C)4 I'KINCJPLES OF AEROSTATION. 



discovered. It is very elastic, ami from six to ten or eleven times 

 lighter than common air; and consequently tins compound 

 uill rise in the ItMXWpbere, ami continue to ascend till it attain a 

 height -at which the atmosphere, is of the same specific gravity a* 

 itself; where it will remain or float with the current of air, as long 

 as the inflammable air does not escape through the pores of its co- 

 vering. And this is an inflammable air-balloon. Another way is to 

 make use of common air, rendered lighter by warming it, instead of 

 the inflammable air. Heat, it is well known, rarefies and expands 

 common air, and consequently lessens its specific gravity; and the 

 diminution of its weight is proportional to the heat applied. If 

 therefore the air, inclosed in any kind of a bag or covering, be heated, 

 and consequently dilated, to such a degree, that the excess of the 

 weight of an equal bulk of common air, above the weight of the 

 heated air, be greater than the weight of the covering and its ap- 

 pendages, the whole compound mass will ascend in the atmosphere, 

 till, by the diminished density of the surrounding air, the wholt 

 becomes of the same specific gravity with the air in which it floats ; 

 where it will remain, till, by the cooling and condensation of the 

 included air, it shall gradually contract and descend again, unless 

 the heat is renewed or kept up. And such is a heated air-balloon, 

 otherwise called a Montgolfier, from its inventor. Now it has been 

 discovered, by various experiments, that one degree of heat, accord- 

 ing.to the scale of Fahrenheit's thermometer, expands the air about 

 one five.hundreth pact ; and, therefore, that it will require about 

 500, or nearer 484 of heat, to expand the air to just double its 

 bulk : which is a degree of heat far above what it is practicable to 

 give it on such occasions. And, therefore, in this respect, common 

 air heated is much inferior to inflammable air, in point of levity and 

 usefulness for aerostatic machines. Upon such principles then de- 

 pends the construction of the two sorts of air-balloons. But before 

 treating of this branch more particularly, it will be proper to give a 

 short historical account of this late. discovered art. 



SECTION II. 



History of Aerostation. 



VARIOUS schemes for rising in the air, and passing through it, 

 have been devised and attempted, both by the ancients and 'mo- 

 dems, and that upon different principles, and with various success. 



