174 
PROFESSOR OSBORNE REYNOLDS ON ROLLING-FRICTION. 
the two laws propounded by Coulomb, viz. that for the same material the resistance to 
rolling is proportional to the weight of the roller, and inversely proportional to its 
diameter. In addition to these laws, however, it appears clear to me that there must 
be another law connecting rolling-friction in some way with the softness of the tires of 
the wheels and the road. In addition to the instance of india-rubber tires already 
mentioned, there are several other phenomena connected with wheels which point to 
such a law, and can be explained by the recognition of the slipping under the roller. 
Steel and Iron Bails. 
The very great advantage in point of durability of steel rails over iron has been a 
matter of much surprise, it not being sufficiently accounted for by the greater hardness 
of the steel, supposing it to be subjected to the same wearing action as the iron. This 
is at once explained, however, by the recognition of the fact that hardness tends to 
reduce the slipping and hence the wearing action, as well as to enable the rail the 
better to withstand the wear to which it is subjected. 
That rails should wear at all in places where they are straight and where brakes are 
not applied is a matter which calls for an explanation, and this, so far as I am aware, 
has not hitherto been given ; mere crushing, however much it might deform the rail, 
would not cause such a reduction of weight as actually takes place. The explanation 
of this phenomenon also at once follows the recognition of the slipping which attends 
rolling. 
A little consideration also serves to show that the scaling of wrought-iron rails is the 
result of the repeated lateral extension of the surface of the rail under the action of the 
wheel. The systematic way in which this takes place shows that it is due to something 
more than the mere imperfection in the iron. There is no doubt that the grain of the 
iron has a great deal to do with it ; but considering the multitudinous ways in which 
iron is used and that this is the only one in which scaling takes place, it is clear that it 
must be due to some cause directly connected with the action to which the rail is 
subjected. Now every time a wheel passes over a point in a rail it tends to slide the 
upper strata of the rail over those beneath them, and thus causes tangential stress. If 
the rail were homogeneous this would hardly cause it to scale ; but owing to the grain in 
the iron some strata are stronger than others, and the weaker strata are called upon to 
do more than their share of the yielding, and so become still weaker and eventually 
give way. 
There are other phenomena which, having been hitherto unnoticed or unexplained, 
might be shown to arise from the slipping which takes place during rolling ; but perhaps 
those I have mentioned are sufficient to show that the effects of the action are not 
altogether without' practical importance. 
