192 INFLUENCE OF THE FOREST ON PRECIPITATION. 
effect of clearing when sucli operations take place within those 
basins. But it must be observed that lakes without a visible 
outlet are of very rare occurrence, and besides, where no 
superficial conduit for the discharge of lacustrine waters exists, 
we can seldom or never be sure that nature has not provided 
subterranean channels for their escape. Indeed, w T hen we 
consider that most earths, and even some rocks under great 
hydrostatic pressure, are freely permeable by water, and that 
fissures are frequent in almost all rocky strata, it is evident 
that we cannot know in what proportion the depression of the 
level of a lake is to be ascribed to infiltration, to percolation, 
or to evaporation.* Further, we are, in general, as little able 
to affirm that a given lake derives all its water from the fall 
of rain within its geographical basin, or that it receives all the 
water that falls in that basin except what evaporates from the 
ground, as we are to show that all its superfluous w T ater is 
carried off by visible channels and by evaporation. 
Suppose the strata of the mountains on two sides of a lake, 
east and west, to be tilted in the same direction, and that those 
of the hill on the east side incline toward the lake, those of 
that on the west side from it. In this case a large proportion 
of the rain which falls on the eastern slope of the eastern hill 
may find its way between the strata to the lake, and an equally 
large proportion of the precipitation upon the eastern slope of 
the western ridge may escape out of the basin by similar chan¬ 
nels. In such case the clearing of the outer slopes of either 
or both mountains, while the forests of the inner declivities 
remained intact, might affect the quantity of water received by 
the lake, and it would always be impossible to know to what 
territorial extent influences thus affecting the level of a lake 
* Jacini, speaking of the great Italian lakes, says: “ A large proportion 
of the water of the lakes, instead of discharging itself by the Ticino, the 
Adda, the Oglio, the Mincio, filters through the silicious strata which 
underlie the hills, and follows subterranean channels to the plain, where 
it collects in the fontanili, and being thence conducted into the canals of 
irrigation, becomes a source of great fertility .”—La Proprietd Fondiaria , 
etc., p. 144:. 
