214. Mr. Thompson’s Experiments 
light as to he rendered inti rely, ufelefs : and for fome purpofes it 
feems probable, that it would not be the work for being even 
heavier than it is now made. Vents are abfolutely neceflary in 
fire-arms, and in large pieces the windage mull be confiderable, 
in order that the bullets, which are not always lo round as they 
Ihould be, may not flick in the bore; and thofe who have been 
prefent at the firing of heavy artillery and large mortars with 
fliot and fhclls, liiuft have obferved, that there is a fenfible fpace 
of time elapfes between the lighting of the prime and the ex- 
piofion ; and that, during that interval, the flame is continually 
ifluing out at the vent with a hilling noile, and with a prodi- 
gious velocity, as appears by the height to which the ftream of 
fire mounts up in the air. It is plain, that this lols muft be 
greater in proportion as the Ihot that is dilcharged is heavier ; 
and I have often fancied, that I perceived a fenfible difference in 
the Vim| that elapfed between the firing of the prime and the 
explofion, when bullets were difeharged, and when the p.ece 
lias been fired with powder only ; the time being apparently 
longer in the former cafe than the latter. 
Altnoft all the writers upon gun- powder, and particularly 
thofe of the laft century, gave different recipes for powder that 
' is defigned for different ufes. Thus the F rench authors men- 
tion poudre a moufquet, poudre ordinaire de guerre , poudre de 
chap, and poudre d' artifice -, all of which are compofed of fait 
petre, fulphur, and charcoal, taken in different proportions. 
Is it not probable, that this variety in the compofition of pow- 
der was originally introduced in confequence of obfervation 
r that one kind of powder was better adapted for particular 
' purpofes than another, or from experiments made on purpofe 
to afcbVfitiu ' the fact ? ' There is one circumlhwe that would 
lead' us to fuppofe that that was the cafe. — That kind of pow- 
