114 GENERAL SCIENCE 



PRESSURE EXERTED BY THE ATMOSPHERE, AND EFFECT OF 

 PRESSURE ON GASES 



When masses of air have a velocity of many miles an hour 

 as winds we have evidence of the great pressure exerted by 

 the air. We can feel it push against us as we are hurried 

 along by it in its course, or as we struggle against it when 

 breasting a gale. It is calculated that when a wind is 

 blowing at the rate of twenty miles an hour its pressure upon 

 any surface, such as the side of a building, is approximately 

 two pounds per square foot. At sixty miles per hour this 

 pressure becomes eighteen pounds, and at one hundred 

 miles per hour fifty pounds per square foot. The pressure 

 increases as the square of the increase in velocity. Winds 

 approaching this greatest velocity are known as hurricanes, 

 and their destructive violence can readily be understood. 



When the atmosphere is calm, however, we do not ordi- 

 narily appreciate the fact that it exerts any pressure upon 

 exposed surfaces. This pressure may be thought of as the 

 weight of a column of air reaching downward from the upper- 

 most level of the atmosphere, and having a cross-section the 

 same as that of the body pressed upon. The weight of the 

 mercury contained in a barometer tube is used to measure 

 the value of atmospheric pressure. At sea-level the height of 

 the mercury column averages about thirty inches (or 76 cm.), 

 and this height is taken as the standard for atmospheric 

 pressure. As the weight of a column of mercury thirty 

 inches high and one square inch cross-section is about fourteen 

 and one-half pounds, the pressure of the atmosphere at sea- 

 level must be fourteen and one-half pounds per square inch. 

 This amount of pressure by any gas is often spoken of as 

 "one atmosphere." In making barometers great care is 

 exercised that the space above the mercury in the tube shall 

 be free of air. 



