Page:Physical Geography of the Sea and its Meteorology.djvu/116

 90 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 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 consequently each wind must either be, like the trade-winds (§ 222), drawing down and sucking in air from 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 the 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. The 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 hurricane to stir up and agitate, to mix and mingle the whole in one homogeneous mass. By this perpetual stirring up, this continual