115 



is right not to accept, in the system of the world, any but known 

 forces, or forces susceptible of being verified experimentally 

 when in the supposed mode of action/^^ 



32. There is, however, no actual necessity for carrying our 

 investigations to the extreme limit of the terrestrial atmosphere, 

 for on the earth's surface motion ceases, if not wholly, at least 

 partially, which is sufficient for our purpose. To show this I 

 need only quote the authority of Sir John Herschel, who says, 

 " In the collision of inelastic bodies, vis viva is necessarily and. 

 invariably destroyed. The destruction may be total, or may 

 fall short of totality in any proportion, according to the direct- 

 ness of the impact and the proportion of the moving masses ; 

 but whenever contact occurs between such bodies, vis viva 

 disappears, and, once lost, is gone for ever.'^ f In the face of 

 such statements and facts as the foregoing, to talk of the con- 

 servation or persistence of energy is a mere waste of words. 



33. I must not, however, forget that Dr. Tyndall denies this 

 position of Sir John, and says, " It was formerly universally 

 supposed that by the collision of unelastie bodies force was 

 destroyed. Men saw, for example, when two spheres of clay, 

 or painter's putty, or lead, were urged together, that the motion 

 possessed by the masses prior to impact was more or less anni- 

 hilated. They believed in an absolute destruction of the force 

 of impact. Until recent times, indeed, no difficulty was expe- 

 rienced in believing this, whereas at present the ideas of force 

 and its destruction refuse to be united in ordinary philosophic 

 minds.'' % No new experiments, it will be observed, have been 

 made to render the former belief untenable. All the known 

 facts are as they were, but the exigencies of of a system require 

 denial, and therefore the annihilation must be denied. No 

 word has been uttered to shake Sir John's position, except to 

 exclude his mind from association with those philosophic ones 

 that think with Dr. Tyndall. But even at the risk of being 

 classed amongst the readers to whom his ^' Fragments " have 

 been given, i.e., the Unscientific People," we would remind 



^ " Ainsi cette theorie ne met en action que des forces connnes, 

 I'attraction du soleil, celle que la comete exerce sur ses propres particules, 

 la chaleur du soleil et la repulsion due a cette clialeur." (p. 353.) 



" Mon dernier travail avait pour but de lever tous les doutes a ce sujet ne 

 montrant que le milieu resistant ne pouvant exister qvCh la condition de 

 circuler autour du soleil suivant les lois de Kepler, et que son action n'etait 

 pas constamment resistante, comme le supposait M. En eke." (p. 354.) 



" 5°. II convient de n'accepter, dans le systeme du mondo, que des forces 

 connues, ou des forces susceptiblcs d'etre verifiees expuiimentaleuient jnsque 

 dans le mode d'action suppose." (p. 704.)—" Comptes Renduff,'' 1860, vol. i, 



t " Familiar Lectures," p. 465. 



X " Fragments of Science," p. 12. 



