Ob PHYSICAL GEOGRAPHY OF THE SEA, AXD ITS IIETEOEOLOGY. 



233. Now suppose the surface from wliicli this vapour rises, or 

 Cold belts. on which it is condensed, be sufficiently large to 



produce a rush of wind fr'om afar ; suppose it, moreover, to be an 

 oblong lying east and west somewhere, for example, in the tem- 

 perate zone of the northern hemisphere. The wind that comes 

 rushing in from the south side will be in the category of the counter 

 trades of the southern hemisphere (§ 228), viz. : going from larger to 

 smaller parallels, and giving rise to ascending columns ; while that 

 fr'om the northern side, moving in the opposite direction, is, like 

 the trade-winds (§ 223), bringing down air from above. 



234. By the motion of the clouds upper cmTents of wind are 

 -^ih"hMrmb^^'^"% ^^scerned in the sky. They are an-anged in layers 

 offices. ' 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 cur- 

 rents 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 

 thefr 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), di-awinoj down 

 and sucking in air from above, or, hke the counter trades (§ 226), 

 crowding out and forcing it off into the upper cuiTents. 



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

 Tendency of air forcos of diiUTnal rotatiou, or by the pressure of the 

 to m(\^e1n ui'rpLme VTind whou it blows towards a common centre, as in 

 of a great circle. j^ cyclonc. lu uo caso 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 spfrals about a point 

 on the sm^face of the earth, or in loxodromics about its axis. In 

 either case the pushing up or pulling down of the combing, cm'd- 

 ling air fr'om layer to layer is going on. 



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

 The results upon its fiorm'e of tlio caith all uuitc iu requirinsf every wind 



circulation ot ihis .^ , in • ,^ i o • i- n '' p 



tendency. that blows Cither to lOTce au' u]) trom tiie suriace 



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



