96 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK i. 



could not hurt anyone. Ballast is nevertheless a very limited re- 

 source, which is exhausted rapidly ; in many ascents the necessity 

 of diminishing the rapidity of descent or fall has been accomplished 

 by throwing over the sides of the car any heavy bodies clothing, 

 provisions, instruments, &c. 



In order to descend, a certain quantity of gas is allowed to escape. 

 The enyelope partly emptied, the volume of the balloon diminishes and 

 the air displaced becoming less, the globe descends until it finds 

 itself in a stratum of greater density, which compensates for the 

 loss of ascending power. 



FIG. 59. Valve of the balloon Entreprenant: 



To render the escape of gas more easy and more regular, the balloon 

 lias at the top an opening which holds a valve, fixed in by springs. 

 A string, which passes through the balloon and out at the neck, 

 within reach of the aeronaut, enables him to open this at pleasure. 



It is necessary to moderate the descent, without which the fall 

 would become dangerous, as the velocity goes on increasing. " If we 

 descended at one bound from a great height," said M. Banal, " the 

 velocity that would be acquired on reaching the ground would be 

 frightful, and the aeronaut would be destroyed by the fall. Hence 

 the descent is accomplished in ' cascades,' that is to say, first a distance 



