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 ai'ising 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 necessaiy 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-f". 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. Tlie 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 



* Experiments on the Strength of Materials: — Pliilosopbical Transactions 1817. 

 f Leslie's Experimental Philosophy. 



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