ual houses, or to common reservoirs
from which it is distributed to the various buildings.

[Illustration: FIG. 148.--The elevated mountain lake serves as a
source of water.]

Figure 148 illustrates in a simple way the manner in which a mountain
lake may serve to supply the inhabitants of a valley. The city of
Denver, for example, is surrounded by mountains abounding in streams
of pure, clear water; pipes convey the water from these heights to the
city, and thus a cheap and adequate flow is obtained. Such a system is
known as the gravity system. The nearer and steeper the elevation, the
greater the force with which the water flows through the valley pipes,
and hence the stronger the discharge from the faucets.

Relatively few cities and towns are so favorably situated as regards
water; more often the mountains are too distant, or the elevation is
too slight, to be of practical value. Cities situated in plains and
remote from mountains are obliged to utilize the water of such streams
as flow through the land, forcing it to the necessary height by means
of pumps. Streams which flow through populated regions are apt to be
contaminated, and hence water from them requires public filtration.
Cities using such a water supply thus have the double expense of
pumping and filtration.

195. The Pressure of Water. No practical business man would erect a
turbine or paddle wheel without calculating in advance the value of
his water power. The paddle wheel might be so heavy that the stream
could not turn it, or so frail in comparison with the water force that
the stream would destroy it. In just as careful a manner, the size and
the strength of municipal reservoirs and pumps must be calculated. The
greater the quantity of water to be held in the reservoir, the heavier
are the walls required; the greater the elevation of the houses, the
stronger must be the pumps and the engines which run them.

In order to understand how these calculations are made, we must study
the physical characteristics of water just as we studied the physical
characteristics of air.

When we measure water, we find that 1 cubic foot of it weighs about
62.5 pounds; this is equivalent to saying that water 1 foot deep
presses on the bottom of the containing vessel with a force of 62.5
pounds to the square foot. If the water is 2 feet deep, the load
supported by the vessel is doubled, and the pressure on each square
foot of the bottom of the vessel will be 125