Prof. 0. Reynolds on Rolling-Friction. 77 



part of which is devoted to the examination and illustration of 

 the exact manner in which the deformation at the point of contact 

 occurs, and the influence of friction upon it. The latter part of 

 the paper contains an account of numerous experiments, and their 

 results, which were undertaken as part of this investigation. 



The first series of experiments relate to the resistance which an 

 iron roller experiences on surfaces of different hardness. Cast iron, 

 glass, brass, boxwood, and india-rubber were tried. Extreme care 

 was taken to make the roller and the surfaces true ; and this was so 

 far successful that on cast iron the roller would roll in either direc- 

 tion when the surface had an inclination of one in five thousand, 

 or, roughly, a foot in a mile. Comparing the different surfaces, we 

 see that the resistance increases with the softness, although appa- 

 rently not in the simple proportion ; on boxwood the resistance is 

 nearly double as great as] on the harder surfaces, and on india- 

 rubber from six to ten times as great. 



The second series of experiments were to ascertain the actual 

 extent of slipping on india-rubber, both with a cast-iron roller and 

 also with an india-rubber tire glued on to the roller, and rolled on 

 hard surfaces and on plates of india-rubber of different thicknesses. 



These experiments bear out the arguments expressed in the first 

 part of the paper ; in fact the arguments were based on the ex- 

 periments. There is no intention to imply that the whole of the 

 resistance to rolling is in all cases due to the causes already men- 

 tioned. Under ordinary circumstances the irregularities of the 

 surfaces and the crushing of the material beneath the roller are 

 the chief causes. And, besides these, two other causes are discussed 

 in the paper as having been brought to light by the experiment, 

 viz. the communication of heat between the compressed material and 

 that which surrounds it, which prevents the material immediately 

 expanding to the same volume as it previously occupied, and the 

 viscosity of the material, which also renders it slow to expand. 

 Both these causes are, however, rather connected with the effect 

 of the speed of the roller on the resistance than with the residual 

 resistance, which, so far as the surfaces are perfectly true and per- 

 fectly hard, appears to be due to the friction which accompanies 

 the deformation, and is hence called rolling-friction. 



No attempt has yet been made to investigate the laws of rolling- 

 friction, although the author hopes to continue the investigation in 

 this direction as soon as he has obtained the necessary apparatus. 



At the end of the paper attention is called to certain phenomena 

 connected with railway-wheels, which it is thought now, for the 

 first time, receive an explanation. Thus the surprising superiority 

 of steel rails over iron in point of durability is explained as being 

 due as much to the fact that their hardness prevents the wearing- 

 action, ?'. e. the slipping, as that it enables them better to withstand 

 the wear. Also the slipping beneath the wheel explains the wear 

 of the rails in places where brake are not applied ; and the severe 

 lateral extension beneath the wheel is thought to explain the scaling 

 of wrought-iron rails. 



