upon the Collljion of Bodies . r 
lifhed in the Philofophical TranfaCtions, vol. LI. for the year 
1 759 *• 
]t may alfo be faid, that fince we have no bodies perfectly 
eladic, or perfectly uneladic and fojt, why fhould we expeCt and 
bodies perfectly uneladic and hard? Why may not the efteCts be 
fucli as fhould refult from a fuppodtion of their being imper- 
ially elafic joined with their being imperfectly hard ? But here 
I mud: obferve, that the fuppolition appears to be a contra- 
diction in terms. 
We have bodies which are fo nearly perfectly eladic, that thb 
laws may be very well deduced and confirmed by them; and 
the fame obtains with refpeCt to non-eladic foft bodies ; but con- 
cerning bodies of a mixed nature, which are by far thegreated 
number, fo far as they are wanting in eladicity, they are foft , 
and bruife, yield , or leave a mark in collidon ; and fo far as 
they are not perfectly foft they are eladic, and obferve a mix- 
ture of the law relative to each ; but imperfeCtly eladic bodies, 
imperfeCtly hard come in reality under the fame defeription 
as the former mixed bodies : for fo far as they are imperfeCtlv 
hard they are foft, and either bruife and yields or leave a mark 
in the droke ; and fo far as they want perfeCt eladicity, 
they are non-eladic ; that is to fay, they are bodies imperfeCtly 
eladic, and imperfeCtly foft ; and in faCt I have never yet feen 
any bodies but what come under this defeription. It feems, 
therefore, that refpeCting the hardnefs of bodies they differ in 
degrees of it, in proportion as they have a greater degree of 
tenacity or cohefion ; that is, are further removed from perfeCt. 
* “ The effect, therefore, of overfhot wheels, under the fame circumftance of 
“ quantity and fall y is at a medium double to that of the underfhot : and as a 
“ confequence thereof, that non-elaflic bodies , when aiding by their impulfe or collif on, 
“ communicate only a part of their original power; the other part being fpent in 
“ changing their figure in confluence of the f robe Phil. Tranf, voi. LI. p. 133. 
foftnefs. 
