10 ELEMENTS OF GENERAL SCIENCE 



an inch for each 900 to 1000 feet increase in altitude. The 

 average pressures for various elevations are shown in the 

 following table: 



ELEVATION PRESSURE 



Sea level 30 inches 



910 feet 29 inches 



1850 feet 28 inches 



2820 feet 27 inches 



3820 feet 26 inches 



4850 feet 25 inches 



5910 feet 24 inches 



7010 feet 23 inches 



8150 feet 22 inches 



9330 feet 21 inches 



10,550 feet 20 inches 



13,170 feet 18 inches 



16,000 feet 16 inches 



8. 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 out upon the road, may 

 be much harder by noon, as if more air had been put into it. 

 The same thing will occur if the machine is allowed to stand 

 in the sun. The additional pressure may indeed become so 

 great as to burst the tire. Of course, it is very clear that no 

 air has been added to that which was in the tire in the morn- 

 ing, for if the rubber will retain the air which is in the tire, 

 it will be equally effective in preventing more from entering. 

 Since such an increase of pressure as we have been describing 

 occurs only when the tire has been heated by contact with 

 the hot road or by direct exposure to the sun, it is fair to 

 conclude that when the air is heated it has greater tendency 

 to expand, and that it therefore presses harder against the 

 walls of the tire and would expand if it were not confined by 

 the tire. 



The notion that air expands when heated ought to be tested 

 before we accept it finally. This may be done in many ways. 



