CHAPTER VI
LIQUID LEVERS AND GEARS
MANY an inventor has strayed off into the delusive pursuit of perpetual motion because he did not know that the pressure in a body of water at any given point is equal in all directions, upward, downward, or laterally.
A cubic foot of water weighs 62½ pounds. Take a hollow column with an internal cross-sectional area of one square foot and if it be filled with water to a depth of ten feet there will be a weight of 625 pounds of water in the column and hence a pressure of 625 on the bottom of the tube or 4.34 pounds on every square inch of the bottom. But the water presses on the sides of the tube as well and the amount of this pressure depends upon the depth or “head” of water and not upon the quantity of water. At the bottom of the tube the pressure on the side walls is 625 pounds per square foot or 4.34 pounds per square inch; at a depth of one foot the pressure on the side walls is 62.5 pounds per square foot or .434 pound per square inch; at a depth of two feet it will be .864 pound per square inch, etc. The pressure on each square inch depends not upon the mass of the water but upon its depth. If the column of water had a cross-sectional area of a mile or a thousand miles, the pressure at a depth of one foot would always be .434 pound per square inch. (Of course there are slight variations from this figure due to salt or other substances dissolved in water or to changes in density produced by variations of temperature, but we need not consider such minute differences here.) That is why a dam which is strong enough to hold back the waters of a pond will be just as able to hold back the waters of the whole ocean if it be placed in a sheltered bay where ocean waves cannot tear it to pieces. The ocean, despite its enormous mass, can exert no more pressure per foot of depth than the water in a cistern.