ON PASSIVE STRENGTH AND FRICTION. 155 



cation of any force by Avhich the can'iage is to be drawn along a horizontal 

 road of the same materials. 



It is obvious that an inclined plane, on which a weight rests by means of 

 an adhesion proportionate to the pressure, can never be forced backwards by 

 any increase of that pressure, since the resistance increases in the same pro- 

 portion, and continues always sufticient to prevent the relative motion of the 

 weight and the inclined plane. Two such planes, put together, would con- 

 stitute a wedge, which would be equally incapable of giving way to a pressure 

 applied to its opposite surfaces, each of them possessing similar properties 

 with respect to friction. Thus, if the friction or adhesion were exactly one 

 eighth of the pressure, the height of the inclined plane would be one eighth of 

 its length, and the back of the wedge one fourtli. Such a wedge Avould 

 therefore possess a perfect stability with respect to any forces acting on its 

 inclined surfaces. But the effects of agitation, and the minute tremors pro- 

 duced by percussion, have a great tendency to diminish the force of adhesion, 

 by interrupting the intimacy of contact; and where a pin, a nail, or a screw 

 is required to retain its situation with firmness, the inclination of the surfaces 

 must be smaller than the angle of such a wedge as is barely capable of afford- 

 ing a sufficient resistance in theory. 



It appears, therefore, that the force of lateral adhesion, acting between two 

 bodies in contact, is of great importance in all mechanical arts; the firmness 

 of architecture and of carpentry depends in great measure on it. This kind of 

 resistance being equally powerful, when the force is applied in the direction of 

 the surface, to whatever part of the surface it may tend, it follows that any 

 body which is subjected to friction on all sides, will retain its situation with 

 the same force, that was used in overcoming the friction, in order to bring it 

 into that situation, or rather with a greater force, since the lateral adhesion is 

 generally a little greater than the friction: so that a cylindrical wire cannot 

 be withdrawn from a perforation in a board, by any direct force less than that 

 which was employed in introducing it; and this kind of stability, together 

 with that of a wedge or nail resisting a lateral pressure, constitutes the se- 

 curity of the lighter structures of carpentry, while those of architecture re- 

 ceive a great part of their firmness from the accumulation of weight, which 



