e varies in different parts of
a water system.]

When the respective flows at _A_, _B_, and _C_ (Fig. 151) are measured,
they are found to be far lower than the pressures which columns of
water of the heights _A_, _B_, and _C_ have been shown by actual
demonstration to exert. This is because water, in flowing from place
to place, expends force in overcoming the friction of the pipes and
the resistance of the air. The greater the distance traversed by the
water in its journey from reservoir to faucet, the greater the waste
force and the less the final flow.

In practice, large mains lead from the reservoir to the city, smaller
mains convey the water to the various sections of the city, and
service pipes lead to the individual house taps. During this long
journey, considerable force is expended against friction, and hence
the flow at a distance from the reservoir falls to but a fraction of
its original strength. For this reason, buildings situated near the
main supply have a much stronger flow (Fig. 152) than those on the
same level but remote from the supply. Artificial reservoirs are
usually constructed on the near outskirts of a town in order that the
frictional force lost in transmission may be reduced to a minimum.

[Illustration: FIG. 152.--The more distant the fountain, the weaker
the flow.]

In the case of a natural reservoir, such as an elevated lake or
stream, the distance cannot be planned or controlled. New York, for
example, will secure an abundance of pure water from the Catskill
Mountains, but it will lose force in transmission. Los Angeles is
undertaking one of the greatest municipal projects of the day. Huge
aqueducts are being built which will convey pure mountain water a
distance of 250 miles, and in quantities sufficient to supply two
million people. According to calculations, the force of the water will
be so great that pumps will not be needed.

197. Why Water does not always flow from a Faucet. Most of us have
at times been annoyed by the inability to secure water on an upper
story, because of the drawing off of a supply on a lower floor.
During the working hours of the day, immense quantities of water are
drawn off from innumerable faucets, and hence the quantity in the
pipes decreases considerably unless the supply station is able to
drive water through the vast network of pipes as fast as it is drawn
off. Buildings at a distance from the reservoir suffer under such
circumstances, because w