FRICTION. 



circumstances remain the same, the diffi- 

 culty of bending a rope increases with the 

 square of its diameter, as also with its 

 tension ; and it decreases according las 

 the radiuof the curvature of the body to 

 which it is adapted increases Of the 

 simple mechanical powers, the lever is 

 the least subject to friction. In a wheel, 

 the friction upon the axis is as the weight 

 that lies upon it, as the diameter of the 

 axis, and as the velocity of the motion. But 

 upon the whole this sort of friction is 

 not very great, provided the machine is 

 well executed. In common pullies, es- 

 pecially those of a small size, the friction 

 is very great. It increases in proportion 

 as the diameter of the axis increases, as 

 the velocity increases, and as the diame- 

 ter of the pully decreases. With a move- 

 able tackle or block of 5 pullies, a pow- 

 er of 150 pounds will barely be able to 

 draw up a weight of 500 pounds. The 

 screw is subject to a great deal of fric- 

 tion ; so much so, that the power which 

 must be applied to it, in order to produce 

 a given effect, is at least double that 

 which is given by the calculation, inde- 

 pendent of friction. But the degree of 

 friction in the screw is influenced consi- 

 derably by the nature of the construction, 

 for much of it is owing to the tightness 

 of the screw, to the distance between its 

 threads, and to the shape of the threads; 

 the square threads producing, upon the 

 whole, less friction than those which are 

 sharp. The friction which attends the 

 use of the wedge exceeds, in general, 

 that of any other simple mechanical pow- 

 er. Its quantity depends so much upon 

 the nature of the body upon which the 

 wedge acts, besides other circumstances, 

 that it is impossible to give even an approx- 

 imate estimate of it. The friction of me- 

 chanical engines not only diminishes the 

 effect, or, which is the same thing, occa- 

 sions a loss of power, but is attended 

 with the corrosion and wear of the prin- 

 cipal parts of the machine, besides pro- 

 ducing a considerable degree of heat, and 

 even actual fire ; it is, therefore, of great 

 importance in mechanics to contrive 

 means capable of diminishing, if not of 

 quite removing, the effects of friction. 



The methods of obtaining the impor- 

 tant object of diminishing the friction 

 are of two sorts, viz. either by the inter- 

 position of particular unctuous or oily 

 substances between the contiguous mov- 

 ing parts, or by particular mechanical 

 contrivances. Olive-oil is the best, and 

 perhaps the only substance that can be 

 used in small works, as in watches and 





clocks, when metal works against metal, 

 But in large works the oil is liable to 

 drain off, unless some method is adopt- 

 ed to confine it. Therefore, for large 

 works, tallow is mostly used, or grease 

 of any sort, which is useful for metal, as 

 well as for wood. In the last case tar is 

 also frequently used. The mechanical con- 

 trivances which have been made, and are 

 in use, for the purpose of diminishing the 

 effects of friction, consist either in avoid- 

 ing the contact of such bodies as produce 

 much friction, or in the interposition of 

 rollers, viz. cylindrical bodies, between 

 the moving parts of machines, or between 

 moving bodies in general. Such cylin- 

 ders derive, from their various size and 

 application, the different names of rollers, 

 friction wheels, and friction rollers. Thus 

 in mill- work, and other large machines, 

 the wooden axis of large wheelsterminate 

 in iron gudgeons, which turn in wood, or 

 more frequently in iron or brass, which 

 construction produces less friction than 

 the turning of wood in w*ood. In the 

 finest sort of watch-work the holes are 

 jewelled, viz. many of the pivots of the 

 wheels, &c. move in holes made in rubies, 

 or topazes, or other hard stone, which, 

 when well finished, are not liable to 

 wear, nor do they require much oil. In 

 order to understand the nature of rollers, 

 and the advantage with which their use is 

 attended, it must be considered, that 

 when a body is dragged over the surface 

 of another body, the inequalities of the 

 surfaces of both bodies meet, and oppose 

 each other, which is the principal cause of 

 the friction or obstruction ; but when one 

 body, such as a cask, a cylinder, or a ball, 

 is rolled upon another body, the surface 

 of the roller is not rubbed against the 

 other body, but is only successively ap- 

 plied to, or laid on, the other, and is then 

 successively lifted up from it. There- 

 fore, in rolling, the principal cause of 

 friction is avoided, besides other advan- 

 tages : hence a body may be rolled upon 

 another body, when the shape admits of 

 it, with incomparably less exertion than 

 that which is required to drag it over the 

 surface of that other body. In fact, we 

 commonly see large pieces of timber, and 

 enormous blocks of stone, moved upon 

 rollers that are laid between them and 

 the ground with ease and safety, when it 

 would be almost impossible to move them 

 otherwise. 



FRICTION, is a jterm made use of in 

 medicine, and implies the act of rubbing 

 a diseased part with oils, or other sub- 

 stances, Friction is also applied to the 



