MECHANICAL USES OF WATER AND AIR 139 



such density that the displaced air equals the weight of 

 the balloon and gas, and it remains at that elevation unless 

 something disturbs it. To reach a greater height ballast may 

 be thrown out, and it is possible to descend by allowing the 

 gas to escape. The dirigible balloon may be steered upward 

 and downward in the air, as a submarine may be in the 

 water. As the density of the air varies at different levels, 

 there are limits to the control of the balloon by use of 

 rudders, since the ballast cannot be readily adjusted. 



156. Densities. In the preceding paragraphs attention has 

 been called to the fact that an object in the water is buoyed 

 up by a force equal to the weight of the water it displaces. 

 Use may be made of this fact in several ways. We Are 

 enabled to determine the volume of irregular objects by this 

 means, for it is evident that if an object loses 5 grams of 

 weight when immersed in water, it must displace 5 grams of 

 water. But water weighs 1 gram per cubic centimeter, and 

 it therefore takes 5 cubic centimeters of water to weigh 

 5 grams. The object must have a volume of 5 cubic centi- 

 meters 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 vol- 

 ume equals the weight per cubic centimeter, or the density. 

 The density 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. 



