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 pounds, and if the water is 10 feet deep, the load borne by each square foot will be 625 pounds.  The deeper the water, the greater will be the weight sustained by the confining vessel and the greater the pressure exerted by the water.

[Illustration:  FIG. 149.—­Water 1 foot deep exerts a pressure of 62.5 pounds a square foot.]

Since the pressure borne by 1 square foot of surface is 62.5 pounds, the pressure supported by 1 square inch of surface is 1/144 of 62.5 pounds, or .43 pound, nearly 1/2 pound.  Suppose a vessel held water to the depth of 10 feet, then upon every square inch of the bottom of that vessel there would be a pressure of 4.34 pounds.  If a one-inch tap were inserted in the bottom of the vessel so that the water flowed out, it would gush forth with a force of 4.34 pounds.  If the water were 20 feet deep, the force of the outflowing water would be twice as strong, because the pressure would be doubled.  But the flow would not remain constant, because as the water leaves the outlet, less and less of it remains in the vessel, and hence the pressure gradually sinks and the flow drops correspondingly.

In seasons of prolonged drought, the streams which feed a city reservoir are apt to contain less than the usual amount of water, hence the level of the water supply sinks, the pressure at the outlet falls, and the force of the outflowing water is lessened (Fig. 150).

[Illustration:  FIG. 150.—­The pressure at an outlet decreases as the level of the water supply sinks.]

196.  Why the Water Supply is not uniform in All Parts of the City.  In the preceding Section, we saw that the flow from a faucet depends upon the height of the reserve water above the tap.  Houses on a level with the main supply pipes (Figs. 148 and 151) have a strong flow because the water is under the pressure of a column A; houses situated on elevation B have less flow, because the water is under the pressure of a shorter column B; and houses at a considerable elevation C have a less rapid flow corresponding to the diminished depth (C).