502 Scientific Intelligence. 



gave some very striking figures and described some highly inter- 

 esting phenomena associated with blows produced in various 

 ways. An account of only a few typical cases may be here pre- 

 sented. 



Suppose that each of two equal billiard balls has a speed of 

 eight feet per second and that they are moving towards each 

 other along their line of centers. At the very instant of touch- 

 ing there is, of course, no pressure between the balls, but as the 

 centers continue to approach, each sphere becomes flattened at 

 the region of contact. This region is circular and it rapidly 

 increases in area until the balls as wholes are brought to rest, that 

 is, until the work done against the elastic forces of restitution is 

 equal to the original kinetic energy of the system. For the case 

 in question the distance of approach is 14/1000 of an inch' and 

 the force equals 1,300 lbs. The circle of contact has a diameter 

 of one-sixth of an inch, so that the average pressure amounts to 

 27 tons per square inch. The distribution of pressure, however, 

 is not uniform, the pressure at the center of the areas of contact 

 being 1 1 / 2 times as great as the average pressure. The subse- 

 quent behavior of the spheres is of no interest in this connec- 

 tion. If very hard, hollow steel balls, having the same mass as 

 the ivory spheres, are caused to collide with a relative speed of 

 16 feet per second, the distance of approach will be less, the area 

 of contact smaller, and the maximum pressure much greater than 

 for the billiard balls. This pressure when averaged over the 

 circle of contact attains a value of 280 tons per square inch. 

 These results of theoretical computation for steel balls have been 

 verified by comparing the calculated time of contact with the 

 interval obtained experimentally by the aid of appropriate elec- 

 trical apparatus.! The time of contact for the ivory spheres, men- 

 tioned above, was 1/4000 of a second. 



A case involving an inelastic substance is afforded by the 

 impact of an elongated lead rifle bullet against a hard steel plate. 

 Under the enormous pressures developed lead flows very freely, 

 so that, in the absence of any lateral support, each cross-sectional 

 disc of the bullet maintains its speed practically unchanged until 

 it comes in direct contact with the steel. The pressure exerted 

 by the bullet is, probably, sensibly constant, since it depends 

 upon the square of the speed, but not upon the length or diam- 

 eter of the projectile. Increase in diameter only alters the area 

 over which the force is applied, and increase in length the time 

 during which it acts. As a practical example, consider a Lee- 

 Metford bullet moving with the normal speed of 1,800 feet per 

 second. This projectile is 1 1 /4 inches in length, it has a mass 

 of about 0*03 lb., and it would be stopped in 1 / 18000 of a second. 

 The force required to destroy the 1*7 lb. -second units of momentum 

 would be 15 tons. Since the area of cross-section of the bullet 

 is 1/14 of a square inch, the mean pressure would amount to 210 

 tons per square inch. 



Passing over several interesting cases involving the propaga- 



