222 ACTIOM OP POBESTH ON THE SURFACE-PLOW OF BAIN WATER. 



reservoirs, whence it is made to project itself by artisian wells ; 

 finally, the remainder flows away by a thousand streamlets towards 

 the thalweg, feeding the brooks, and streams, and rivers. 



But these portions do not stand to each other in any definite pro- 

 portion. Their relative importance varies not only, between one 

 water-course and another, with the nature of the ground, according 

 as this may be more or less favourable to the flow, or to the infil- 

 tration, but also in the case of the same water-course according as 

 the season restrains or promotes evaporation. In summer the drier 

 Soil and warmer air are more greedy ; they drink up or absorb the 

 waters ; in winter the moist air and damp ground allow the sheet of 

 rainfall to flow away. 



The only general and certain law is that over the area of the river's 

 basin these are complemental to one another, their sum is equal to 

 the quantity of rainfall. And if, for example, after evaporation has 

 taken away its portion, the infiltration be complete, the flow will be 

 nought. 



It is the infiltration which is here brought under consideration. 



Following out this division he says that it is by springs that the 

 permeability of the soil may be said to be revealed to us, and if their 

 regime be connected with meteorology, their existence depends on 

 geological conditions. And in illustration he says : — 



" Mount Ventoux — separated from the Alps by deep valleys, and 

 isolated on all sides, — is a mass of very permeable cleft limestone, 

 resting on an impermeable base which slopes toward the south-east. 

 So constructed, the mountain is a veritable filter, through which the 

 rain disappears at once ; and excepting in the cases of any violent 

 storms of rain, the superficial ravines are always dry. The sub- 

 terranean waters protected from evaporation descend slowly by the 

 internal fissures of the rock, and stopped at last by the impermeable 

 bed, they are directed towards the south-east, and flow out in springs 

 and streams sufficiently powerful to give motion to water-mills. 



" The fine fountain of Vaucluse owes its origin to analogous 

 circumstances : it is a veritable river, which is formed as other rivers 

 are by the means of a great many imperceptible tributaries ; but it 

 presents this peculiarity, that its basin instead of being open to the 

 vault of heaven, is subterranean, and covered by a great thickness of 

 permeable limestone. The area of the basin is nearly 100,000 

 hectares, * the delivery of the fountain is at low water from 10 to 12 



* Bouvitr, Annates dts Fonts et OhauiaSes- (1855, p. 367.) 



