184 A. S. Kimball— Siding Friction on an Inclined Plane. 



the velocity increases, rapidly at first but more slowly after- 

 ward. 2d. With the same velocity, the coefficient of friction 

 is greater the greater the inclination of the plane, within the 

 limits of the experiments. Sd. The coefficient of friction in 

 each experiment tends toward a constant quantity. 4th. This 

 constant seems to be the same in each experiment. 



No simple expression which will show the variations in the 

 coefficient of friction has yet been found ; indeed, I have not 

 thought best to attempt to formulate the work till certain 

 errors, which will be referred to, have been corrected. It was 

 found impossible to procure a plank with a perfectly uniform 

 surface. The one used in the experiments given showed at 

 the same inclination and velocity a coefficient which slightly 

 but regularly increased from one end to the other. The end 

 which gave the lower coefficient was placed uppermost. ^ The 

 obvious result of this was to make the coefficients in Table B 

 at high velocities greater than they otherwise would have been. 

 This fac.t also explains the apparent anomaly in columns 3 and 

 4 of the same table, where the coefficients at high velocities 

 are seen to fall below the corresponding coefficients iu col- 



In experiment 4 the slide had the velocity 120 at a distance 

 of 40 inches from the upper end of the plane ; in experiment 2 

 it did not acquire that velocity until it had passed over a dis- 

 tance of 60 inches, and consequently was on a rougher portion 

 of the plane. The uniformity of the plane was tested by start- 

 ing the slide at different points along its length, and comparing 

 the curves on the smoked glass. These experiments have not 

 been corrected for the resistance of the atmosphere. The 

 effect of such a correction would be to diminish still more the 

 coefficients at high velocities. 



As the inclination of the plane increases the normal pressure 

 decreases. Thinking that this change of pressure might explain 

 a pan of the difference due to a change of inclinations, we 

 made three experiments at the same inclination, with weights 

 of 18, 80 and 140 lbs., in the box. At the end of one second 

 we found the velocities in the three cases to be as 1, I'lS and 

 1"32, showing a less resistance in the case of the greater load, 

 and corresponding to a decrease of about 2^ per cent in the 

 coefficient of friction. This seems to be insufficient to explain 

 the change in the coefficient when the inclination of the plane 

 is changed. But it is interesting as showing that in the case 

 of pine on pine friction is not strictly proportional to the nor- 

 mal pressure. 



As soon as possible we propose to repeat these experiments, 

 extending the range of velocities, also to try the effect of a 

 change of pressure, with a view to formulate deviations from 



