1912] 



on the Pressure of a Blow. 



2Ti 



compressed springs, their original energy of motion having been 

 completely transformed into strain energy in their substance. The 

 reason of the high intensity of pressure developed is that this strain 

 energy is concentrated into a very small volume of ivory near to the 

 point of contact. The balls then begin to separate, and the whole 

 process of compression is gone through in reverse order, the strain 

 energy being transformed back into energy of motion by the pressure. 

 Finally the balls rebound unstrained, with nearly the velocity with 

 which they approached. 



Distance of Afiftroaxh, ZhousandZhs of an, inch. 

 Fig. 1. 



If for the ivory balls we substituted hollow balls of steel having 

 the same mass, the pressure produced by the blow would be greater, 

 because the steel is much more rigid than ivory and gives less under 

 a given force. Thus the distance of approach is less, the circle of 

 contact smaller, and the maximum intensity of pressure much greater. 

 It reaches 280 tons per square inch averaged over the surface of con- 

 tact. Such a pressure could only be sustained without permanent 

 effect by a very hard steel. Ordinary mild steel would begin to flow 

 when the pressure passed about 100 tons, a permanent flat would be 

 left by the blow, and the balls would rebound with less velocity than 

 that of approach. The theory whose results I have given does not, 



u 2 



