112 GENERAL SCIENCE 



however; it can be proved true. To calculate the pressure 

 in terms of some unit we have only to take area X 

 depth X weight of unit volume. At a given depth a liquid 

 presses in every direction with exactly the same force. 



Pressure on the Sides of a Tank. It is quite easy to 

 calculate the pressure on the side of a tank, since the 

 pressure at any point of unit area is equal to the weight 

 of the column of water which has for its area this same 

 unit and for its height the depth of the water. Since the 

 pressure increases in direct proportion to the depth below 

 the free surface of the liquid, the total pressure on the 

 side of the tank will be found by multiplying the area of 

 the side of the tank by the average depth of all the units 

 of area below the free surface of the liquid (area X aver- 

 age depth X weight of unit volume). The pressure 

 against one square centimeter of the wall of a tank con- 

 taining water, with the upper edge of the square centi- 

 meter 10 centimeters below the surface of the water, will 

 be IX 10. 5X1 gram. What will be the pressure if 

 inches and pounds be substituted for centimeters and 



grams ? 



1 cubic foot of water weighs 62.5 pounds 

 1 cubic centimeter of water weighs 1 gram 



Transmission of Pressure by Liquids. We have 

 learned that pressure in a free liquid depends simply 

 upon the depth and density of the liquid. From this 

 Pascal (1623-1662), a French scientist and philosopher, 

 deduced a very surprising and important conclusion which 

 is now known as Pascal's Law. 



In the city water system the pressure in one part of 

 the system is transmitted to all parts of the city. This 

 pressure is often obtained by means of a reservoir located 

 on some high spot of land and into which the water is 



