154 LECTURE XIII- 



faces : but Mr. Coulomb has observed that in many cases there is, besides 

 this force, another resistance, amounting to several pounds for each square 

 foot of the surface, which is independent of the pressure; and by calculatina: 

 these forces separately, we may probably always ascertain the whole resist- 

 iince with sufficient accuracy. This constant portion is usually much smaller 

 than that which varies with the weight, and in all common cases it may be 

 safely neglected, and the friction of stone on stone may be called equal to 

 one half of the pressure, that of wood on wood one third, and that of metal 

 on metal one fourth; and this may serve as an estimate sufficiently accurate 

 for calculating the effiscts of machines; although, if their parts were, perfectly 

 adjusted to each other, and all the surfaces well polished, the friction would 

 not in general exceed one eighth of the pressure, whatever might be the na- 

 ture of the materials. The application of unctuous substances lessens the 

 friction in the first instance; but unless they are frequently renewed, they 

 sometimes tend rather to increase it. 



The simplest mode of ascertaining the magnitude of the friction of two 

 bodies, is to incline their common surface to the horizon until the one begins 

 to slide on the other: this point determines the magnitude of their adhesion; 

 but in order to find that of their friction when they are in motion, they must 

 be first separated, and then allowed to move on each other, while the vhole 

 apparatus is gently agitated. The friction will then be to the pressure, as the 

 height of the inclined plane to its horizontal length, Avhcn the inclination is 

 barely such as to allow the continuance of any motion which is imparted to 

 the substance placed on the plane. 



It follows from the doctrine of the resolution of force, that when any body 

 is to be drawn along a horizontal surface, which produces a resistance propor- 

 tionate to the pressure, a part of the force may be advantageously employed 

 in diminishing the pressure produced by the weight of the body ; hence, in 

 order for the most advantageous application of the force, its direction must 

 be inclined to the horizon, and it may be demonstrated, that the inclination 

 must be the same with that of a plane on which the relative weight of the 

 body is precisely equal to the friction. Thus if we can determine the incli- 

 nation of a road which is barely sufficient for a carriage to descend on it by 

 its own weight, tlie same inclination will be the best possible for the appli- 



