upon the Collifion of Bodies . 
33 ) 
perfectly unelaftic, and perfe£tl y fifty and to bodies perfectly 
unelaftic, and perfedlly hard. To avoid prolixity, I fhall confider 
in each, only the limple cafe of two bodies which arc equal in 
weight or quantity of matter finking one another. Refpedting 
thofe which are perfectly elaftic, it is univerfally agreed, that 
when two fuch bodies ftrike one another, no motion is loft ; but 
that in all cafes, what is loft by one is acquired by the other : 
and hence, that if an elaftic body in motion ftrikes another at 
reft, upon the ftroke the former will be reduced to a ftate of 
reft, and the latter will fly off with an equal velocity. 
In like manner, if a non-elaftic Joft body ftrikes another at 
reft, they neither of them remain at reft, but proceed together 
from the point of collifion with exactly one half of the velo- 
city that the firft had before the ftroke ; this is alfo univer- 
fally allowed to be true, and is fully proved by every good 
experiment upon the lubjedt. 
Refpedting the third fpecies of body, that is, thofe that are 
non-elaftic, and yet perfedlly hard ; the laws of motion relating 
to them, as laid down by one fpecies of philofophers, have 
been rejected by another ; the latter alledging, that there are 
no fuch bodies to be found in nature whereon to try the experi- 
ment ; but thofe who have laid down and afligned the dodtrine 
that would attend the collifion of bodies ot this kind (if they 
could be found) have univerfally agreed, that if a non-elaffic 
hard body was to ffrike another of the fame kind at reff, that, 
in the fame manner as is agreed concerning non-elaffic foft 
bodies, they neither of them would remain at reff, but would 
in like manner proceed from the point of collifion, withexadtly 
one half of the velocity that the firft had before the ftroke : in 
fhort, they lay it down as a rule attending all non-elaffic bodies, 
whether hard or foft, that the velocity after the ftroke will 
Y y 2 be 
