'35 1 Mr. smeaton’s Fundamental Experiments 
of the firft mover, yet in this cafe the other half is not lojf 
but fufpended , ready to be re- exerted whenever it is fet at 
liberty ; and that it is really and bona fide one half and neither 
more or lefs, appears from this mi controverted fimple principle, 
that the power of reftitution of a perfect fpring is exactly equal 
to the power that bends it. And this may, in a certain degree, 
be (hewn to be faCt by experiment, if there were any need of 
Inch a proof ; for if, when the bodies are at reft after the laft 
\ 
experiment, the two rods are lathed together near the bottom 
with a bit of thread, and then the ratchets unpinned and 
removed ; on cutting the thread with a pair of feiftfars they 
will each of them rebound, C towards M, and D towards N ; 
and if they rebounded reipeCtively to z andjq the mechanical 
power exerted would be the fame as it was after the ftroke, 
when the mean of their two afeents was up to the mark z ; 
but here it is not to be expected, becaufe not only the motion 
loft by the friction of the ratchets is to be deducted, becaufe 
it had the effeCt of real non-elafticity ; but alfo the elafticity 
that feparated them in the ftroke, which was loft in the vibra- 
tions that lucceeded ; neither of which hindered the mean 
afeent to be to z ; but yet, under all thefe diladvantages in the 
machine (if not unreafonably ill made) the rod ef will afeend 
to d , and gh to a : and hence 1 infer, as a pofitive truth, that 
in the collifion of non elaftic foft bodies, one half of the me - 
chanic power ref ding in the friking body is If in the Jlroke. 
RefpeCting bodies unelaftic and perfectly hard, we muft 
infer, that fince we are unavoidably led to a conclufion con- 
cerning them, which contradicts what is efteemed a truth ca- 
pable of the ftriCteft demonftration ; viz . that the velocity of 
the center of gravity of no fyftem of bodies can be changed 
by any collifton betwixt one another, fomething muft be 
aiYumed that involves a contradiction. This perfectly holds, 
i according 
