16 



ON THE SEPABATE PBESSURES OF THE AQUEOUS AND 



small extent of water from which evaporation takes place, 

 hence the atmosphere is there comparatively dry and clear, and 

 the dew-point generally low compared with the temperature. 

 But the southern hemisphere, beyond the latitude of say 50°, 

 is almost wholly sea. Evaporation consequently takes place 

 from a large surface of water, and much vapour is sent into the 

 atmosphere, making the dew-point high for the temperature. 

 The air is consequently generally filled with mist, in this 

 respect resembling the hazy tropical regions ; and the exten- 

 sive condensation warms the air in the middle and upper parts 

 of the atmosphere, and makes the whole mass of air swell and 

 expand, leaving it lighter than in the other hemisphere. The 

 solar rays which on the broad extent of land of the northern 

 hemisphere heat the land to a high temperature, are in the 

 south united with water, and pass in vapour into the atmo- 

 sphere. The quantity of aqueous matter in the air in the 

 south must therefore be large, whilst the total atmospheric 

 pressure is found to be small ; the gaseous pressure must 

 consequently be small. This state of the southern atmosphere, 

 however, seems consequent on — not the large proportional 

 surface of water alone, but also on the few elevations that 

 exist there to bring on local condensation. Spread over 

 Europe and the western part of North America, extending 

 into the polar regions, there is a large number of mountains 

 up which winds carry vapour, where it is extensively 

 condensed, and these elevated localities become atmospheric 

 vortices, into which more vapour is borne by the winds fi'om 

 great distances, to be there condensed. The northern atmos- 

 phere is thus relieved from a considerable part of its vapour, as 

 it always is, by extensive condensation. It is well known, for 

 instance, that a thunder storm clears the air of vapour. In 

 summer, when evaporation is active, the air in a short time is 

 charged nearly up to the point of saturation, when from some 

 cause which we need not trace a thunder storm occurs. The 

 vapour is first carried sufficiently high to commence conden- 



