On the prefent State of Aerojlation, 29 



ing air, fo if the balloon were originally but a quarter full 

 (by which means room would be left for this fubfequent ex- 

 panfion of the gas), the machine would mount proportionally 

 higher than the three miles and a half, by as much more as 

 the relatis'e denfity of the gas is become lefs. Thus, if, as is 

 now allowed, we reckon the volume of all elaftic fluids to be 

 inverfely as the preffure, and that the balloon was at firft but 

 a quarter full (yet of half its weight of power of afcenfion), 

 it would not Itop till arrived at feven miles height, and the 

 machine would be at its full diftenfion, the denfity being here 

 leflened four times what it was at the moment of afcenfion. 

 But if the machine (whofe power of afcenfion is equal to half 

 its whole weight) was at firft half full, the gas would be ex- 

 panded to twice its volume at three miles and a half; the 

 balloon ^vould then be full, and all further rife would be pre- 

 vented by the air efcaping at the neck by all the fubfequent 

 rarefaftion ; and if this was prevented by tying the neck, the 

 machine would burft. But we are prevented from knowing 

 to what precife height a balloon will rife, from the yet vmac- 

 counted-for circumftance of the temperature of the contained 

 air always much exceeding that of the furrounding medium : 

 and as there is not that comcidence in the expanfions of dif- 

 ferent gafes by temperature, as i's the cafe with refpeA to 

 barometrical prefiTure ; fo, till we are better acquainted with 

 the caufe of this diverfity of temperature, and the precife ex- 

 panfion of the gas by the varying degrees of the fame, we 

 muft reft fatisfied with a rough calculation in fuppofing 

 the temperature of the internal and external air to corre- 

 fpond *. 



The annexed table (extrafted from Mr. Cavallo's treatife) 

 (hows the proportion between the furfaces and capacities of 

 fpheres of various diameters, remembering that the fpecific 



• From the mode of inflating aL-roftatic machines, and the diflblving 

 power of all elaftic fluids, the contained gas will always hold in foiuticn 

 more or lefs water in an extremely divided ftate. In tiic elevated and cold 

 regions of the atmofphere this vapomus fluid will be condenfed in finali 

 drops in the inner concavity of the balloon; but the before-combined yet 

 now free caloric (to which the fluid owed its vaporous fixie, and which, 

 in every condenfation of a fluid, or chang<. of ftate of its aggregation from 

 aeriform elafticity to a fluid or fohd (late, is eiven o\(t,) will be prevented 

 from cfciping to the external air by the non-conduftiiig quality of the en- 

 velope : it will therefore fcrve to augment the temperature of the hydroty.n 

 gas; while in a warmer zone (or when a large and v.liitc cloud is fo fnii- 

 ated as to become a mirror to the balloon, ftrongly refle<i^ling the rays of 

 the fun upon the fame,) the contrary clfefts will enfue — the water will 

 reaflume the vaporous ftate, and the temperature of the gas be propor- 

 tijonaliy reduced, as far as relate: i t» it> contained water. 



quantity 



