like the trade-winds (§ 222), drawing down and 

 sucking in air fi'om above, or, like the counter trades (§ 226), 

 crowding out and forcing it off into the upper currents. 



235. Tendency of air when put in motion to move in tJie plane of a 

 great circle. — This tendency to move in great circles is checked 

 by the forces of diurnal rotation, or by the pressure of the wind 

 when it blows towards a common centre, as in a cyclone. In no 

 case 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, curdling air from 

 layer to layer is going on. 



236. Tlie results upon its circulation of this tendency. — Thus the 

 laws of motion, the force of gravity, and the figure of the earth . 

 all unite in requiring every wind that blows either to force air 

 up from the surface into the regions above, or to draw it down 

 to the earth from the crystal 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 hurri- 

 cane to stir up and agitate, to mix and mingle the whole in one 

 homogeneous mass. By this perpetual stirring up, this continual 



