M. A. de la Rive on Glaciers. 169 



18 metres of height. Hence, it is easy to perceive that this 

 quantity of water is much larger than that which would be 

 necessary to saturate the whole atmospheric column with a 

 base of one hectare of surface ; for if it be true that this 

 column be somewhat higher than 1800 metres, on the other 

 hand, its mean temperature is much below 10°. Thus, in 

 supposing that a portion only of the continents had been 

 covered with forests, it is not less true that the water ab- 

 sorbed by these forests has been more than sufficient to pro- 

 duce a notable diminution in the humidity of the surface of 

 the earth and of the atmosphere, and consequently in the 

 evaporation, as in the cold produced by this evaporati< n, and 

 in the quantities of rain and snow ; effects which, in their 

 combination, must have produced a gradual retreat of the 

 glaciers. This retreat has ceased when, at the termination 

 of a certain time, an equilibrium has been established be- 

 tween the action of the causes which determined the absorp- 

 tion of the meteorological water and the action of those 

 which produced its formation. At the same time, there is 

 no doubt that if, from any cause, the vegetation were to dis- 

 appear from the whole surface of the earth at the same time, 

 the existing phenomena would assimilate themselves to those 

 of the epoch which immediately preceded and accompanied 

 the formation of glaciers, and would still reproduce them- 

 selves, although with less intensity, the soil being devoid of 

 that humidity which resulted from its recent emergence. 



It is almost unnecessary to observe that this general in- 

 fluence which the presence of great forests must exert upon 

 the meteorological state of the atmosphere, after the appear- 

 ance of the. most recent formations, is not to be confounded 

 with the local influence which the actual clearance and 

 establishment of forests produces, and which is of an oppo- 

 site nature with the foregoing ; they are, in fact, contrary 

 phenomena, whose discrepancy it is easy to account for. 

 Finally, whilst assigning to the cause which 1 have pointed 

 out the principal agency in the retirement of glaciers, and in 

 the determination of the existing meteorological condition of 

 the atmosphere, 1 am far from pretending that it is the only 

 one ; and I am far from denying the influence of those causes 



