AND THE PREVAILING WINDS OVER THE GLOBE. 587 



the surface winds of north temperate regions towards and from the polar 

 regions. 



An examination of the isobaric and wind charts for the months shows, as has 

 been already pointed out, that where there is a mean low pressure, such as 

 occurs in the north of the Atlantic in the winter months, and in the centre of 

 Asia in the summer months, thitherward the winds tend in all directions in an 

 inmoving spiral course ; and where there occurs a mean high pressure, as in the 

 centre of Asia in winter, and in the Atlantic between Africa and the United 

 States in summer, out of this space the winds flow in all directions, or they 

 appear to be thrown out from the space of high pressure in a manner exactly 

 the reverse from that by which they are drawn inward upon a space of low 

 pressure. These spaces of low and high pressures may therefore be regarded as 

 the true poles of the winds, which blow at the surface of the earth, towards which, 

 and from which, the great movements of the atmosphere proceed. From the 

 unequal distribution of land and water, it results that the poles of the pressure 

 and movements of the atmosphere are, as in the case of the poles of temperature, 

 very far from being coincident with the north pole. 



The causes which bring about an unequal distribution of the mass of the earth's 

 atmosphere may be considered to be chiefly two, viz., the temperature primarily ; 

 and, secondarily, the moisture of the atmosphere, in their relations to the 

 geographical distribution of land and water. From the relations of land and 

 water to temperature, the summer temperature of continents greatly exceeds 

 that of the ocean in the same latitudes. Hence the abnormally high temperatures 

 which prevail in Asia, Africa, and North America during summer, in conse- 

 quence of which the air becomes specifically lighter, and ascends, as from a 

 furnace, in vast columns thousands of miles in diameter. In this way the 

 summer pressure of continents is diminished, the amount of the decrease being 

 greatest in Asia, the largest continent, and least in Australia, the smallest. At 

 Barnaul, in Asia, the pressure in July is 0418 inch below the annual average;* 

 whereas at Deniliquin, in Australia, the pressure in January is only 0154 inch 

 below the annual average : at Great Salt Lake, in North America, it is inter- 

 mediate, being 0-333 inch. 



In the remarks which follow on the vapour of the atmosphere, the principles 

 laid down in the two following extracts are assumed: — 1. "Air charged with 

 vapour, or vaporised air, is specifically lighter than when without the vapour ; 

 or, in other words, the more vapour any given quantity of atmospheric air has in 

 it, the less is its specific gravity."f 2. " It appears, therefore, that the explanation 

 suggested by Dr Joule is correct ; and that the condensation of vapour in ascend- 



* Some part of this diminished pressure in Asia is doubtless due to the condensation of the 

 vapour of the south-west monsoon. 



t Dalton's Meteorological Observations and Essays, 2d ed. Manchester, 1834, p. 100. 

 VOL. XXV. PART II. 7 N 



