ELEMENTS OF GENERAL SCIENCE 



FIG. 11. A barograph 



This instrument automatically records the air pressure. A, compound aneroid 

 barometer; B, pointer, at the tip of which is C, the pen ; />, paper-covered cylin- 

 der which is rotated by a clock contained within it. As the pressure changes, th( 

 pen moves up or down, thus tracing its course upon the rotating cylinder of paper 



at any given altitude, but at elevations of less than a mile 

 above sea level the pressure decreases by an inch for each 

 900 to 1000 feet increase in altitude. The average pressures 

 for various elevations are shown in the following table : 



ELEVATION 

 Sea level 

 * 910 feet 

 1850 feet 

 2820 feet 

 3820 feet 

 4850 feet 

 5910 feet 



PRESSURE 

 30 inches 

 29 inches 

 28 inches 

 27 inches 

 26 inches 

 25 inches 

 24 inches 



ELEVATION 



7010 feet 



8150 feet 



9330 feet 



10,550 feet 



13,170 feet 



16,000 feet 



PRESSURE 

 23 inches 

 22 inches 

 21 inches 

 20 inches 

 18 inches 

 16 inches 



12. Effect of temperature. It is often noticed that a pneu- 

 matic tire which is only moderately firm in the morning, 

 when the bicycle or automobile is first taken upon the road, 



