262 Mr . Thompson’s Experiments 
I now regretted exceedingly the lofs of a fmall pocket ther- 
mometer, which I had provided on purpofe to meafure the 
heat of the barrel, but it was accidentally broken by a fall the 
day before I began rny experiments ; and being fo far from 
London, l had it not in my power to procure another : I was 
therefore obliged to content myfelf with determining the heat 
of the piece as well as I could by the touch. 
Being much {truck with this accidental difcovery of the great 
degree of heat that pieces acquire when they are fired with 
powder without any bullet, and being defirous of finding out 
whether it is a circumftance that obtains univerfally, I was 
very attentive to the heat of the barrel after each of the fuc- 
eeeding experiments ; and I conftantly found the heat fenfibly 
greater when the piece was fired with powder only, than when 
the fame charge was made to impel one or more bullets. 
Though the refult of thefe experiments was totally unex- 
pected, and even contrary to what I fhould have foretold if I 
had been afked an opinion upon the fubjeft previous to making 
them ; yet, after mature confederation, I am now convinced* 
that it is what ought to happen, and that it may be accounted 
for very well upon principles that are clearly admiflible. 
It is certain, that a very fmall part only of the heat that a 
piece of ordnance acquires in being fired is communicated to it 
by the flame of the powder; for the time of its aCtion is fo 
fhort -(not being, perhaps, in general longer than about -4 ^.th 
or -A-^th part of a fecond) that if its heat, inftead of being 4 
times, as Mr. robins fuppofes, was 400 times hotter than red- 
hot iron, it could not fenfibly warm fo large a body of metal 
as goes to form one of our large pieces of cannon. And be- 
lides, if the heat of the flame was fufficiently intenfe to pro- 
duce fo great an effeCf in fo fhort a time, it would certainly be 
4 fufficient 
