SCIENCE OF COMMON THINGS. 71 



Pressure of air. Vacuum. 



438 If the air were condensed, so as to occupy no more space than the 

 same weight of water, to how great an tkvation above the earth would it 



extend ? 



To an elevation of thirty-four feet. 



439 In what direction is the pressure of the atmosphere exerted f 



It is the nature of a fluid to transmit pressure in every 

 direction equally ; therefore the air presses upwards, 

 downwards, laterally, and obliquely, with the same 

 force. 



440 How great a pressure is exerted by the air upon the body of a 

 man or animal having a surface of 2000 square inches f 



Not less than 30,000 pounds, or about 15 tons. 



441 Why is not the individual crushed beneath so enormous a load? 



Because the atmosphere presses equally in all direc- 

 tions^ and our bodies are filled with liquids capable of 

 sustaining pressure, or with air of the same density as 

 the external air ; so that the external pressure is met 

 and counteracted by the internal resistance. 



44S What would be- the effect upon a man or animal if at once 

 relieved of all atmospheric pressure ? 



All the blood and fluids of the body would be forced 

 ~by expansion to the surface, and the animal would burst. 



443 What do we mean by a vacuum? 



A space devoid of all matter ; in general, we mean 

 by a vacuum, a space devoid of air. 



444 Can a perfect vacuum be produced artificially f 



No ; but confined spaces may be deprived of air 

 sufficiently for all experimental or practical purposes. 



445 Are there any instances of a vacuum in nature? 



There is no positive certainty that the spaces which 

 exist between the various planets and other hea- 

 venly bodies, are occupied with any material sub- 

 stance. 



446 Is the existence of air necessary to the production of sound? 



It is ; in a vacuum there can be no sound ; and on 

 the top of high mountains, where the air is greatly 

 rarefied, as on Mont Blanc, the report of a pistol can 

 hardly be heard. 



