12 Introduction to the Study of Science 



pump and the vessel weighed. The difference between the 

 two weights is equal to the weight of a cubic foot of air, that is, 

 one and one-fifth ounces. This accounts for the fact that the 

 air exerts pressure upon the surface of the earth. 



Pressure of air at sea level. The depth of the ocean of air 

 is anywhere from fifty to two hundred miles. The largest bulk, 

 however, is within the fifty-mile boundary, and beyond this is 

 only one twenty-thousandth of the total volume of the air. 

 By careful experiment and calculation it has been learned that 

 the air pressure at sea level amounts to 14.7 pounds to each 

 square inch of surface. 



Questions. Is this more or less than a ton to a square foot? 

 Assume that the surface area of the body is 12 square feet. What is 

 the amount of air pressure upon the body? Why do we not feel 

 this tremendous pressure? Why do we feel its reduction at high 

 altitudes? Do these observed facts indicate that the pressure of the 

 air is equal in all directions? Summarize the points which support 

 your answer. 



10. Measuring air pressure. The pressure of the air is not 

 shown on weather maps as " pounds per square inch," but in 

 the height in inches of a column of mercury which balances the 

 pressure of the air. If water were used instead of mercury for 

 a barometer, the pressure would be stated in feet and inches ; 

 for water is about one fourteenth as heavy as mercury. Mer- 

 cury is exactly 13.6 times as heavy as water, volume for volume. 

 We shall consider a water barometer later. We have now to 

 find out how the air pressure may be determined by the height 

 of a column of mercury. 



Exercise. A cubic inch of mercury weighs .4908 pound. The 

 pressure of the air at sea level upon a square inch of surface is given 

 as 14.7 pounds. A column of mercury with a cross section of one 

 square inch, which would balance or equal the pressure of the air at 

 sea level, must be 14.7 + .4908 or 29.95 inches high, or in round 

 numbers 30 inches high. What do you find to be the average pressure 

 given on the weather maps? Would you infer from these facts that 

 30 inches is the "normal atmospheric pressure"? 



