WATER PRESSURE 135 



takes' 5 cubic centimeters of water to weigh 5 grains. The 

 object must have a volume of 5 cubic centimeters in order to 

 displace 5 cubic centimeters of water. 



The facts regarding buoyancy are useful also in enabling us 

 to find the densities of different substances. By density is 

 meant the weight of a unit volume of a substance one cubic 

 inch or one cubic centimeter. We might secure the weight of 

 one cubic centimeter of iron by cutting a cube of that size and 

 weighing it, but this would be a difficult process. Instead, we 

 can take any irregular piece of iron, weigh it in the air and 

 again in water, and from the weights calculate its density. The 

 loss of weight in water is equal to the number of grams of 

 water displaced, and is therefore equal to the volume. The 

 weight in air divided by the volume equals the weight per 

 cubic centimeter or the density per cubic centimeter. The den- 

 sity in ounces per cubic inch can also be secured, but the process 

 is less convenient for use, and the density in grams per cubic 

 centimeter is practically always used. 



In the case of liquids the density is usually ascertained by 

 weighing a measured quantity of the liquid. 



Below are a few common substances with their densities 



TABLE OF DENSITIES FOB REFERENCE 

 (Grams per cubic centimeter) 



Ebony ... 1.2 Oak wood . . 0.8 Zinc .... 7.1 



