312 LECTURE XVIII. 



equally divided, according to the positions of the arms. (Plate XVII. Fig. 

 220, 221.) 



A man can carry in general a weight four or five times as great as that 

 •which he can raise continually in a vertical direction witli the same velocity: 

 so that we may consider the resistance to be overcome as a kind of friction 

 which amounts to about a fourth or a fifth of the weight. If we attempted to 

 draw a weight along a horizontal surface, the resistance of the surface would 

 often not only impede the motion, but also injure the texture of the substance 

 to be moved. This injury may, however, be avoided by the interposition of a 

 simple frame or dray, and the dray may be armed with a substance subject to 

 little friction, as with -iron: the friction may also be somewhat further dimi- 

 nished by making the outline of 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 diminishing 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 consider- 

 ably 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 increase the ad- 

 hesion. 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, however, 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 

 tliat of the rod. 



Friction may in general be considerably diminished by the interposition of 



