C 44 » ] 
by firing it in an exhaufted Receiver, and obferving 
how much the mercurial Gage fubfides thereby, 
making a proper Allowance for the Increafe of its 
Elafticity from the Heat of the included hot Iron. 
But then, as the Subfiding of the Mercury is not 
mcafured till the Flame of the Powder is extinguifhed, 
and the Fluid is reduced fomewhat near the Tem- 
perature of the external Air, it is evident, that the 
Elafticity thus eftimated is much fhort of what it 
really was in the Inftant of Explofion ; and therefore, 
to obtain that Elafticity, which is the Force fought, 
it is neceftary to make fome Eftimate of the Increafe 
of the Elafticity of the Fluid by the Fire and Flame 
of the Explofion. For this Purpofe it is examined in 
the Fifth Propofition, how much the Elafticity of 
common Air is increafed by a Degree of Heat equal 
to that of Iron beginning to grow white hot ; and it 
is found, at a Medium, to be thereby augmented 
fomething more than Four times ; whence, as the 
Fluid produced by any Quantity of Gunpowder takes 
up, when comprefted by the Weight of the incum- 
bent Atmofphere, a Space fomething lefs than 250 
times the Bulk of the Powder 5 it follows, that if its 
Elafticity in the Inftant of Explofion be fuppofed to 
be increafed in the fame Proportion with that of the 
Air laft-mentioned, it becomes by this means about 
1000 times greater than the Preflure of the Atmo- 
fphere j that is, conceiving it to be contained in that 
Space only which the Powder occupied before it was 
fired. 
Thofe who have not been converfant in thefe Ex- 
periments, may poftibly fuppofe, that the Elafticity of 
the Powder at the Inftant of Explofion may be im- 
L 1 1 2 mediately 
