PROFESSOR OSBORNE REYNOLDS ON ROLLING-FRICTION. 
173 
The effect of Heat and Viscosity to cause Friction. 
While making the experiments which have been described, two other causes of 
resistance to rolling besides friction suggested themselves to me, and were to a certain 
extent verified. The first of these is the transference of heat which takes place within 
both the plate and the roller in the neighbourhood of the point of contact. As the 
roller moves forward it is continually compressing the material in front of the point of 
greatest pressure, and this material expands again so soon as the roller is past. During 
compression there will be a change in the temperature of the material compressed, 
which change will be readjusted again as the material expands, supposing that in the 
interval between compression and expansion there has been no heat communicated to or 
taken from the portion of material affected. But since the change of temperature 
caused by compression will place the part compressed out of accord with that imme- 
diately surrounding it, a transference of heat will necessarily take place. The quantity 
of heat thus transferred will depend on the length of the interval, i. e. the speed of the 
roller, and on the conducting-power of the material. 
This transference will cause resistance to the roller, for the material will not expand 
to the same temperature, and hence to the same volume, as that from which it was 
compressed, and hence it will take more work to compress it than it will give out in 
expanding. 
It does not, however, follow that the greater the transference of heat the greater the 
resistance ; for if a sufficient time be allowed the transference of heat will readjust the 
temperature as fast as expansion takes place. There is some speed, therefore, for which 
the resistance arising from this cause will be a maximum. If, therefore, the material 
be a good conductor and the motion slow, .the transference of heat will prevent any 
variation of temperature during either compression or expansion. When such was the 
case the resistance would increase with the speed, a fact which was very evident when 
the rolling took place on india-rubber ; for it was possible to give the plane such an 
inclination that the motion of the roller was scarcely perceptible, and any increase in 
the inclination was followed by a corresponding increase in the speed of the roller. 
As already stated, there is another cause of resistance ; and this may partly explain the 
result : this is viscosity. 
If we stretch a piece of india-rubber, or any material, when released it does not 
immediately come back to its original length, but at once comes back a certain distance 
and then recovers the rest more or less slowly. Hence as the roller moves forward the 
compressed material will require time for its complete expansion, and hence will offer 
less resistance to the roller when the motion is slow than when it is rapid. 
Conclusion . 
The foregoing remarks must be regarded as relating only to the nature of rolling- 
friction. I have not attempted to ascertain the laws which connect its magnitude with 
the various circumstances which affect it. As far as they go I can see no reason to doubt 
