DETERMINATION OF HEIGHT BY BAROMETER 131 



Determination of Height by a Barometer. Experiment 65. 



Carry an aneroid barometer from the bottom of a high building 

 to the top. Note the reading of the barometer at the bottom 

 and again at the top. Why is the barometer lower at the top of 

 the building? 



As the pressure of air at any surface is due to the weight 

 of the air above that surface, it happens that as we go up 

 the pressure decreases, since there is a continually de- 

 creasing weight of air above. If the rate of this decrease 

 is determined, then it is possible to determine the elevation 

 by ascertaining the pressure. 



Although the height of the barometer is continually vary- 

 ing with the changing air conditions, yet if these conditions 

 remain about the same, it may roughly be estimated that 

 the fall of re of an inch in the height of the mercury column 

 indicates a rise of about 57 feet, and that the fall of a milli- 

 meter indicates a rise of about 11 meters. These values 

 are fairly reliable for elevations less than a thousand feet, 

 under ordinary temperatures and pressures. 



At the height of 25 miles the barometric column would 

 probably not be more than ^ of an inch high. Several 

 measurements made in different ways indicate that the air 

 is at least 100 miles in depth, probably more. Nearly 

 three fourths of the atmosphere, however, is below the top 

 of the highest mountain. The highest altitude ever reached 

 by man was about 7 miles. 



To study air conditions small balloons to which meteoro- 

 logical instruments are attached have been sent to a height 

 of 21 miles. It is found that the minimum temperatures 

 occur at a height of from 6 to 10 miles. Conditions affect- 

 ing weather, however, seem to extend to a height of not 

 much over 3 miles. 



