ON RAISING AND REMOVING WJEIGHTS, 213 



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 ac- 

 curacy. Tallow is liable to lose its lubricating quality, unless it be frequently 

 renewed. Between surfaces of wood, soap is sometimes applied, but more 

 commonly black lead, which becomes highly polished. The advantages of 

 canals, and of navigation in general, are principally 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 tlie appli- 

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

 moves between them without any friction: but it has still to overcome 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 Avant of elasticity in the materials concerned. The continued change 

 of place of the rollers is often a material objection to their employment; their 

 action may in some cases be prolonged by fixing wheels on their extremities, 

 as well as by some other arrangements; but thcKse methods are too compli- 

 cated to afford much practical utility. Rollers may also be placed betwceu 

 two cylinders, the one convex and the other concave, and the friction may 

 in this manner be wholly removed, whatever may be the magnitude of the 

 rollers. (Plate XVII. Fig. 222, 223.) 



The effect of friction in any machine being always diminished, in propor- 

 tion as the velocity of the parts sliding on each other is diminished, it is ob- 

 vious that by reducing £lie dimensions of the axis of a wheel as much as possi- 

 ble, we also reduce the friction. When the pressure on the axis is derived 

 principally from the weight of the wheel itself, the friction may be lessened 

 by placing the wheel in a horizontal position, and making the axis vertical; 



