414 On the Collision of Hard Bodies. (Dec. 
&c. or lime-stone, or the tops of underlying, stratified mountains, 
such as some of the newest floetz trap formations of Werner, appear 
above the general stratified formations which surround them, they 
have individually one steep and one flat side, and are much larger, 
than any other inequalities that are found among the stratified for- 
mations. 
The undulated figure of the earth’s inequalities, as has been 
shown, tovk its rise from the sinking of the ground lower in one 
part than in another, and the rented surfaces were formed by its 
sinking below natural valieys, so much that the high ground was 
forced to separate into parts, to permit the continuation of that 
sinking. Therefore all the earth’s features are owing to the unequal 
contraction of its matter. 
ArtTicLe IV. 
On the Collision of perfectly Hard Bodies. By Mr. John Gough. 
(To Dr. Thomson.) 
SIR, 
Bopres are divided into elastic and inelastic, from a very obvious 
difference in their effects after collision : they are also distinguished 
by the epithets of hard and soft, from their comparative pliantness. 
Hardness and softness are terms with which every one is acquainted ; 
but the human mind constantly endeavours, in the contemplation 
of the qualities of things, to conceive them in a state of perfection; 
for it thereby acquires precise definitions, which are afterwards used 
in comparing the same qualities as they occur in nature. For this 
reason philosophers define that body to be perfectly hard which is 
so constituted as to resist all change of figure when acted on bya 
finite force. On the contrary, those bodies are said to possess the 
quality of hardness in an imperfect degree which undergo any 
alteration in their shapes from collision or pressure ; which changes 
are evidently produced by an internal motion amongst their consti- 
tuent particles ; and this motion is as evidently effected in time in 
consequence of an external force being applied to the mass. After 
mathematicians have divided bodies into the kinds stated above, 
they proceed to lay down the laws of collision by help of these de- 
finitions; but their demonstrations do not appear to be conducted 
with the same perspicuity, nor even with equal correctness in all the 
cases ; ] mean in those relating to hard bodies. For theorems have 
been invented, expressing the interchange of motion, arising from 
the collision of bodies which are perfectly elastic, and of such as 
are imperfectly so; and the results are found, upon comparison, to 
differ essentially. But when the same interchange comes to be in 
vestigated in the case of hard bodies, no notice is taken of perfect 
