MECHANICS. 



plane, in the same situation as if there 

 were no friction, and the least accession 

 which the power receives will draw up 

 the weight. Thus it appears, that when 

 there is friction, a less power is neces- 

 sary to sustain a given weight on an in- 

 clined plane, and a greater power is 

 necessaiy to move it up the plane than 

 would be necessaiy were there no fric- 

 tion. 



In like manner, in the case of the 

 single moveable pulley, if there were 

 none of the resistances already men- 

 tioned, any power exceeding one pound 

 would draw up the weight. But if the 

 resistances be supposed to exist before 

 motion can ensue, we must add to the 

 power a sufficient quantity to balance all 

 the resistances. The least addition to 

 the power after that will produce 

 motion. 



We, therefore, infer in general, that 

 when a machine of any kind is used 

 simply to sustain a weight, or to balance 

 a resistance, the resisting forces act in 

 conjunction with the power, and are of 

 mechanical advantage. In many in- 

 stances the resisting powers constitute 

 the whole power and efficacy of the in- 

 strument. Thus, when screws are used 

 to bind together the parts of any struc- 

 ture, the friction of the screw with the 

 substances in which it is inserted pre- 

 vents its return, and constitutes its 

 whole efficacy. In like manner, in the 

 use of nails, which are nothing but nar- 

 row wedges, the friction prevents the 

 recoil of the instrument, and gives it all 

 its binding power. 



The ordinary use of the wedge itself 

 presents at once an instance of the sta- 

 tical advantage, and the dynamical dis- 

 advantage, of a resisting force. "When 

 a wedge is used for any purpose, as to 

 split timber, the great friction which is 

 produced between its faces and the sur- 

 face of the timber, opposes a consider- 

 able resistance to the effect of the force 

 of percussion applied to the back of the 

 wedge to urge it forward, and it accord- 

 ingly advances with much less force and 

 effect than it would if the friction were 

 removed. But after it has advanced, 

 this same friction prevents its recoil, in 

 the interval between two successive 

 strokes on its back. In this case, there- 

 fore, were it not for the effect of friction, 

 we should be compelled to urge the 

 wedge by pressure instead of the far 

 more efficacious force of percussion. 

 Much more power is gained here by the 

 effect of the friction in preventing the 



recoil during the intermission of the 

 force, than is lost by its resistance to 

 the advance of the wedge. 



(4). The laws which govern resisting 

 forces are derived wholly from experi- 

 ment, nor have we any simple and ge- 

 neral principles from which they can be 

 deduced by mathematical reasoning. It 

 is to be regretted, that even among the 

 best-conducted experiments which have 

 been instituted, some considerable dis- 

 crepancies are observable, and differ- 

 ences of opinion subsist between the 

 most respectable authorities respecting 

 many particulars connected with the 

 properties and laws of these forces. We 

 shall give a general account of those 

 properties in which philosophers have 

 most generally agreed, stating those 

 cases distinctly in which the results of 

 different systems of experiments are 

 materially at variance. 



Although, as W T C have just stated, the 

 laws of resisting forces are derived 

 wholly from experiment, yet even here 

 the general principles of the science are 

 far from being useless. They serve as 

 a guide in the selection of the experi- 

 ments best calculated to develop those 

 laws which are the subject of inquiry, 

 as well as to shew the inconclusiveness 

 of some experiments on which we might 

 otherwise be induced to rely ; and they 

 also enable us to deduce from the re- 

 sults of experimental inquiry numerous 

 useful practical results. 



We shall commence with the consider, 

 ation of friction, by far the most im- 

 portant of those resisting forces which 

 we shall have to investigate. 



CHAPTER II. Of the Friction of one 

 Surface sliding over another. 



(5). FRICTION necessarily supposes the 

 surface of one body moving or tending 

 to move upon the surface of another. It 

 also necessarily supposes the one sur- 

 face to be urged against the other with 

 some sensible degree of pressure. Under 

 these circumstances, the cause of fric- 

 tion is the want of perfect smoothness, 

 or polish in the surfaces which are in 

 contact. The small asperities which are 

 spread over each surface, become, when 

 the surfaces are in contact, inserted, as 

 it were, among each other ; and upon any 

 effort to move the surfaces one upon 

 the other, these asperities and inequali- 

 ties oppose each other, and resist the 

 tendency to motion. The manner in 



