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mediately known by the Firft fudden Defcent of the 
Mercury : But many Circumftances concur to render 
this Method impracticable ; amongft the reft it muft 
be remembered, that fome Air is conftantly left in 
the Receiver, which is heated by theBlaft, and unites 
its EfFe&s in the Firft Inftant with the Aftion of the 
Powder: Befides, the Firft Defcent may be varied, by 
varying the Tube, although all things elfe remain 
unchanged. 
By the Method hitherto deferibed, it is collected, 
that the Elafticity of the Fluid produced from fired 
Gunpowder, when contained in the Space which 
was taken up by the Powder before the Explofion, 
is about 1000 times greater than the Elafticity of 
common Air, or, which is the fame thing, 1000 
times greater than the Preflure of the Atmofphere. 
But, befides the Determination of the Quantity of 
Fluid produced from a given Quantity of Powder, 
(the Method on which this Deduction is founded) 
there is another Method of difeovering the fame 
thing, which, though lefs obvious, is yet (as hath 
been already obferved) more accurate: That is, by 
examining the adtual Velocities communicated to 
Bullets by the Explofion of given Charges in given 
Cylinders 5 and this is the Subject of the 7th, 8th, and 
9th Propofttions. 
And Firft, it is evident, that this Examination can- 
not take place, unlels a Method of difeovering the 
Velocities of Bullets be previoufly eftablilhed. Now 
the only known Means of effe&ing this was, either 
by obferving the Time of the Flight of Bullets through 
a given Space j or by finding their Ranges when they 
were projected at a given Angle, and thence com- 
puting 
