Not only does the flow vary with the elevation of the house, but it varies with the location of the faucet within the house.  Unless the reservoir is very high, or the pumps very powerful, the flow on the upper floors is noticeably less than that in the cellar, and in the upper stories of some high building the flow is scarcely more than a feeble trickle.

[Illustration:  FIG. 151.—­Water pressure 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 while the diminished pressure is ordinarily powerful enough to supply the lower floors, it is frequently too weak to force a continuous stream to high levels.  At night, however, and out of working hours, few faucets are open, less water is drawn off at any one time, and the intricate pipes are constantly full of water under high pressure.  At such times, a good flow is obtainable even on the uppermost floors.