The Supply and Uses of Water 227 



92. Measuring water pressure. A matter of practical 

 concern to any one about to install a private water supply 

 system, or to an engineer in charge of the installation of a 

 municipal system, is how to determine what the pressure of the 

 water will be for any given part or place of the supply system. 

 There must be a minimum rate of flow at every point of delivery, 

 and this cannot be left to chance or mere guesswork. A water 

 supply that gives only a trickle at any place in a system is 

 worse than none. Only the barest outline of the chief factors 

 of this problem can be given here ; but this may serve as in- 

 troductory to a complete study of it such as may be found in any 

 treatise on the mechanics of fluids, under the title hydraulics. 



A cubic foot of water weighs about 62.5 pounds. A tank 

 containing water to the depth of twelve inches sustains by its 

 floor a pressure of 62.5 pounds for each square foot. The 

 pressure on each square inch is y^ of 62.5 pounds, or 0.434 

 of a pound. If now the height of the column of water is in- 

 creased to two feet, it makes the pressure upon each square 

 foot 2 X 62.5, or 125 pounds. What is the pressure upon each 

 square inch ? If the water were eight feet deep, the pressure 

 would be 8 X 62.5 pounds per square foot or 8 X 0.434 for 

 each square inch. 



Water incompressible. The weight, it is to be noted, does 

 not change for a unit volume at different depths. Water is 

 practically incompressible, and therefore at any depth or with 

 any head or fall the pressure may be computed by multiplying 

 its pressure at the depth of one foot by the total depth in feet. 

 Pressure is always proportional to depth. 



93. Pressure in moving water. The conditions just stated 

 hold for still water pressure but do not entirely apply to water 

 flowing through pipes or conduits. Experience shows that the 

 flow of water from pipes near the distributing reservoir is full 

 and forceful, while, at a distance, unequal and inadequate 

 pressure is the rule. Even though the supply reservoir is ele- 

 vated above all places in the distributing system, the difference 



