MECHANICS. 



already explained, and obtained results 

 from numerous trials, which fully con- 

 firmed the consequence he had deduced. 

 He found that the motion of C D on the 

 plane, produced by a given weight, S, 

 was always more accelerated as the 

 surface of contact was diminished, the 

 pressure being the same ; from whence 

 it followed, on the principles already 

 explained, that the friction was dimi- 

 nished. 



For example, a body was taken, 

 whose face was to its edge in the ratio of 

 22 to 9. The same weight, S, which 

 moved it through 33 inches in two 

 seconds, placed upon its face, moved it 

 through 47 inches in the same time, 

 when placed upon its edge. 



Again, when the face was to the edge 

 as 32 to 3, the spaces through which it 

 was moved in two seconds were 32 

 inches and 37| inches. Numerous 

 other experiments were instituted, and 

 attended with similar results. 



(13.) Most of the experiments by 

 which the proportionality of the friction 

 to the pressure had been established, 

 were conducted on a principle different 

 from that adopted by Vince. In these 

 the friction was generally measured by 

 the force necessary to put the body C D 

 in motion, being placed at rest upon 

 the horizontal plane. Vince, however, 

 makes several objections to this method 

 of measuring the friction. In the first 

 place, he objects, that the force neces- 

 sary to put the body in motion must be 

 necessarily greater than the friction. 

 This objection, in strict theory, is un- 

 doubtedly valid, but, practically consi- 

 dered, will, we conceive, be found to 

 have but little weight. It is very true 

 that the force which is equal to the 

 friction, is that weight which exactly 

 keeps the friction in equilibrium, and 

 without putting the body in motion, 

 puts it in a state in which the smallest 

 additional force imaginable will produce 

 motion. If the experiment be nicely 

 executed, therefore, the weight which is 

 found just to produce motion will ex- 

 ceed the friction by a quantity so small 

 as to produce no sensible effect on the 

 results of the investigation. 



It is further objected by Vince, that 

 the force which opposes the motion of 

 the body from a state of rest is not 

 friction alone, but friction and cohesion 

 conjointly, the latter in general greatly 

 predominating over the former. In 

 confirmation of this, he instituted several 

 experiments, by which he proved that 



the force necessary to put a body in 

 motion was much greater than the force 

 which is necessary to continue that 

 motion uniform; assuming that the 

 latter must be the true measure of the 

 friction. 



That the resistance which a body 

 resting upon another offers to a force 

 tending to put it in motion, is greater 

 thai; the friction of the same body when 

 moving on the other, is a fact which was 

 distinctly noticed, and very accurately 

 investigated by Coulomb. But this 

 resistance is ascribed by him entirely 

 to friction, and accordingly one of the 

 principles which he lays down as esta- 

 blished by experiment is that the friction 

 of bodies at rest is greater than the 

 friction of the same bodies in motion. 



Coulomb found that this friction of 

 bodies at rest (we shall call it friction 

 for the present), is increased to a cer- 

 tain limit with the duration of their 

 contact. That is to say, wlien one body 

 rests upon another, the friction of their 

 surfaces increases for a certain length of 

 time, until it reaches its greatest value ; 

 after this, it remains constant ; and 

 whatever length of time the bodies are 

 permitted to rest in contact, the friction 

 is not increased. The length of time in 

 which the friction reaches its greatest 

 amount was found to be different in 

 different bodies. AY hen the boolies are 

 both wood, it is one or two minutes ; 

 when they are both metal, it is so short 

 as not to be perceptible. When wood 

 is placed upon a metallic surface, the 

 friction continues to increase for several 

 days. 



It is, therefore, agreed on all hands, 

 that the resistance of bodies at rest is 

 much greater than the friction when one 

 body moves upon another ; and the only 

 question to be decided is, whether this 

 resistance be entirely friction, or the 

 mixed effects of friction and cohesion ; 

 and if so, what proportion the cohesion 

 bears to the friction. Professor Vince 's 

 reasoning on these points appears to be 

 far from conclusive. He gratuitously 

 assumes in the first place that the re- 

 sistance of bodies at rest is the mixed 

 effect of friction and cohesion. Thus 

 far we should be inclined to go with 

 him ; because, if we grant the existence 

 of such a force as cohesion, we can 

 scarcely deny that it must be mixed more 

 or less with friction in resisting the mo- 

 tion of the one body upon the other. But 

 then another difficulty arises respecting 

 the results of Vince s own experiments. 



