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 the pump is to be operated by an engine, the strength of the latter would have to be 8.7 H.P.  An 8.7-H.P. pumping engine working at full speed every second of the day and night would be able to supply the town with the necessary amount of water.  When, however, we consider the actual height to which the water is raised above the pumping station, and the extra pumping which must be done in order to balance the frictional loss, it is easy to understand that in actual practice a much more powerful engine would be needed.  The larger the piston and the faster it works, the greater is the quantity of water raised at each stroke, and the stronger must be the engine which operates the pump.

In many large cities there is no one single pumping station from which supplies run to all parts of the city, but several pumping stations are scattered throughout the city, and each of them supplies a restricted territory.