4 PEEFACE. 



heat appeared, which, though not in terms touching on the mutual 

 and necessary dependence of all the Physical Forces, yet bears 

 most importantly upon the doctrine. 



While my third edition was going through the press I had 

 the good fortune to make the acquaintance of M. Seguin, who 

 informed me that his uncle, the eminent Montgolfier, had long 

 entertained the idea that force was indestructible, though, with 

 the exception of one sentence, in his paper on the hydraulic ram, 

 and where he is apparently speaking of mechanical force, he has 

 left nothing in print on the subject. Not so, however, M. Seguin 

 himself, who in 1839, in a work on the ' Influence of Railroads, 7 

 has distinctly expressed 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 considered 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 proportion- 

 ate 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 and the force annihilated. Montgolfier went a step far- 

 ther, 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 photography, 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 announcing a 

 theory that motion was the form, as he quaintly termed it, of 

 heat. Rumford and Davy adopted this view, the former with a 

 fair approximate attempt at numerical calculation, but no one of 

 these philosophers seems to have connected it with the inde- 

 structibility of force. A passage in the writings of Dr. Roget, 



