106 ON THE POSSIBILITY OF PREVENTING DAMAGES BY FROST. 



The influence of this radiated heat is diminished by the 

 circumstance that it meets the short grass which generally 

 covers the ground in places where preventives against the 

 damage of frost might be used. This source of heat is without 

 any influence and may therefore be neglected. 



IV. — Evaporation. 



Every hour of the day water evaporates from the ground 

 and the vegetation with more or less intensity, and thereby heat 

 is consumed in considerable quantities. The degree of 



evaporation determines the degree of humidity in the atmosphere. 



The aqueous vapour thus formed is mixed mechanically 

 with the surrounding atmosphere, exercising a pressure which 

 may be measured by the weight of a column of mercury of a 

 certain height in the same way as the pressure of the atmosphere 

 is measured. The evaporation only continues until the 

 surrounding air is saturated with vapour. 



Experiments show that the pressure of the aqueous vapour 

 in case of saturation depends solely on temperature. 



The degree of evaporation again, and hence the quantity 

 of vapour formed, depends on the followiug circumstances : — 



(a) It is proportional to the evaporating surface. 



(b) It is proportional to the difference between the highest 

 pressure and the pressure ruling at the moment. 



(c) It is also inversely proportional to the pressure of air. 



(d) The quantity of moisture depends finally on the 

 pureness and temperature of the evaporating fluid, as well as 

 upon the temperature of the surrounding atmosphere and its 

 pureness. 



The vapour can sometimes remain in this form even if the 

 temperature has fallen beneath the dew point, just as a fluid is 

 heated above the boiling point without turning into steam. 

 These phenomena have their origin in the play of the forces of 

 molecules, which play remains without influence if the air con- 

 tains particles of dust, as is usually the case. 



If we follow the changes in the moisture of the atmosphere 

 during a clear day we find the amount of vapour rising and 

 falling with the temperature. The changes vary greatly in dif- 

 ferent regions of the earth. We must make a distinction between 

 a place on the sea coast and a place in a country without lakes. 



The smallest quantity of moisture is found in the atmos- 

 phere about sunrise, increasing until 9 a.m., then falling till 



