3Q6 Mr. Thompson’s Experiments 
actions, whofe operations have a contrary tendency, fo compen- 
iate each other, that their effects when united are not fenfible. 
But when this is the cafe, if any one of the caufes of error is 
removed, thofe which remain will he detected. 
When any given charge is loaded with a heavy bullet, more 
of the powder is inflamed in any very lhort fpace of time than 
when the bullet is lighter, and the aftion of the powder ought 
of courfe to be greater on that account ; but then a heavy bul- 
let takes up more time in pafling through the bore than a light 
one, and eonfequently more of the elaftic fluid, generated from 
the powder, efcapes by the vent and by windage. It may hap- 
pen, that the augmentation of the force, on account of one of 
•thefe circumftancesj may exactly counterbalance the diminution 
of it ariling from the other; and if it fliould be found upon 
trial that this is the cafe in general, in pieces as they are now 
■conftm&ed, and with all the variety of fhot that are made ufc 
of in practice, it would be of great ufe to know the fa£t : and 
poffibly.it might anfwer as well, as far as it relates to the art of 
gunnery, as if we were perfectly acquainted with, and were 
able to appreciate, the effect of each varying circumftance under 
which an experiment can be made. But when, concluding too 
haftily from the refult of a partial experiment, we fuppofe 
with Mr. robins, that becaufe the fum total of the action or 
prefiure of the elaftic fluid upon the bullet, during the time of 
its paflage through the bore, happens to be the fame when bul- 
lets of different weights are made ufe of (which collective 
preffure is in all cafes proportional to, and is accurately mea- 
sured by, the velocity, or rather motion, communicated to the 
bullet;) that therefore the preffure in any given part is always 
exa&ly the fame, when the quantity of powder is the fame 
with which the piece is fired ; and from thence endeavour to 
prove. 
