340 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



657. An accumulation of atmosphere over one part of the earth's 

 A meteorological law. surface impHos a dcpressiou over some other part, 

 precisely as the piling up of water into a wave ahove 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 mean height of the barometer in the calm belts of the 

 Thebarometerin the tropics is greater (Plate I.) than it is in any other 

 calm belts. latitude. The mean height of the barometer m 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 difference is permanent. 

 It is sufficient to put both systems of trade-mnds in motion, and 

 to create an mdraught of air flowing perpetually towards the equa- 

 torial calm belt from the distance of two thousand miles on each 

 side of it. 



659. In like manner, as we go from either tropical calm belt 

 Winds with «or</ii«^ towards the nearest pole, the barometric pressure 

 southing Libera, bccomcs Icss and less. The meteorological law just 

 announced requires the prevaihng wind on the polar side of these 

 calm belts to be from them and m the direction of the poles ; and ob- 

 servations (Plate I.) show that such is the case. Di^dding the winds 

 in each hemisphere into winds with northing and winds with southing 

 in them as has been in Chapter XXI. and Plate XV., actual obser- 

 vation shows (§ 852) that they balance each other in the southern 

 hemisphere between the parallels of 35^ and 40°, and in the north- 

 ern between the parallels of 25° and 50° ; that between these 

 parallels the average annual prevalence of wmds with northing and 

 of winds with southing in them is the same, the difference (Plate 

 XY.) being so small as to be apparently accidental ; that, proceed- 

 ing 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 prevalent. 

 'Eov^, in each case, the prevaihng winds blow (§ 657) from the high 

 to the low barometer (Plate I.). 



660. The fact of two barometric ridges encircling the earth, as 

 The barometic ridges, the high barometer of the tropical calm belts do, 

 and as they may be called (Plate I.), suggests a place of low 



