138 



ELEMENTS OF GENERAL SCIENCE 



made out of very heavy material, it is so shaped that it dis- 

 places a great deal of water. The iron ship floats for the 

 same reason. The vessel therefore sinks in the water only 

 deep enough to displace an amount of water equivalent to 

 its own weight. 



155. Submarines and balloons. A submarine boat is so 

 constructed that no water can enter it, even if it is wholly 

 submerged, excepting at the will of the occupants. If the 

 occupants wish the boat to dive, water is admitted into 

 special compartments in the interior until the weight of the 



FIG. 71. Floating objects 



The object at the right weighs 10 grams and displaces 10 grams of water. When 



a weight of 10 grams is placed upon it, as in the middle, it displaces twice as 



much water as at first. When the total weight is 30 grams, as at the left, the 



displacement is three times as great as at first 



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. The depth 

 to which a submarine may safely descend is limited by the 

 ability of the boat's hull to resist the pressure of the water. 

 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, the balloon being 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. It rises until it reaches a layer in the air of 



