FILLING THE BALLOON. 



305 



air. The envelope being partly filled with coal-gas-heated air and hydrogen, 

 is much lighter than the surrounding atmosphere, and rises to a height 

 according as the density of the air strata diminishes. The density is less 

 as we ascend, and the buoyant force also is lessened in proportion. So 

 when the weight of the balloon and its occupants is the same as the power 

 of buoyancy, it will come to a stand, and go no higher. It can also be 

 understood that as the pressure of the outside becomes less, the expansive 

 force of the gas within becomes greater. We know that gas is very com- 

 pressible, and yet a little gas will fill a large space. Therefore, as the 



Fig. 300. Filling a balloon. 



balloon rises, it retains its rounded form, and appears full even at great 

 altitudes ; but if the upper part were quite filled before it left the ground, 

 the balloon would inevitably burst at a certain elevation when the external 

 pressure of the air would be removed, unless an escape were provided. This 

 escape is arranged for by a valve at the top of the balloon, and the lower 

 part above the car is also left open very often, so that the gas can escape 

 of itself. When a rapid descent is necessary, the top valve is opened by 

 means of a rope, and the balloon sinks by its own weight. Mr. Glaisher 

 advises for great ascensions a balloon of a capacity of 90,000 cubic feet, 

 and only filled one-third of that capacity with gas. Six hundred pounds of 



ballast should be taken. 



20 



