PREFACE TO THE FIFTH EDITION. ix 



his uncle's and his own views on the identity of heat 

 and mechanical force, and has given a calculation of 

 their equivalent relation, which is not far from the 

 more recent numerical results of Mayer, Joule, and 

 others. 



Several of the great mathematicians of a much 

 earlier period advocated the idea of what they termed 

 the Conservation of Force ; but although they consi- 

 dered that a body in motion would so continue for ever, 

 unless arrested by the impact of another body and, 

 indeed, in the latter case, would, if elastic, still continue 

 to move (though deflected from its course) with a force 

 proportionate to its elasticity yet with inelastic bodies 

 the general and, as far as I am aware, the universal 

 belief was, that the motion was arrested on impact and 

 the force annihilated. Montgolfier went a step farther, 

 and his hydraulic ram was to him a proof of the truth 

 of his preconceived idea, that the shock or impact 

 of bodies left the mechanical force undestroyed.* 



Previously, however, to the discoveries of the voltaic 

 battery, electro-magnetism, thermo-electricity, and pho- 

 tography, it was impossible for any mind to perceive 

 what, in the greater number of cases, became of the 

 force which was apparently lost. The phenomena of 

 heat, known from the earliest times, would have been 

 a mode of accounting for the resulting force in many 

 cases where motion was arrested, and we find Bacon 

 enouncing a theory that motion was the form, as he 

 quaintly termed it, of heat. Rumford and Davy 



* See also a paper by Mr. Rankinc, On the Dynamical Principles of JVeu'fon, 

 in the Engineer, Oct. 26, 1866. 



