MECHANICS. 



which this effect is produced, may be 

 conceived by placing two brushes with 

 the points of the hairs presented towards 

 each other, and in this position pressing 

 them together in the direction of the 

 hairs, so that the hairs of each brush 

 insert themselves between those of the 

 other. Any attempt to move the one 

 brush upon the Bother in a direction at 

 right angles to tne hairs, will be found 

 to be opposed by a considerable resist- 

 ance. The inequalities with which sur- 

 faces subject to friction are covered, 

 act upon one another in the same man- 

 ner as the hairs of the brushes ; and as 

 it is obvious, that the greater the force 

 with which the brushes are pressed to- 

 gether, the greater the lateral resistance, 

 so also it is invariably found that the 

 greater the pressure with which surfaces 

 are urged one against the other, the 

 greater will be the friction. 



(6). There are three species of fric- 

 tion, or, to speak more properly, there 

 are three ways in which one surface can 

 move upon another, in each of which 

 the friction acts differently. The first 

 is, when one body resting on a plane 

 base, slides upon the plane surface of 

 another body. The second is, when one 

 body being cylindrical, rolls upon the 

 surface of another body. The third is, 

 when a solid cylinder is inserted in a 

 hollow cylinder of a greater diameter, 

 and being pressed in any direction with 

 a certain force revolves within it. We 

 shall consider successively friction act- 

 ing in these several ways. 



(7.) That under the same circum- 

 stances the friction of one surface mov- 

 ing upon another is proportional to the 

 pressure with which the surfaces are 

 urged together, seems to be a law pretty 

 satisfactorily established by experiment. 

 Considering then, for the present, this 

 law as exact, we shall explain the na- 

 ture of the experiments by which it may 

 be established. 



Let AB (fig. 1,) be a perfectly level 

 plane, the surface of which is one of 

 those whose friction is to be examined. 

 Let CD be a piece of the substance, the 

 lower surface of which it is proposed to 

 move in contact with that of the plane, 

 this surface and that of the plane being 

 those between which the friction is to 

 be examined. The upper part of C D 

 is adapted to receive weights, so as to 

 vary at pleasure the pressure of C D on 

 the plane AB. Attached to C D, at 

 D, is a thread, which is carried parallel 

 to the plane over a fixed pulley, JP, and 



Fig.l. 



has a dish, S, suspended from it, 

 adapted to receive weights, the effect 

 of which will be to draw C D along the 

 plane. Now, let C D be loaded so that 

 the \veight of itself and its load shall be 

 one pound. Let fine sand flow into 

 the dish, S, until it is of just sufficient 

 weight to move C D. The weight of 

 the sand, including that of the dish, 

 will then represent the friction. Sup- 

 pose that this is half a pound. Now 

 let C D be loaded with another pound, 

 so that the pressure upon the plane will 

 be twice its former amount. It will be 

 found that it will require half a pound 

 more to be placed in the dish, S, in 

 order to put C D in motion. Thus, 

 when the pressure is doubled, the fric- 

 tion is also doubled. Again, let a third, 

 fourth, and fifth pound be successively 

 added to CD, it will be found to be 

 necessary to add a third, fourth, and 

 fifth half pound to the weight in S, in 

 order to overcome the friction. And in 

 the same manner the experiment may 

 be continued, demonstrating that in 

 whatever proportion the pressure is in- 

 creased, the friction will be increased in 

 exactly the same proportion. 



Such is the result of the experiments 

 of Coulomb and Ximenes, instituted on 

 a large scale, and submitted to a great 

 variety of trials. There was, however, 

 in an extreme case, found to be a slight 

 deviation from this law. For when the 

 pressures used were extremely intense, 

 it was found that the friction did not 

 increase in quite so great a proportion 

 as the pressure. The deviation from 

 the law, however, was so veiy inconsi- 

 derable, and happened only in such ex- 

 treme cases, that it might for the most 

 part be neglected. 



The friction being then considered to 

 be proportional to the pressure when 

 the surfaces are given, a very remark- 



