514 Modes cf inflating Balkons. [Book V. 



and impels ic upwards; but as the denfity of the atmo- 

 fphere decreafes, on account of the diminifhed preffure 

 of the fuperincumbent air, and the elaftic property 

 which it poffefTes, at different elevations above the 

 earth, an air balloon can rife only to a height in which 

 the furrounding air will be of the fame fpecific gravity 

 with itfelf. When it is in this fituation, it will either 

 float, or be carried in a direction with the wind or cur- 

 rent of air which it may happen to encounter in thofe 

 upper regions. 



The whole theory of aeroftation depends upon this 

 ' principle ; for the fame effect is produced, whether 

 we make the air lighter, by introducing heat into it, 

 or inclofing a quantity of gas lighter than the com- 

 mon airj both will afcend on the fame principle. 

 Philofophers have found by experiments, that a cubic 

 foot of air weighs about five hundred and fifty-four 

 grains, and that it is expanded by every degree of heat 

 marked on Fahrenheit's thermometer, about one 

 five-hundredth part of the whole ; by hearing, there - 

 fore, a quantity of air to five hundred degrees, we 

 double its bulk when the thermometer (lands at 54* 

 in the open air, and consequently its weight will be 

 ciiminifhed in the fame proportion. 



With refpect to the mode of inflating a balloon with 

 headed air, nothing more is neceffary than the injec- 

 tion of heat into the machine, by burning combuflibles 

 under it. The air for filling the inflammable air- 

 balloons 'may be obtained in feveral ways, but the 

 bed methods are, by applying acids to certain metals, 

 or, by expofing a quantity of water with certain mi- 

 neral fubftances, in a clofe vellel, to a ftrong fire. 

 M. Lavoifier, for this purpofe, made the fleam of 

 boiling water pafs through the barrel of a gun kept 

 5 red- 



