CHAP, v.] BALLOONS. 87 



CHAPTER V. 



BALLOONS AERIAL NAVIGATION. 



I. APPLICATION OF THE PRINCIPLE OF ARCHIMEDES TO THE 

 VERTICAL ASCENSION OF BODIES IN THE ATMOSPHERE. 



A BODY immersed in a fluid loses in weight a weight equal to 

 that of the fluid which it displaces. This principle, which is 

 known to have been discovered by Archimedes, applies to gases as 

 well as to liquids, and hence it is that many light bodies smoke, 

 vapour, and clouds rise and remain suspended in the air, instead of 

 falling to the surface of the earth as would happen on a planet devoid 

 of a gaseous envelope or atmosphere. 



In order to bring about this ascent, it is necessary that the 

 weight of the body be less than that of the portion of air which it 

 displaces. At the surface of the earth, the air weighs 1-29 at the 

 temperature of and under a pressure of O m< 76, that is to say, the 

 weight of a cubic metre of air is then l ku> 29. Under the same 

 physical circumstances, a cubic metre of hydrogen gas has a density 

 about fifteen times less, as it only weighs O kil 090. Let us imagine such 

 a volume inclosed in an impermeable envelope ; the loss of weight 

 which it will undergo in the air will be l kiL 29, and as the weight 

 of the gas is only O kil> 09, it will be raised in the vertical direction 

 with a power equal to the difference of these weights. Part of this 

 buoyancy or ascending power will be used to balance the weight of 

 the solid envelope, and the remainder will serve to raise the system 

 to a certain height in the atmosphere. As the strata of this latter 

 have a density which decreases with height, the ascending power will 

 go on diminishing gradually until it entirely ceases. At this point, 

 the balloon will cease to rise, and if its movement continues it will 



