12 Selim Lemsteöm. 



column of mercuiy of a certain heiglit iii the sarne way as tlie pressure of 

 the atmosphère is measured. ïlie evaporatioii only continues until the suiTound- 

 ing air is saturated with vapour. 



Expei-iments show that the pressure of the aq. vapur F, in case of satu- 

 ration, dépends exchisively on température, whicli will hest he illustrated hy 

 the following example: 



Pressure of aq 



We give liere helow a statement of the circmnstances of moisture in ge- 

 neral, in order to hâve an opportunity of pointing ont that part which is of 

 greatest conséquence in the question of trost. 



The atmosphère contains aq. vapour in very différent degrees changing from 

 near zéro to this point of saturation, which cannot he exceeded. It very rarely 

 occurs that the degree of humidity sinks to o and just as rarely dors it 

 attains maximum. 



Above the surface of water the space might he saturated with moistui-e 

 in a calm air, but the winds, hy mixing différèrent layers of atmosphère, i)ro- 

 duce a more equal distribution of it. Expérience shows that evaporation from 

 ground covered with turf is greater than from a surface of water which again 

 surpasses a surface of earth. 



The intensity of evaporation, and hence the quantity of vapour foi'med, 

 dépends on the following circumstances : 



a) it is proportional to the evaporating surface; 



h) it is proportional to the différence F f or the diffé- 

 rence between the highest pressure and the pressure ruling at 

 the moment; 



r) it is also inversely proportional to the pressure of air; 



