49 Inquiry concerning the Source of 



steel borer was fixed ? or by the small neck of gun -metal 

 by which the hollow cylinder was united to the cannon ? 

 These suppositions appear more improbable even than 

 either of those before mentioned ; for Heat was contin- 

 ually going off, or out of the machinery, by both these 

 passages, during the whole time the experiment lasted. 



And, in reasoning on this subject, we must not forget 

 to consider that most remarkable circumstance, that the 

 source of the Heat generated by friction, in these experi- 

 ments, appeared evidently to be inexhaustible. 



It is hardly necessary to add, that anything which any 

 insulated body, or system of bodies, can continue to fur- 

 nish without limitation, cannot possibly be a material sub- 

 stance ; and it appears to me to be extremely difficult, if 

 not quite impossible, to form any distinct idea of any- 

 thing capable of being excited and communicated in the 

 manner the Heat was excited and communicated in these 

 experiments, except it be MOTION. 



I am very far from pretending to know how, or by 

 what means or mechanical contrivance, that particular 

 kind of motion in bodies which has been supposed to 

 constitute Heat is excited, continued, and propagated; 

 and I shall not presume to trouble the Society with mere 

 conjectures, particularly on a subject which, during so 

 many thousand years, the most enlightened philosophers 

 have endeavoured, but in vain, to comprehend. 



But, although the mechanism of Heat should, in fact, 

 be one of those mysteries of nature which are beyond 

 the reach of human intelligence, this ought by no means 

 to discourage us or even lessen our ardour, in our at- 

 tempts to investigate the laws of its operations. How 

 far can we advance in any of the paths which science has 

 opened to us before we find ourselves enveloped in 



