88 FIRST YEAR COURSE IN GENERAL SCIENCE 



to collapse and press against his body. As fast as the diver 



uses the air, it escapes through a valve into the water, fresh 



ai^ being pumped to him all the while. 



95. Laws of Pressure in Liquids. Experiments prove 



three things about the pressure of liquids: 



First, that the pressure varies directly with the depth. The 



pressure of the water upon a body ten feet below the surface 

 is twice as great as upon a body five 

 feet below. 



Second, that the pressure varies 

 with the density of the liquid. The 

 pressure upon a body ten feet below 

 the surface of the ocean is greater than 

 upon a body ten feet below the surface 

 of a pond, because the salt water of 

 the ocean is more dense than the fresh 

 water of the pond. 



Third, that at any given point in 

 the liquid the pressure is the same in 

 all directions. At any point upon a 

 body under water, there is the same 

 pressure from above, below, and from 

 each side. 



98. Direction of Pressure. Pres- 

 sure in a liquid is exerted at right 

 angles to the surface of the body upon 

 which it is acting. Suppose a cubical 

 block, two centimeters on each edge, 

 is placed under water so that its top 

 is three centimeters below the surface 

 of the water. The bottom of the 



block is five centimeters below the surface and the average 



depth of each side is four centimeters. 



In accordance with the first law of pressure in liquids, the 



pressure on all six sides can be found. The total downward 



FIG. 38. PRESSURE 

 UPON A SUBMERGED 

 BODY 



1. Which is greater, 

 the upward or the down- 

 ward pressure upon this 

 block? 2. Would the 

 block remain (naturally) 

 in this position: (a) if its 

 density were greater than 

 that of water? (6) if it 

 were less? (c) if it were 

 the same as that of water? 



3. Give the reasons for 

 one of these answers. 



4. Explain from these 

 answers why a body ap- 

 parently weighs less in 

 water than in air. 



