540 Farm Mechanics. 



face with a uniform velocity. Suppose the bight to which 

 the end of the plane has been raised is just "iV of its base, 

 then the friction of motion is 10 per cent, that is, it will 

 require a force equal to ijtr of the weight of the load to 

 maintain motion. 



It has been found that usually the sliding friction be- 

 tween non-lubricated surfaces increases directly as the 

 weight but is independent of the area of surface, provided 

 the weight is not great enough to cut or tear them. Gen. 

 Morin found, in his experiments, that wood without lubri- 

 cant showed a friction of 25 to 50 per cent., but when soap 

 was used between the surfaces the friction was reduced to 

 from 4 to 20 per cent. In the case of metal on wood, with- 

 out lubricant, he found it to be from 50 to 60 per cent. 

 With the smoothest and best lubricated surfaces he found 

 it as small as 3 to 3.6 per cent. 



679. Rolling Friction. When one solid surface rolls 

 over another, no matter how smooth they may be, there is 

 always friction, but the amount is much less than that of 

 sliding, and the fundamental reason is the same as that 

 which permits a load to be carried over bare ground on 

 wheels' with less friction than when carried upon a sled. 

 The roller bearings in common use on the grindstone illus- 

 trate the smaller friction due to the use of rollers, but we 

 get the most perfect example of reduced friction in the 

 ball bearings of bicycles. 



680. Friction Between Liquids. The laws governing the 

 friction of liquids are very different from those of solids. 

 In the first place it increases with the square of the veloc- 

 ity, instead of directly as the velocity in the case of solids, 

 and it decreases with increase of temperature of the liquid, 

 often to a very important extent. 



The liquids used as lubricants have generally compara- 

 tively high viscosity, as fluid friction is technically called, 

 and they vary between wide limits among themselves, es- 

 pecially when they experience wide changes of tempera- 



