192 On the Measure of 
nonelastic, Mr. Smeaton understood a contra- 
diction to be involved in the very supposition 
of the existence of such bodies. It has never 
been contended that any such are to be found 
in nature. But it is very generally argued, 
with Mr. Maclaurin, that “ there is the same 
objection [of non-existence] against admitting 
and treating of bodies of a perfect elasticity.*” 
fn reply to this I would observe that, the 
objection does not appear to be of the same 
weight against perfectly elastic, as against 
perfectly non-elastic hard bodies. For, we 
have substances which approach very nearly 
to perfect elasticity ; but we can find no sub- 
stance of which the qualities approach to 
hardness and non-elasticity united. In gene- 
ral the elasticity encreases as the hardness 
encreases, and no substance has ever been pro- 
duced that can be called hard, without posses- 
sing, at the same time, great clasticity. _ 
It does not appear that the possible exist- 
ence of a perfectly hard non-elastic body was_ 
obvious to the first discoverers of the laws of 
percussion. Huygens appears to have under- 
stood a hard body to be one that is perfectly 
elastic. His 6th law of percussion is as fol- 
lows, “ Summa productorum factorum a mole 
eujuslibet corporis dwi ducta in quadratum 
* Account of S:r Isaac Newton’s discoveries, p. 93. 
