upon Gun-powder, &c. 265 
iron, there is no doubt but they would hand fire as well as iron 
guns ; and if fteel, or any other metal, either fimple or com- 
pounded, fhould upon trial be found to anfwer for that pur- 
pofe better than iron, it might be ufed inftead of it;, and 
even if gold was made ufe of for lining the vent, I imagine it 
might be done in fuch a manner as that the expence would not 
be very confiderable, at the fame time that the thicknefs of the 
gold fhould be fufficient to withftand the force of the flame for 
a very great length of time. 
But to return to the heat acquired by guns in firing. It 
being pretty evident that it is not ail communicated by the 
flame, there is but one other caufe to which it can be attri- 
buted, and that is the motion and friflion of the internal parts - 
of the metal among themfelves, occafioned by the fudden and ? 
violent effort of the powder upon the infide of the bore, and 
to this caufe I imagine the heat is principally if not almoft 
intirely owing. It is well known, that a very great degree of 
heat may be generated in any hard and denfe body in a fhort 
fpace of time by fridlion, and in a ftill fhorter time by colli- 
fion. 46 For if tw T o denfe hard elaftic bodies be ftruck againft 
“ each other with great force and velocity, all the parts of 
“ fuch bodies will every moment be clofely comp refled, and 1 
“ being rigid will re-a£t with equal force. Hence a quick and 
“ powerful contraflion and expanfion will arife in every part, 
“ refembling that fwift kind of vibrations obferved in frretched : 
“ firings ; how great thefe vibrations are may be learnt from 
“ the inftance of a bell, when ftruck with a Angle blow, by 
64 which the whole bulk, however vaft, will for a long time 
“ expand,, and contraft itfelf in infinite ellipfe's. And when 
<c the attrition above defcribed is produced, with what force and 1 
44 velocity are all the particles of the rubbed body comprefled, 
2 46 fhaken,. 
