10 



MECHANICS. 



Does the motion of one body upon the 

 other altogether destroy the cohesion ? 

 If not, why should the resistance of 

 bodies in motion be entirely ascribed to 

 friction, while the resistance of the same 

 bodies at rest is ascribed to the united 

 effects of friction and cohesion ? We 

 shall not, however, pursue this ob- 

 jection. 



Vince next assumes that all that quan- 

 tity by which the resistance of bodies 

 at rest exceeds the friction of the same 

 bodies when moving one upon the 

 other, is the effect of cohesion. It is 

 evident, that in this there is a tacit as- 

 sumption that the friction of bodies at 

 rest is equal to the friction of bodies in 

 motion ; for the whole resistance of 

 bodies at rest arises from the sum of 

 the effects of their friction and cohesion ; 

 and he assumes that if from this sum 

 the friction in motion be subtracted, 

 the remainder will be the friction at rest. 



If the proportion of the parts of the 

 resistance which are to be assigned to 

 friction and cohesion be introduced into 

 the investigation at all, it would have 

 been desirable that experiments should 

 be instituted to determine this. We 

 would be led expect from such experi- 

 ments a very different result from that 

 assumed by Vince, and should antici- 

 pate that in most cases the cohesion 

 would be found to bear a very small 

 proportion to the friction; and that, 

 therefore, the friction at rest would still 

 be found to be much greater than the 

 friction in motion. 



The quantity of cohesion might, we 

 conceive, be thus determined : Let a 

 string be attached to the body, which 

 rests apon the plane at a point imme- 

 diately over its centre of gravity, and 

 let this string be carried in a vertical 

 direction over a fixed pulley, and let a 

 weight be suspended from it exactly 

 equal to the weight of the body which 

 rests upon the plane. This weight will 

 equilibrate with that of the body, and 

 the force with which the body will then 

 be attached to the plane will be that of 

 the cohesion alone. Now, let small 

 weights or fine sand be added to the 

 weight which equilibrates that of the 

 body, until the body be just lifted from 

 the plane. This additional weight may 

 be taken as the measure of the cohe- 

 sion ; and if it be subtracted from the 

 weight which just moves the body upon 

 the plane acting in a direction parallel 

 to the plane, the remainder will evi- 

 dently be the true value of the friction 



at rest. We have not had an oppor- 

 tunity of instituting experiments on this 

 principle, but are strongly disposed to 

 predict that in most cases the cohesion 

 would be found to bear a very small 

 ratio to the friction. 



That the duration of the contact 

 should increase the friction is not diffi- 

 cult to conceive, inasmuch as the effect 

 of the pressure acting for a certain time 

 is to make various asperities and in- 

 equalities of the surfaces insert them- 

 selves among each other more effec- 

 tually than they could if one surface 

 were moving over the other. We may 

 illustrate this as in a former instance, 

 by two brushes placed one upon the 

 other. If a weight be placed upon the 

 upper brush, the pressure will cause the 

 hairs of the one to insinuate themselves 

 and descend between the hairs of the 

 other. This process will, however, 

 proceed gradually ; and, after the lapse 

 of a certain interval, will cease. A con- 

 siderable force will then be requisite to 

 put the one brush in motion over the 

 other ; but when in motion, the same 

 pressure will fail to produce so great an 

 intermixture of the hairs, since in no 

 one position of the brushes will suffi- 

 cient time be given to produce so great 

 an effect. Thtse effects, if minutely 

 examined, will be found to have the 

 most exact analogy and correspondence 

 with the properties of friction, as de- 

 termined by the experiments of Cou- 

 lomb ; and they illustrate, if not explain, 

 the phenomenon of the friction at rest 

 being greater than the friction in motion. 



The experiments of Coulomb were 

 not, however, confined to the investi- 

 gation of the quantity of the friction 

 when the bodies under examination are 

 put in motion from a state of rest. He 

 also examined the friction of bodies in 

 motion, and determined that friction is 

 an uniformly retarding force, in nearly 

 the same manner as Professor Vince. 

 He also examined, in different sub- 

 stances, the proportion between the 

 friction at rest and the friction in 

 motion, and found that this proportion 

 is different with different bodies. la 

 woods, the friction at rest he found to 

 amount to half the pressure, while the 

 friction in motion only amounts to an 

 eighth of it. Between wood and metal, 

 the friction at rest was found to be one- 

 fifth of the pressure, and the friction in 

 motion one-twelfth of it. Between 

 metals, there was no sensible difference 

 observed between the two frictions. 



