292 EVAPORATION AND CONDENSATION. 



contain an amount of vapour equal to the pres- 

 sure of only half an inch of mercury, while it 

 is capable of sustaining twice that quantity, it 

 will require a reduction of temperature below 

 59 to cause precipitation. Hence it is that the 

 atmosphere often undergoes great reductions of 

 temperature without producing rain. But when 

 the air is full of vapour the process of evaporation 

 is arrested ; which explains why a cold dry air is 

 more favourable to evaporation than warm air 

 that is already saturated, especially if the former 

 be in a state of rapid motion. 



Dr. Dalton found that when boiling water 

 was exposed to a current of air that carried off 

 its vapour as fast as formed, vaporization went 

 on a third faster than in a room where the air 

 was still. Hence it is, that north-east and east 

 winds, which, in the west of Europe are generally 

 dry, and far below the point of saturation, sel- 

 dom condense the vapour of France and Eng- 

 land ; but, on the contrary, often redissolve the 

 clouds already formed, producing clear weather. 

 During summer, when the atmosphere has been 

 for some time comparatively still, it is soon 

 saturated with vapour, which is indicated by 

 heavy dews, and the formation of clouds termed 

 cumuli, when the weather becomes hot and sul- 

 try ; because the caloric which is usually carried 

 off by evaporation and winds, accumulates on 

 the surface of the earth and heats the superin- 



