INTERESTING THINGS ABOUT THE AIR 



11 



Since their small size makes it possible to carry 

 them conveniently, they may be used to ascer- 

 tain the approximate height of mountains and 

 the elevations reached by balloons or aeroplanes. 

 Some aneroids are so constructed that a pen 

 attached to the pointer is caused to write on a 

 piece of paper which is kept moving by means 

 of a clock in such a manner as to make a record 

 of the readings. Such an instrument is called a 

 barograph (fig. 11). 



11. Altitude and air pressure. It was stated 

 in section 8 that the pressure of the air is due 

 to the weight of the air above. It must be 

 plain that if we ascend in a balloon, we are 

 leaving some of the air below us, and the 

 pressure of the air at our elevation will be 

 less by a proportionate amount. In the case of 

 our bladder-glass experiment, if the experiment 

 were conducted at a place 1000 feet up in the 

 air, as in a balloon, the rubber would be pressed 

 down by a force equal to the weight of the air 

 above it; but there would be 1000 feet less 

 depth of air above it than there would be if it 

 were at the earth's surface, and the pressure 

 would be correspondingly less. The same thing 

 would be true on a mountain 1000 feet high, 

 as has been noticed by those 

 who have ascended moun- FIG. 10. A standard 



tains. It is this fact which barometer 



makes it possible to USe the A mercurial barometer 



with thermometer and 



scales. The graduations 

 allow both barometer 

 and thermometer to be 



barometer to measure alti- 

 tudes (fig. 12). 



It is not possible to State read. This is the stand- 

 any exact rule as to the J|jj! d fo of barom f ter 



pressure W.hich will be found States Weather Bureau 



