~PHYSICALanp LITERARY. 949 
bodies; and refts it.wholly upon this faa, 
_. that equal changes of motion are produced 
upon collifion ; which. is, in other words, that, 
whatever force is loft by the one body, an e- 
qual force is generated in the other. And 
indeed it. appears probable, that the law has 
been invented chiefly to account for this ef- 
fect: if fo, it is extremely rafh in philofo- 
phers to apply it, as they do, in every cafe 
where there is the leaft appearance of action 
and reaction, as if it were an univerfal law 
of nature that muft obtain in every cafe what- 
ever.. It was not difcovered, it would feem, 
that the equality of motion before and after 
percuffion, is the genuine effect of the wis 
refiftentiae. ‘Therefore, to account for this 
phoenomenon, a new law behoved to be in- 
vented; which, to difguife the matter, muft 
be confidered as a general law: for, had it 
been plainly fpoke out, what in effect is faid 
by Dr. Keill, that this law applies only to the 
_ percuffion of bodies; every perfon would be 
_ fenfible, that accounting for the law of per- 
cuffion in this manner, was doing no more 
_ than repeating the fact itfelf in different 
_ words. For to fay that theadtions of two bodies 
‘i in percuffion are equal and in oppofite direti- 
ons, 
—_— eee eee !C«SS 
* ~~. 
