96 OUTLINES OF NATURAL PHILOSOPHY. 



garded as constant, whatever be the extent of the 

 surfaces that touch. Journal de Pliys. vol. xxvn, 

 p. 296, at the end. 



From some very accurate experiments, Mr VINCE 

 concluded, that friction increases in a less ratio 

 than that of the pressure. Trans. 1785, p. 172. 

 BOSSUT has remarked the same, when the weights 

 become very great, and gives as a proof, the small 

 inclination of the planks on which a ship slides 

 when it is launched. Mecanique, 256. 



160. Friction may be distinguished into two 

 kinds, that of Sliding, and that of Rolling Bodies. 

 The force of the latter is very small compared 

 with that of the former. 



Though the friction of rolling is small in respect of 

 that of sliding, great force can be given to it by the 

 application of pressure. In the passing of metals 

 between rollers or cylinders, strongly compressed 

 by springs, the friction is that of rolling, yet it is 

 sufficient to communicate the motion of one of the 

 cylinders to the plate of metal between them, in op- 

 position to the great force which resists the exten- 

 sion of the plate. 



161. The preceding facts appear to prove, that 

 friction arises from the asperities at the surfaces of 

 bodies, the eminences of the one sinking into the 

 cavities of the other, so that a certain force, when 

 it acts parallel to the surfaces, is necessary to dis- 

 engage the parts, by forcing the one body to rise 



2 over 



