2.J 2 Mr. Thompson’s Experiments 
trials, that the velocities of like mnjket bullets, when they are 
difcharged from the lame piece by different quantities of the 
.fame kind of powder, are very nearly in the hub-duplicate 
ratio of the weights of the charges. Whether this law will' 
hold good when applied' to cannon balls, and bomb fhells of 
large dimenfions, I dare not at prefent take upon me to decide r 
but, for feveral reafons that might be mentioned, I’ am rather 
of opinion, that it will not ; at leaf! not with that degree of 
accuracy which obtained in thefe experiments. 
Of the effect of placing the vent in different parts of the charge 
There have been two opinions with refpedt to the manner, in 
which gun-powder takes fire. Mr. robins fuppofes that the 
progrefs of its inflammation is fo extremely rapid, “ that all 
“ the powder of the charge is fired and converted into an elaflic 
“ fluid* before the bullet, is fenfibly moved from its place 
while others have been of opinion, that, the progrefs of the in- 
flammation is much flower, and that the charge is feldom or 
never completely inflamed before the bullet is out of the gun. 
The large quantities of powder that are frequently blown, 
out of fire arms un-inflamed, feem to favour the opinion of' 
the advocates for the gradual firing ; ; but Mr, robins endea- 
vours to account for that circumftance upon different principles, 
and fupports his opinion by fhewing that every increafe of the 
charge within the limits of practice produces a proportional 
increafe of the velocity of the bullet, and that when the pow- 
der is confined by a great additional weight, by firing two or 
more bullets at a time inftead of one, the velocity is not fenfl- 
bly greater than it ought: to be according to his theory. 
4 
IT 
