ON RAISING AND REMOVING WEIGHTS. 163 



the dray a little convex below, so that a slight agitation may be continually 

 produced during its motion. Sometimes the simple expedient of placing a 

 load on two poles of elastic wood, the thickest ends of which are supported 

 by the horse, and the thinner drag on the ground, is of use both in dimi- 

 nishing the friction by confining it to a smaller and smoother surface, and 

 in equalising the motion by the flexibility of the poles. 



It often happens that agitation of any kind enables us to lessen consi- 

 derably the friction between two bodies, especially when they are elastic. 

 If we wish, for instance, to draw a ring along an iron rod, by a thread 

 which is nearly perpendicular to it, we may exert all our strength in vain 

 if we apply it by slow degrees, since the increase of force continues to in- 

 crease the adhesion. But if we pull the ring suddenly, and then slacken 

 the thread, it rebounds from the rod by its elasticity, and in this manner it 

 slides readily along by a continuance of alternations. In such a case, how- 

 ever, it would be more natural, if the thread were sufficiently heavy, to 

 give it a serpentine motion which would draw the ring in a more oblique 

 direction. It is said that when a screw is fixed very firmly in a piece of 

 iron, it may be extricated much more easily while the iron is filed in some 

 neighbouring part. The agitation thus produced probably operates in a 

 manner somewhat similar to that of the rod. 



Friction may in general be considerably diminished by the interposition 

 of oily substances, where the surfaces are of such a nature as to admit of 

 their application. Thus common oil, tallow, or tar, are usually interposed 

 between metals which work on each other. It is necessary to attend to 

 the chemical properties of the oil, and to take care that it be not of such 

 a nature as to corrode the metals employed, especially where the work 

 requires great accuracy. Tallow is liable to lose its lubricating quality 

 unless it be frequently renewed. Between surfaces of wood, soap is some- 

 times applied, but more commonly black lead which becomes highly 

 polished. The advantages of canals and of navigation in general, are prin- 

 cipally derived from the facility with which the particles of fluids make 

 way for the motion of bodies floating on them. 



The interposition of rollers or of balls bears some resemblance to the 

 application of fluids. Supposing the surfaces to be flat and parallel, a 

 roller moves between them without any friction : but it has still to over- 

 come the resistance occasioned by the depression which it produces in the 

 substance on which it moves, and which is greater or less according to 

 the softness and want of elasticity of the substance, If the substance were 

 perfectly elastic, the temporary depression would produce no resistance, 

 because the tendency to rise behind the roller would be exactly equivalent 

 to the force opposing its progress before ; and the actual resistance only 

 arises from a greater or smaller want of elasticity in the materials con- 

 cerned. The continued change of place of the rollers is often a material 

 objection to their employment ; their action may in some cases be pro- 

 longed by fixing wheels on their extremities, as well as by some other 

 arrangements ; but these methods are too complicated to afford much 

 practical utility. Rollers may also be placed between two cylinders, the 

 one convex and the other concave, and the friction may in this manner 



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