DIFFUSION OF WATER IN THE SOIL. 
439 
It is undoubtedly true that loose soils, stripped of vegeta¬ 
tion and broken up by the plough or other processes of cul¬ 
tivation, may, until again carpeted by grasses or other plants, 
absorb more rain and snow water than when they were cov¬ 
ered by a natural growth ; but it is also true that the evapora¬ 
tion from such soils is augmented in a still greater proportion. 
Bain scarcely penetrates beneath the sod of grass ground, but 
runs off over the surface; and after the heaviest showers a 
ploughed field will often be dried by evaporation before the 
water can be carried off by infiltration, while the soil of a 
neighboring grove will remain half saturated for weeks to¬ 
gether. Sandy soils frequently rest on a tenacious subsoil, at 
a moderate depth, as is usually seen in the pine plains of the 
United States, where pools of rain water collect in slight de¬ 
pressions on the surface of earth, the upper stratum of which 
is as porous as a sponge. In the open grounds such pools are 
very soon dried up by the sun and wind ; in the woods they 
remain unevaporated long enough for the water to diffuse itself 
laterally until it finds, in the subsoil, crevices through which 
it may escape, or slopes which it may follow to their outcrop 
or descend along them to lower strata. 
The readiness with which water not obstructed by imper¬ 
meable strata diffuses itself through the earth in all directions 
-—and, consequently, the importance of keeping up the supply 
of subterranean reservoirs—find a familiar illustration in the 
effect of paving the ground about the stems of vines and trees. 
The surface earth around the trunk of a tree may be made per¬ 
fectly impervious to water, by flag stones and cement, for a 
distance greater than the spread of the roots ; and yet the tree 
will not suffer for want of moisture, except in droughts severe 
enough sensibly to affect the supply in deep wells and springs. 
Both forest and fruit trees grow well in cities where the streets 
and courts are closely paved, and where even the lateral access 
of water to the roots is more or less obstructed by deep cellars 
and foundation walls. The deep-lying veins and sheets of 
water, supplied by infiltration from above, send up moisture 
by capillary attraction, and the pavement prevents the soil 
