XXIV INTEODtJCTION. 



no man can define better than he did. " I am very far from pre- 

 tending to know how, or by what means or mechanical contri- 

 vances, that particular kind of motion in bodies, which has been 

 supposed to constitute heat, is exerted, continued, and propagated, 

 and I shall not presume to trouble the Society with new conjec- 

 tures. But although the mechanism of heat should in part be one 

 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 ardor in our attempts 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 

 those thick mists, which on every side bound the horizon of the 

 human intellect." 



Rumford's experiments completely annihilated the material hy- 

 pothesis of heat, while the modern doctrine was stated in explicit 

 terms. He moreover advanced the question to its quantitative and 

 highest stage, proposing to find the numerical relation between 

 mechanical power and heat, and obtained a result remarkably near 

 to that finally established. The English unit of force is the foot- 

 pound, that is, one pound falling through one foot of space ; the 

 unit of heat is one pound of water heated 1 F. Just fifty years 

 subsequently to the experiment of Rumford, Dr. J. P. Joule,* of 

 Manchester, England, after a most delicate and elaborate series of 

 experiments, determined that 772 units of force produce one unit 

 of heat ; that is, 772 pounds falling through one foot produces suf- 

 ficient heat to raise one pound of water 1 F. This law is known 

 as the mechanical equivalent of heat. Now, when we throw Rum- 

 ford's results into these terms, we find that about 940 units of force 

 produced a unit of heat, and that, therefore, on a large scale, and 

 at the very first trial, he came within twenty per cent, of the true 



* JAMES PBESCOTT JOULE, born December 24th, 1818, at Salford, near Manchester, 

 England, where he pursued the occupation of a brewer. Long and deeply devoted 

 to scientific investigation, he became a member of the Manchester Philosophical So- 

 ciety in 1842, and of the Eoyal Society of London in 1850. 



