WATER PRESSURE 



133 



by 1 gram than the pressure downward. Plainly, such an 

 object as we have supposed, if its own weight amounted to 

 nothing, would be pushed up to the surface. If it weighed 

 more than a gram, its weight would overcome the upward 

 push and it would descend to the bottom. 



149. Floating objects. If an object weighs less than the 

 water it displaces, it will float or, if submerged, will rise to the 

 surface. It will not rest with its top even with the surface of 

 the water, but it will project far enough above the surface so 

 that the submerged part will displace an amount of water 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 



weight of which is equal to its own weight (fig. 71). For ex- 

 ample, if the 1-centimeter cube weighed 0.5 gram, it would 

 rise until only one half of it was under water, and it would 

 therefore displace 0.5 gram of water. An iron pail will float 

 on the water because, though it is made out of very heavy 

 material, it is so shaped that it displaces a great deal of water. 

 The iron ship floats for the same reason. The weight of the 

 ship and its cargo is less than the weight of the water dis- 

 placed would be if the vessel were sunk with the gunwales at 

 the water level. The vessel therefore sinks in the water only 

 deep enough to displace an amount of water equivalent to its 

 own weight. 



150. Submarines and balloons. A submarine boat is so 

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



