hile 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.

In order to overcome the disadvantage of a decrease in flow during the
day, standpipes (Fig. 153) are sometimes placed in various sections.
These are practically small steel reservoirs full of water and
connecting with the city pipes. During "rush" hours, water passes from
these into the communicating pipes and increases the available supply,
while during the night, when the faucets are turned off, water
accumulates in the standpipe against the next emergency (Figs. 151 and
154). The service rendered by the standpipe is similar to that of the
air cushion discussed in Section 184.

[Illustration: FIG. 153.--A standpipe.]

198. The Cost of Water. In the gravity system, where an elevated
lake or stream serves as a natural reservoir, the cost of the city's
waterworks is practically limited to the laying of pipes. But when the
source of the supply is more or less on a level with the surrounding
land, the cost is great, because the supply for the entire city must
either be pumped into an artificial reservoir, from which it can be
distributed, or else must be driven directly through the mains (Fig.
154).

[Illustration: FIG. 154.--Water must be got to the houses by means of
pumps.]

A gallon of water weighs approximately 8.3 pounds, and hence the work
done by a pump in raising a gallon of water to the top of an average
house, an elevation of 50 feet, is 8.3 x 50, or 415 foot pounds. A
small manufacturing town uses at least 1,000,000 gallons daily, and
the work done by a pump in raising that amount to an elevation of 50
feet would be 8.3 x 1,000,000 x 50, or 415,000,000 foot pounds.

The total work done during the day by the pump, or the engine driving
the pump, is 415,000,000 foot pounds, and hence the work done during
one hour would be 1/24 of 415,000,000, or 17,291,666 foot pounds; the
work done in one minute would be 1/60 of 17,291,666, or 288,194 foot
pounds, and the work done each second would be 1/60 of 288,194, or
4803 foot pounds.

A 1-H.P. engine does 550 foot pounds of work each second, and
therefore if