A manufacturer usually stores quantities of water for manufacturing purposes in a tank at the top of each of the different buildings of the plant, but in case the factory or mill is near a stream, the water is stored in a dam, at a convenient height. The pressure of the water against the sides of the tank or dam is exerted perpendicularly to the surface on which it bears. Every pound of water in a tank or dam at some height above the point where the water is to be used possesses a certain amount of potential energy due to its position. To illustrate: W lbs. of water raised a definite height H possess the capacity of doing work which is equal to the weight of water in pounds multiplied by the height in feet. The result is W X H ft.-lbs.

To estimate the energy in the reservoir of a city or town so as to know the exact water pressure, it is necessary to know the perpendicular height from the water level in the reservoir to the point of discharge. The perpendicular height is called the "head." Mechanics, engineers, etc., often speak of a "head of water," meaning the pressure that water exerts. "A head of 50 ft.," for instance, is the pressure (due to its weight) of a column of water 50 ft. high.

The pressure per square inch at any point in a body of water equals the depth in feet below the surface, or the head times .434. If P is pressure per square inch and H is head,

Then P = H X .434 and H = P / .434

To find the head when pressure is given the rule is: Divide the pressure by .434.