266 Mr. Thompson’s Experiments 
“ fhaken, and loofened to their very intimate fubbance*?” And 
in proportion to the fwiftnefs of this vibration, and the violence 
of the attrition and friction, will be the heat that is produced. 
A piece of iron that would fuftain the preffure of any weight, 
however large, without being warmed, may be made quite hot 
by the blow of a hammer ; and even foft and un-elabic bodies 
may be warmed by percufiion, provided the velocity with which 
their parts are made to give way to the blow is fufficiently rapid. 
If a leaden bullet is laid upon an anvil, or any other hard body, 
and in that fituation it is ftruck with a fmart blow of a ham- 
mer, it will be found to be much heated ; but the fame bullet in 
the fame fituation may be much more flattened by preffure, or 
by the broke of a very heavy body moving with a fmall velo- 
city, without being fenfibly warmed. 
To generate heat therefore the action of the powder upon the 
liifide of the piece mub not only be fufficient to brain the metal, 
and produce a motion in its parts, but this effedt mub be ex- 
tremely rapid ; and the heat will be much augmented, if the 
exertion of the force and the duration of its adtion are momen- 
taneous ; for in that cafe, the fibres of the metal (if I may ufe 
the exprebion) that are violently bretched, will return with 
their full force and velocity, and the fwift vibratory motion and 
attrition before defcribed will be produced. 
The heat generated in a piece by firing is therefore as the 
force by which the particles of the metal are brained and com- 
preffed, the fuddennels with which this force is exerted, and the 
(hortnefs of the time of its adfion ; that is to fay, as the 
brength of the powder and the quantity of the charge, the 
quicknefs of its inflammation, and the velocity with which the 
generated fluid makes its efcape. 
* Vide shaw’s tranflation of boerhaave’s Chemiilry, vol. I. p. 249. 
Now 
