CHAP, v.] BALLOONS. 95 



water, the gas -is introduced by a tube into the lower part of the 

 envelope, and by degrees the balloon is filled by the action of the 

 elastic force of the gas. 



In place of pure hydrogen, ordinary lighting gas, that is, carburetted 

 hydrogen, is most frequently used. The density of this is much 

 greater it is true, as it is as high as 0*63 that of air; 1 the ascending 

 power is therefore then much less. But the advantage of easily ob- 

 taining a considerable quantity of gas in towns renders its use in 

 every respect more advantageous. An English aeronaut, Green, was 

 the first person to substitute ordinary coal gas for hydrogen ; he first 

 inflated a balloon with coal gas. Mr. Glaisher recommends for the 

 same reason the use of gas obtained towards the end of tKe distilling 

 operations. Thus, in his ascent of June 30, 1862, he obtained a gas 

 with a density as low as 0'36, and which, therefore, gave an ascend- 

 ing power of 830 grammes per cubic metre, about two-thirds of that 

 of pure hydrogen. 



We may now state briefly by what means and by what manage- 

 ment the aeronaut ascends and descends at will. We will not speak 

 here of the direction of the balloons, as all movement in a horizontal 

 direction depends only on the aerial current, which draws the balloon 

 along with a velocity nearly equal to that of the mass of air itself. 

 The direction of balloons is entirely denied, at the present time at 

 least, to the aeronaut; his interference is confined to ascending or 

 descending vertically, until he meets with a stratum of air moving in 

 the direction he wishes to follow. 



If the aeronaut travels in a hot-air balloon, by increasing the fire 

 and thus increasing the temperature of the air inclosed in the invelope, 

 he diminishes its density and consequently increases the ascending 

 power of the apparatus. By lessening the fire, or allowing it to go 

 out, the contrary effect is produced, .and the apparatus begins to 

 descend. In gas-balloons the means are no longer the same. To ascend, 

 the aeronaut can only increase the ascending power at the expense of 

 the contents of the car ; he is obliged to throw out ballast, which most 

 frequently consists of sacks filled with sand, and which one of the 

 travellers empties in such a manner as not to endanger persons who 

 might be underneath the balloon : it is always very fine sand which 



1 At and 760 millimetres pressure, the ascending power of common gas is 693 

 grammes per cubic metre ; it is 670 grammes at 10. 



