144 MR. RENNIE’S EXPERIMENTS ON THE FRICTION AND 
On a former occasion an attempt was made to develope some of the pro- 
perties of solid bodies in resisting the action of a disruptive force*, the measure 
of which was represented by the sum and qualities of the particles displaced. 
The connection may be traced in the present inquiry, which relates principally 
to the resistance arising from the displacement, or rupture of the superficial 
asperities of bodies in motion when brought into contact by extreme pressure, 
and is analogous to the cohesive state of a body acted upon by opposite but con- 
trary forces. But the cases investigated by experimentalists have seldom been 
carried to the extent necessary to produce a disrupture of the prominencies, 
being generally confined to the definition of friction as designated by writers on 
mechanics, to be the force expended in raising continually the surface of pressure 
by an oblique action ; the surfaces being represented by a series of inclined 
planes acting against each other in alternate succession. The measure of 
friction therefore being supposed to depend upon the angles of the prominen- 
cies and the elementary structure of the bodies, the effect of polishing could 
only be to diminish those prominencies without altering their curvature or in- 
flections. The expense of force therefore ought still to remain the same in both 
cases^-. In this hypothesis it is reasonable to concur, experiment proving, that 
the amount of friction bears immediate reference to the elementary structure 
of bodies; and although the doctrine of inclined planes admits of a ready com- 
prehension of the causes of this kind of resistance under certain circumstances, 
a very slight investigation of the nature of the bodies themselves will exhibit 
their asperities under every possible configuration. The amount of resistance 
will depend upon the degree of pressure, the approximation or rather the 
engagement of the asperities and concavities, and the nature of the surfaces of 
which fibrous, soft, or hard bodies are composed. To surmount, bend, or 
detach these asperities under the circumstances of pressure, area, and velocity, 
demands a proportionable exertion of force ; and it is by the determination of 
this force under all cases, that we can alone arrive at an estimation of the per- 
formance of machines. 
The nature of friction has excited the attention of most of the writers on 
mechanics, from the period of the first two dissertations of Amontons in the 
k Experiments on the Strengtli of Materials : — Philosophical Transactions 1817. 
f Leslie’s Experimental Philosophy. 
