134 ELEMENTS OF GENERAL SCIENCE 



submerged, excepting at the will of the occupants. It is able 

 to float at the surface like any other ship, and that is its usual 

 position. If the occupants wish the boat to dive, water is 

 admitted into special compartments in the interior until the 

 weight of the boat equals or slightly exceeds the weight of 

 the water it displaces. When they wish to rise to the surface 

 again, some of the water is forced out. If the weight of the 

 boat is kept very close to the weight of the water which it 

 displaces, it is possible to steer it to right or left, up or down. 



A balloon operates on a similar principle. The bag is filled 

 with a gas that is lighter than the air at the surface of the 

 earth. The air buoys up an object with a force equal to the 

 weight of the air displaced, and, as the balloon is lighter than 

 an equal volume of air, it ascends. It will not rise to the top 

 of the air, for the air is less dense at greater altitudes. The 

 balloon rises until it reaches a layer in the air of such density 

 that the displaced air equals the weight of the balloon and gas, 

 and it remains at that elevation unless something disturbs it. 

 Ballast may be thrown out to reach greater height, and it is 

 possible to descend by allowing the gas to escape. 



The balloons commonly used do not have any means of pro- 

 pulsion and cannot be guided. In late years dirigible balloons 

 have been constructed and used to some extent. They are 

 provided with means of propelling them, and may be guided 

 in any direction, like the submarine. Neither the submarine 

 nor the dirigible balloon has passed beyond the experimental 

 stage. 



151. 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 im- 

 mersed in water, it must displace 5 grams of water. But 

 water weighs 1 gram per cubic centimeter, and it therefore 



