252 The Winds. 



m. in. m. 



Jan. —0-031 

 Feb. +0-012 

 Mar. + 0-060 

 April— 0-030 



May —0-029 

 June + 0-017 

 July + 0-013 

 Aug. + 0-010 



Sep. + 0-056 

 Oct. — 0-077 

 Nov. — 0-037 

 Dec. + 0036 



The mean height of the barometer at Greenwich, from 

 1840 to 1858, was 29 - 782 inches, or reduced to the level of the 

 sea, 29*953 inches. The mean annual oscillation above and 

 below this height was very small, and irregularly distributed in 

 the year. A settled condition of the barometer is not only 

 observed in Western Europe and Africa, but also over the 

 greater part of the Mediterranean Sea. Over the whole con- 

 tinent of Asia, in the east of Europe, in Arabia, Nubia, and 

 part of Central Africa, a very remarkable depression of the 

 barometer takes place in summer. A lack of aqueous vapour 

 in summer, to supply the sudden overflow of heated air, evi- 

 dently occasions the barometric depression over this wide area, 

 which embraces the most arid plains and the highest table-land 

 of the world. At the same time, the barometer in the southern 

 hemisphere is high (although the pressure of aqueous vapour 

 is least), showing that air is actually transported to that 

 quarter in a bodily shape. In winter the barometer in Asia 

 is high, whilst the barometer in the southern hemisphere is 

 low, showing that an influx takes place in the opposite direc- 

 tion. These oscillations of the barometer, lately shown to 

 exist by Dove of Berlin, explain the origin of the monsoons 

 upon the Indian Ocean. 



In the same manner that a barometrical depression, and 

 consequent circulation of the air are permanently produced 

 upon the ocean by the sun's heat, an annual overflow 

 of air from the continent of Asia takes place in summer, 

 accompanied by currents flowing inward below, outward above, 

 to and from the heated region as a centre. In winter, when 

 the barometer is low over the southern continents of Africa 

 and Australia, and high over Asia, the conditions are reversed, 

 and the currents then flow in an opposite direction. 



From the great stretch of land (nearly a quarter of the 

 northern tropic) by which the Indian Ocean is barricaded on 

 the north, the impulse of the Asiatic monsoons is alternately 

 due north and south. Owing, however, to the diurnal rota- 

 tion of the earth they deviate from this direction towards the 

 east and west — like the trade and anti-trade winds — and are 

 called from this circumstance the north-east and south-west 

 monsoons. 



The south-west monsoon makes its first appearance in April 

 on the coast of Malabar : with a rainfall of 80 to 170 inches, 



