BETWEEN SUEFACES MOVING AT LOW SPEEDS. 
519 
them with solution of caustic potash ; while in the case of wood the friction was first 
determined when the surfaces were dry ; then they were wetted with water, and the 
friction again measured ; and, lastly, when the water had dried off, oil was applied. In 
every case the same specimens were used in all three determinations. When the bear- 
ings were of wood the fibres were vertical, and the coefficient of friction was approxi- 
mately that corresponding to motion in a plane normal to the fibres. The following is 
a summary of the results : — 
I. Steel on Steel. Dry. 
1. Friction uniform from velocity 0*0002 foot per second to 
greatest velocity observed, 0*0057 foot per second . . ^=0*337 
2. Friction uniform from velocity 0*0002 foot per second to 
greatest velocity observed, 0*0089 foot per second . . 0*350 
3. Friction uniform from velocity 0*0002 foot per second to 
greatest velocity observed, 0*0086 foot per second . . ^=0*365 
Mean ^=0*351 
In this series, although the friction remained sensibly uniform during the time each 
experiment lasted, there was a progressive increase of the coefficient, probably due either 
to a tearing of the surface or to chemical action. The time that elapsed between the 
successive experiments was great relatively to the time taken up by any one of them, 
and a good deal of motion took place between the surfaces in each interval. We have 
observed several cases in which there is a similar progressive increase, but none in which 
there is a progressive decrease in the value of y. 
II. Steel on Steel. Oiled. 
In this case the friction appeared to be somewhat less at the lower than at the higher 
velocities. This result is quite anomalous ; we have observed no other instance in which 
the same thing occurs. The change, however, in the value of [h is not great, and it is 
possible that this result may be due to error of observation ; we therefore state it with 
caution, although the different observations show a remarkably close agreement amongst 
themselves. 
1. For velocity 0 0002 foot per second ^=0*119 
For velocity 0*0046 foot per second ^=0*130 
Here 0*0046 foot per second is the greatest velocity observed. 
2. For velocity 0*0002 foot per second ^=0*116 
For velocity 0*0060 foot per second ^=0*130 
From this to the greatest velocity, 0*0065 foot per second, ^ remains sensibly 
constant. 
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