94 
The last point.tested was the effect of the mass of the impinging block, those 
so far used weighing about one kilogramme, or two and a fifth pounds. The mass 
was now doubled and the radius of curvature being 7 cm., as in the earlier tests, 
a value of .36 was obtained. It was now found that with the height of 
drop 120 cm., or 4 ft., the value of e remained the same as before, namely, .36, 
but that for a drop of 6 ft., the value decreased to .383, and with a much heavier 
mass the value sank to.25. Attention might be here called to the fact that there 
might seem to be some contradiction between the statements that while greater 
height and therefore greater force of impact with the light block on fresh parts of. 
the base plate did not affect the value of e, an increase of the impinging mass 
showed a decrease in the value of e with increasing force of impact, while suc- 
cessive impacts on the same spot showed an increasing value of e. The explana- 
tion would seem to be that whereas beyond a certain limit, increasing force of 
impact produced an initial crushing of the surface from which there is only im- 
perfect recovery, when successive impacts take place on the same spot, at each 
impact there is a greater area of resisting surface encountered, and also less unre- 
stored crushing, and hence a greater velocity of rebound with a consequent 
increase of e. The point seems to merit a more careful investigation than it 
received in these purely preliminary experiments. 
These tests point out the limits as regard mass of impinging block and height 
of fall within which it is safe to work in comparing the coefficients of restitution 
of different materials. It also shows that for a comparison of different materials 
it is necessary to adopt the same curvature of impinging surface and to use a fresh 
part of the base ptate for each impact. Hence in the following comparison the 
masses of the impinging blocks are uniformly one kilogramme, radius of curva- 
ture of the impinging surface 10 em. and the height of falk 120 cm. 
TABLE OF COMPARISON. 
Material. Value of e. Duration of Contact. 
Whitepile vies enc MBit Fitba eae Ges .0017 seconds. 
Wihite poplars i..95%. cht ADEE tices h sod sud ee ae .0016 os 
CON CRNY: 5 coupcis ore nite ce rh oe A eaten 3 th Beet ee .0016 ys 
ierd! maple ie te ree Be Sed Fps eres Be Bee .0012 is 
In the above it will be understood that in all cases the grain of the impinging 
block was vertical and that of the base plate horizontal. The first remark that 
may be made about these results is the surprising closeness of the values of e for 
the different materials, varying between .38 and .45, while the duration of con- 
tact varied between .0012 sec. and .0017 sec. It may also be noted that the values 
of e varied in the inverse order to the duration of contact. 
