BETWEEN SURFACES MOVING AT LOW SPEEDS. 523 
Mean value of p for velocity of 0-0002 foot per second . . . 0-124 
Mean value of [l for higher limit of velocity 0*096 
This change in the value of [h seems to be far greater than experimental errors can 
account for. The three (independent) observations given above agree closely both as 
regards the initial and final values of [h. 
The specimens of greenheart which were used in the above experiments were new, 
that is, their surfaces were freshly cut at the time these experiments were made. They 
were then laid aside without having the oil which had been put on them removed, and 
after an interval of six months the observation was repeated, fresh oil being applied. 
Two determinations were made, giving results which agreed closely in all respects. 
These results were 
For velocity 0-0002 foot per second ^=0-077 
For velocity 0'004 foot per second ^,=0-062 
Greatest velocity observed 0-0055 foot per second. 
Here it is noticeable that although the prolonged exposure to oil had altered the 
bearing-surface of the wood so as greatly to diminish the coefficient of friction, the 
effect of change of velocity was the same as before. From velocity 0-004 foot per 
second upwards the coefficient underwent little or no appreciable change. 
XII. Steel on Greenheart. Wet with water. 
In this case there was an equally marked increase of friction at low speeds. 
1. For velocity 0-0002 foot per second 0-290 
For velocity 0-005 foot per second ^=0-232 
Greatest velocity observed 0-0066 foot per second. 
2. For velocity 0-0002 foot per second |t/,=0-292 
For velocity 0-005 foot per second ^=0-243 
Greatest velocity observed 0-0056 foot per second. 
3. For velocity 0-0002 foot per second ^=0*290 
For velocity 0-006 foot per second |U.=0-237 
Mean value of [h for velocity 0-0002 foot per second . . . 0'291 
Mean value of p for higher limit of velocity 0*237 
The diminution of p as the velocity increases amounts to about twenty per cent, of 
its value at the lower limit. This proportion is almost identical with that observed in 
the case of steel rubbing on oiled greenheart. 
In both of the foregoing cases (XI. & XII.) in which this marked change took place 
in the value of [*, the experiments were at first made under the following conditions : — 
The oil (or water) was liberally supplied to the bearings just before the disk was caused 
to revolve, and the supply was not renewed during the revolution of the disk. It was 
suspected that the increase of p might be due to the absorption of the unguent by the 
