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fundamental Property of the Lever. 
upon this principle, that if two non-elastic bodies meet with 
equal quantities of motion, they will after impact, continue at 
rest; and hence it is concluded, that if a lever which is in 
equilibrio be put in motion, the motions of the two bodies 
must be equal ; and therefore the pressures of these bodies upon 
the lever at rest, to put it in motion, must be as their motions. 
Now in the first place, this is comparing the effects of pres- 
sure and motion, the relation of the measures of which, or 
whether they admit of any relation, we are totally unac- 
quainted with. Moreover, they act under very different cir- 
cumstances ; for in the former case, the bodies acted imme- 
diately on each other, and in the latter, they act by means of 
a lever, the properties of which we are supposed to be ignorant 
of. When forces act on a body, considered as a point, or di- 
rectly against the same point of any body, we only estimate 
the effect of these forces to move the body out of its place, 
and no rotatory motion is either generated, or any causes to 
produce it, considered in the investigation. When we, there- 
fore, apply the same proposition to investigate the effect of 
forces to generate a rotatory motion, we manifestly apply it 
to a case which is not contained in it, nor to which there is a 
single principle in the proposition applicable. The demonstra- 
tion given by Mr. Landen, in his Memoirs, is founded upon 
self-evident principles, nor do I see any objections to his rea- 
soning upon them. But as his investigation consists of several 
cases, and is besides very long and tedious, something more 
simple is still much to be wished for, proper to be introduced 
in an elementary treatise of mechanics, so as not to perplex 
the young student either by the length of the demonstration, 
or want of evidence in its principles. What I here propose to 
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