DRAINAGE. 59

 

again becomes sensible. Much of the vapor
which is formed in the equatorial regions is car-
ried by the winds to high northern latitudes,
where, on condensing, it gives out its heat and
moderates the intense cold which would otherwise
exist. -

152. Solvent Powers.— Water is one of the best
solvents of all common substances. During the
constant washings to which the continents are
subjected by the rains, their surfaces are cleansed
from decaying animal and vegetable matters,
which are partly dissolved and carried by the
rivers into the ocean. The atmospheric waters
in the same way cleanse the air of many of its
impurities.

153. Water is the Main Food of Animals and
Plants.—By far the greater part of the bodies of
animals and plants is composed of water. With-
out large quantities of water no vigorous life can
be sustained in any locality.

Deserts are caused entirely by the absence of
_ water.

——o-089400—_

CHAPTER II.

Drainage.

154. Drainage.— The atmospheric waters, or
those which fall from the atmosphere as rain,
hail, or snow, either sink through the porous
strata and are drained under ground, or run
directly off the surface. Thus result two kinds
of drainage—Subterranean and Surface.

155. Subterranean Drainage-—The water which
sinks through the porous strata continues descend-
ing until it meets impervious layers, when it either
runs along their surface, bursting out as springs
at some lower level, where the layers outcrop, or
it collects in subterranean reservoirs. The origin
of all springs is to be traced to subterranean
drainage.

Underground streams sometimes attain considerable size.
In portions of the Swiss Jura streams burst from the sides
of hills in sufficient volume to turn the wheels of moder-
ately large mills. In a few instances the subterranean
stream can be navigated for considerable distances, as in

the Mammoth Cave of Kentucky, or in the Grotto of
Adelsberg, near Trieste.

156. Surface Drainage—The water which is
drained directly from the surface, either runs
down the slopes in rivulets and rills, which,
uniting with larger streams, are poured directly
into the ocean, or it collects in the depressions of

8

 

basin-shaped valleys, where, having no connection
with the ocean, it can be discharged by evapora-
tion only. Thus arise two kinds of surface drain-
age—oceanic and inland.

157. Springs are the outpourings of subterra-
nean waters. The waters, having soaked through
the porous strata, again emerge at the surface,
either—

(1.) By running along an inclined, impervious
layer of clay, hard rock, or other material until

   

ri Mere iy
Fig. 67, Origin of Springs.

they emerge at some lower level, where the strata
outcrop; or, :

(2.) By being forced upward out of the reser-
voirs into which they have collected by the pres-
sure of compressed gas, highly heated steam, or,
more commonly, by the pressure of a communi-
cating column of water.

It is in the first way that most of the springs of moun-
tainous districts discharge their waters. The tilted and
broken condition of the strata is such as to favor the es-
cape along some of the many layers that crop out on the
mountain-slopes. The springs of plains, which are at some
distance from mountains, discharge their waters mainly by
the methods mentioned under the second heading.

When a well is dug in most porous soils, the water from
the porous strata on the sides runs in and partially fills
the opening. z

158. Classification of Springs—Springs are
most conveniently arranged in different classes
according to peculiarities in the size, shape, and
depth of their reservoirs, and the nature of the
mineral substances composing the strata over which
the waters flow, or in which they collect.

The Reservoirs of springs are the places where