354 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



the trades and the counter trades ; why, on one side of it, the 

 prevailing direction of the wind should be polarward, on tho 

 other towards the equator ; and we also discover the influences 

 which determine their geographical position ; for : — 



657. A meteorological law. — An accumulation of atmosphere 

 over one part of the earth's surface implies a depression over 

 some other part, precisely as the piling up of water into a wave 

 above the sea level involves a corresponding depression below ; 

 and in meteorology it may be regarded as a general law, that the 

 tendency of all winds on the surface is to blow from the place 

 where the barometer is higher to the place where the barometer 

 is lower. This meteorological law is only a restatement of the 

 dynamical truism about water seeking its own level. 



658. The barometer in the calm belts. — The mean height of the 

 barometer in the calm belts of the tropics is greater (Plate I.) 

 than it is in any other latitude. The mean height of the baro- 

 meter in the equatorial calm belt is less than it is on any other 

 parallel between the tropical and equatorial calm belts. The 

 difference for the calm belt of Cancer is 0.25 inch. This differ- 

 ence is permanent. It is sufficient to put both systems of trade- 

 winds in motion, and to create an indraught of air flowing per- 

 petually towards the equatorial calm belt from the distance of 

 two thousand miles on each side of it. 



659. Winds ivitJi northing and ivinds with southing in them. — In 

 like manner, as we go from either tropical calm belt towards the 

 nearest pole, the barometric pressure becqmes le§s and less. The 

 meteorological law just announced requires the prevailing wind 

 on the polar side of these calm belts to be from them and in the 

 direction of the poles; and observations (Plate I.) show that such 

 is the case. Dividing the winds in each hemisphere into winds 

 with northing and winds with southing in them as has been done 

 in Chapter XXI. and Plate XV., actual observation shows (§ 852) 

 that the}^ balance each other in the southern hemisphere between 

 the parallels of 35" and 40°, and in the northern between the 

 parallels of 25^ and 60° ; that between these parallels the average 

 annual prevalence of winds with northing and of winds with 

 southing in them is the same, the difference (Plate XV.) being 

 so small as to be apparently accidental ; that, proceeding from 

 the medial band towards the pole, polar-bound winds become 

 more and more prevalent, and proceeding from it towards the 

 equator, equatorial-bound winds become more and more preva- 



