172 
PROFESSOR OSBORNE REYNOLDS ON ROLLING-FRICTION. 
Experiments on Actual Slipping. 
My object in the second series of experiments was to find by actual measurement how 
far the roller rolled short of its geometrical distance. Since the exceedingly small 
slipping on a hard surface precluded all chance of measuring it, these experiments were 
made on strips of india-rubber glued to wood : these were in general long enough to 
allow of two complete revolutions of the roller. The strips were of different thicknesses. 
This difference of thickness has an effect to vary the degree of indentation and the 
intensity of the pressure, as well as the lateral extension. On the thick india-rubber the 
indentation was considerable ; and, owing to the large bearing-surface thus obtained, the 
intensity of the pressure beneath the roller must have been comparatively small, as must 
also the lateral extension ; whereas with the thin strips the indentation was small, 
but the pressure and consequent lateral extension must have been correspondingly 
great. These considerations serve to explain the differences in the results of the 
experiments, which are given in Tables VII. and VIII. 
Table VII. — Showing the Actual Slipping of a Cast-iron Roller. 
The nature of the Surface. 
The distance travelled. 
The amount 
of 
the slipping. 
In one 
revolution. 
In two 
revolutions. 
A steel bar (polished) 
17-82 
35-64 
•00 
India-rubber, 0-015 inch thick, glued to wood . . 
35-2 
•44 
Ditto, 0-08 inch thick 
34-8 
•84 
Ditto, 0-36 inch thick 
35-15 
•49 
Table VIII. — Showing the Actual Slipping with an India-rubber Tire 0-75 inch thick 
glued on to the Roller. 
The nature of the Surface. 
Distance tra- 
velled in one 
revolution. 
Circumference 
of the 
ring. 
The amount 
of 
the slipping. 
A steel bar 
22-55 
22*5 
-0-05 
India-rubber 0-156 inch thick (clean) 
22-55 
-0-05 
„ ,, (blackleaded) . : 
22-55 
-0-05 
„ 0-08 inch thick (clean) 
22-5 
0-0 
„ „ (blackleaded) .... 
22-52 
-0-02 
„ 0-36 inch thick (clean) 
22-39 
99 
+ 0-11 
„ „ (blackleaded) .... 
22-42 
99 
+ 0-08 
„ 0-75 inch thick (clean) 
22-4 
+ 0-1 
„ „ (blackleaded) .... 
22-4 
” 
+ 0-1 
These experiments show that a hard roller on a soft surface rolls short of its geome- 
trical distance, whereas a soft roller on a hard plane rolls more than its geometrical 
distance, but to a smaller degree, and that when the roller and the plane are of equal 
hardness the roller rolls through less than its geometrical distance, which results are in 
exact accordance with what has previously been explained. 
