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. 5 '’* 
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 unelastic 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." J 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, 3 the “ Unscientific People," we would remind 
* “ Ainsi cette theorie ne met en action qne des forces connues, 
Pattraction du soleil, celle que la comete exerce snr ses propres particules, 
la chaleur du soleil $t la repulsion due a cette chaleur.” (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 qu’a 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. Encke.” (p. 354.) 
“5°. II convient de n’accepter, dans le systeme du monde, que des forces 
connues, ou des forces susceptibles d’etre verifies experimentalement j usque 
dans le mode d’action suppose.” (p. 704.) — “ Comptes Benclus ,” 1860, vol. i. 
t “ Familiar Lectures,” p. 465. 
% “ Fragments of Science,” p. 12. 
