Ou Mi/hf-Frostf'. 13 



d) the quantity of moisture dépends finally on the pnreness 

 and température of the evaporating fluid, as well as on the tem- 

 pérature of the surrounding atmosphère and its pnreness. 



The vaponr can sonietimes reniain in this form, even if the température 

 lias fallen beneath the dew-point. just as a fluid is heated above the boil- 

 ing point without turning into steam. These phenomena hâve their origin in 

 the pla}' of the forces of molécules wliich remains without influence if the air 

 contains particles of dust, the latter being usually the case. 



If we follow the changes in the moisture of the atmosphère of a clear 

 summer-day we find the amount of vapour rising with the température and 

 falling with it. The changes vary greatly in différent regions of the earth. 

 We must make a distinction between a place on the sea and a place in a 

 countrj^ without lakes. 



The smallest quantity of moisture is found in the atmosphère about sun- 

 rise, increasing until 8 or 9 a. m. tlien falling till about 2 p. m., rising again 

 till 8 or 9 p. m. and falling by slow degrees until morning. 



A similar coui'se parallel with température, is taken by the pressure of 

 aq. vapour within a year. 



It is lowest in February, increases speedily until the middle of July and 

 begins to decrease again in August and September. 



The amount of moisture is greatest from May to September, following the 

 changes in the average température tolerably closely. 



The following table shows the changes in round numbers 



This table proves that the average humidity during the summer months 

 greatly exceeds 50 per cent of maximal pressure and in several cases 75 per cent. 



The degree of humidity détermines the dew-point. When now the tem- 

 pérature bas fallen so low that the air is saturated, it cannot remain in form 

 of vapour. if the température is still falling, but turns into water. 



