WATER PRESSURE 



123 



contains 12 cubic inches (fig. 65, a). A cubic inch of water 



62.5 

 will weigh 170Q ' which equals 0.0362, pound, and the entire 



column of 12 cubic inches will weigh 12 x 0.0362 pound, which 

 is exactly 0.434 pound, as in the former calculation. If the 

 depth were not 12 inches, but only 6, the pressure would of 

 course be only one half as great (6 x 0.0362), and if it were 

 only 1 inch deep, it would be 0.0362; that is, 1 x 0.0362 

 pound per square inch. 



From these examples we discover that the pressure is pro- 

 portional to the depth, and that the pressure on any area is 

 equal to the weight of the column 

 of water whose base is the area in 

 question and whose height is equal 

 to the depth of the water. Pressure 

 equals area x depth X weight of unit 

 volume of water. 



136. Pressure on horizontal sur- 

 faces in general. The pressure on 

 any horizontal surface under the 

 water may be obtained in the same 

 way as above. For instance, if we 

 submerge in the water of our tank 

 a cube measuring 1 inch on the 

 edge, and cause it to rest on the 

 bottom, its top will be 11 inches 



beneath the surface of the water (fig. 65, 5). The pressure upon, 

 it will be that of the weight of a column of water 1 inch 

 square and 11 inches high. (How much will it be ?) The 

 same principles will apply to any horizontal surface anywhere 

 under water. 



137. Pressure on the sides of a tank. Since we can easily 

 find the pressure downward at any depth, it will be conven- 

 ient to compare the pressure against the side with the pressure 

 downward. If we submerge any form of pressure gauge in 



FIG. 65. Pressure upon 

 the bottom 



The pressure upon any part of 



the bottom, as a or b, is equal 



to the weight of the water 



above it 



