122 FIRST YEAR SCIENCE 



Instruments called barographs are constructed in which 

 a long lever provided with a pen point is attached to the 

 aneroid and made to record on a cylinder revolved by 

 clockwork. Thus a continual record is made of baro- 

 metric readings. 



58. Determination of Height by a Barometer. Experi- 

 ment 69. 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 

 decreasing weight of air above. If the rate of this de- 

 crease is determined, then it is possible to determine the 

 elevation by ascertaining the pressure. 



Although the height of the barometer is continually 

 varying with the changing air conditions, yet if these con- 

 ditions remain about the same, it may roughly be esti- 

 mated that the fall of -Jg- of an inch in the height of the 

 mercury column indicates a rise of about 57 feet, and that 

 the fall of a millimeter 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 



