upon Gun-powder, &c. 279 
The form I would recommend for the bottom of the bore is 
that of a hemifphere ; and the vent Ihould be brought out di- 
redlly through the fide of the barrel, in a line perpendicular to 
its axis, and pointing to the center of the hemifpheric concavity 
of the chamber. 
In this cafe the vent would be the fliorteft poffible ; it would 
be the lead: liable to be obftrudted, and the piece would be more 
eafily cleaned, than if the bottom of the bore was of any 
other form. All thefe advantages, and feveral others not lefs 
important, would be gained by making the bottom of the bore 
and vent of great guns in the fame manner. 
A new method of determining the velocities of bullets . 
From the equality of aBion and re-aBion it appears, that the 
momentum of a gun muft be precifely equal to the momentum 
of its charge; or that the weight of the gun, multiplied into 
the velocity of its recoil, is juft equal to the weight of the bul- 
let and of the powder (or the elaftic fluid that is generated from 
it) multiplied into their refpedtive velocities : for every particle 
of matter, whether folid or fluid, that iflues out of the mouth 
of a piece, muft be impelled by the aftion of fome power, 
which power muft re-aB with equal force again ft the bottom of 
the bore. 
Even the fine invifible elaftic fluid that is generated from the 
powder in its inflammation cannot put itfelf in motion without 
re-adting againft the gun at the fame time. Thus we fee pieces, 
when they are fired with powder alone, recoil as well as when 
their charges are made to impel a weight of fhot, though the 
'recoil is not in the fame degree in both cafes. ; , . y 
2 P 2 It 
