The Weather 15 



press the same pressures in pounds per square foot, and per square yard. 

 Suppose that upon the outside wall of the schoolroom there is an un- 

 balanced pressure, represented by a reading of 30 inches outside and 

 29 inches inside. What would be the total difference in pressure ? Do 

 you think the wall would withstand the outside pressure ? Give reasons. 



The normal air pressure of your own locality may be deter- 

 mined very simply if you know the elevation. Air pressure 

 decreases within the range of 10,000 feet at the rate of about 

 one tenth of an inch in the height of the mercury column for 

 each ninety feet, or one inch for each nine hundred feet. Thus 

 Spokane, Washington, 1800 feet above sea level, has a normal 

 average atmospheric pressure of twenty-eight inches. Lead- 

 ville, Colorado, 10,600 feet elevation, shows a normal barometric 

 reading of twenty inches, or about ten pounds of air pressure 

 to the square inch. Suppose your locality is 2800 feet above 

 sea level, what will be the normal barometric reading? What 

 the amount of air pressure? 



12. Water barometer. We may now consider a possible 

 water barometer. A cubic inch of mercury is 13.6 times as 

 heavy as a cubic inch of water. If the mercury column is 

 thirty inches high, a water column having the same cross-sec- 

 tional area would be equal to 13.6 times 30 (the height in inches 

 of the mercury column) or 408 (the height in inches of the 

 water column), or 34 feet. That is, the average normal air 

 pressure would support a column of water to the height of 

 thirty-four feet. This emphasizes one advantage of using 

 mercury for the measurement of air pressure. 



Exercise : The suction pump. An interesting application of this 

 fact is found -in the old-style suction pump. It was learned long 

 ago that water could not be " lifted " by such a pump as much as 

 thirty-four feet from a well. And finally it was discovered that the 

 pump does not " lift " the water at all; that as the suction exhausts 

 the air from the pipe and cylinder of such a pump, the pressure of 

 the air upon the free surface of the water in the well forces the water 

 upward into the pipe and cylinder. The water rises to a height at 

 which the weight of the column of water is equal to the pressure of 



