Moving Force. 253 
without producing change of figure. It ap- 
pears then, that if any metaphysical consi- 
deration has been improperly mixed with 
this question, it is the supposed possible ex- 
istence of perfectly hard non-elastic substances. 
But unless we have actual proof of the 
existence of such substances, we can have no 
evidence derived from experience to justify 
the inference, that Aand B may be brought to 
rest without producing change of figure. 
When a physical experiment of any kind is 
made, it is generally understood, that unless 
all the results be collected and examined, 
erroneous conclusions may be formed. If 
an experimenter reject some of the results 
which he obtains, on the supposition, that 
sometimes they may not occur, although 
in fact they constantly occur in deter- 
minate quantities, he cannot reasonably 
demand assent to general conclusions drawn 
from so partial an examination of the facts. 
If this reasoning be well founded, we can- 
not reject the consideration of the changes of 
figure produced by A and B; and if we have 
no experience of a mechanical change of 
figure being produced without moving force, 
nor of bodies destroying each other’s motion 
without producing mechanical change of 
figure, we cannot, in the case before us con- 
4 
