206 IMPOETAlfCE OF SNOW. 



of summer rains. In tlie principal regions occupied by Euro- 

 pean cultivation, and where alone the questions discussed in thi? 

 volume are recognized as having, at present, any practical im- 

 portance, more or less rain falls at all seasons, and it is to these 

 regions that, on this point as well as others, I chiefly confine my 

 attention. 



Irrvportance of Snow. 



Recent observations in Switzerland give a new importance to 

 the hygrometrical functions of snow, and of course to the forest 

 as its accumulator and protector. I refer to statements of the 

 condensation of atmospheric vapor by the snows and glaciers of 

 the Rhone basin. Rendu, in 1844, estimated the condensation 

 by snow and ice to be equal to a precipitation of thirty inches. 

 Agassiz concluded that the evaporation and the condensation 

 balanced each other, — a computation which allows to the latter a 

 value perhaps not greatly inferior to that supposed by Rendu. 

 In 1870, Professors Dufour and Forel of Morges made interest- 

 ing experiments on the subject at Morges in March, and on the 

 Rhine glacier in July and August. Of course the temperature 

 and humidity of the atmosphere influenced the results, but in 

 four experiments at Morges the condensation was foimd to be 

 equal to a precipitation of 410 thousandths of a millimetre to the 

 hour, or nearly a centimetre per day. On the glacier, in twenty- 

 seven observations between the 27th of July and the 3d of Au- 

 gust, the hourly condensation ranged from fifty-one thousandths 

 to twenty-six hundredths of a millimetre. Of course there are 

 many hours, many days even, in the course of a year when, in- 

 stead of a condensation on the surface of snow and ice, there is a 

 very rapid evaporation, and while in some experiments Dufour 

 and Forel found the evaporation during a period of eight favor- 

 able days to be equal only to the condensation during 6Y hours 

 of the following three, in others the evaporation amounted in 

 48 hours to four or five per cent, of the total weight of the ice 

 or snow contained in the vessels exposed. They observe that the 

 condensation is necessarily attended with a disengagement of heat 

 which tends to melt the snow and ice, and it thus augments the 

 flow of water from the glaciers both by depositing moisture and 

 by thawing ice and snow, and at the same time exerts a marked 



