LIMIT TO AERIAL NAVIGATION. 189 



what is called a geometrical progression. For 

 instance, if at the level of the sea a certain 

 quantity of air occupied one square foot of 

 space, at a little more than two miles and-a-half 

 it would fill a space of two square feet ; at about 

 five miles, four ; at eight miles, eight ; at ten 

 miles, sixteen ; at thirteen miles and-a-half, 

 thirty-two ; and at sixteen miles, sixty-four 

 cubic feet. Hence we very soon reach a limit 

 by means of the balloon beyond which we 

 cannot pass. The limit appears to be about 

 22,000 feet; that, at least, is the highest point 

 yet attained in aerial navigation. 



Now, it is a remarkable fact that one of the 

 consequences of this expansion is a diminution 

 of temperature. It has been found by experi- 

 ment that this is due to the fact of expanded 

 air having a greater capacity for heat than com- 

 pressed air. It seems, although it must be 

 carefully remembered the subject is still very 

 obscure, it seems as if expanded air contained 

 more room for heat than compressed air : for it 

 is found that we may, as it were, squeeze out 

 heat from air by a well-fitting syringe, by 

 forcing it violently down ; and a peculiar syringe 

 is constructed, with a piece of German tinder 

 at the bottom, which, when forcibly worked 

 once or twice, produces so much heat by com- 

 pressing the air, as to set fire to the tinder! 



