280 Professor Bertram HojjJcinson [Jan. 26, 



velocity (varying as its sqnare), and not upon the length or diameter 

 of the "bullet. Increase in diameter only alters the area over which 

 the pressure is applied, and increase in length the time during which 

 it is applied. 



If for the bullet of lead we substitute one of hardened steel which 

 will not flow, the problem at once becomes much more complicated. 

 In order to reduce it to its simplest terms, and to bring the theory 

 into such a form that it can be tested in the laboratory, we may sup- 

 pose that instead of the bullet we have a cylindrical steel rod, say 

 I inch in diameter by 10 inches long, with flat ends, and that it strikes 

 quite fair against an absolutely unyielding surface. The latter con- 

 dition could not be fulfilled in practice, because there is no suljstance 

 more rigid than steel. So far as the effects on the rod are concerned, 

 however, it can be fulfilled by making two rods, moving with equal 

 velocities in opposite directions, collide end on ; and this device has 



IMPACT OF STEEL RODS. 



• — to' r i ' ffuncired. VwuiancUhs. 



of a second. 



Porhons of rod -in cotnfwession and. at rest shcuoied 

 RAaZu/e veLocdi/ at i/m/iact 34 feet • 400 tnchei fier second,. 

 Pressure, about fd tons /i^r scfuare it)/Jv 



Fig. 2. 



been used in the laboratory for imitating the effect of impact against 

 an unyielding surface. AVe have to consider how long it takes to stop 

 the rod under such conditions. When the end first strikes it is 

 pulled up dead, just as in the case of the lead bullet, only it does not 

 now flow out sideways. The pressure, however, set up at the end 

 of the rod cannot be instantaneously transmitted through it, and 

 consequently the hind parts do not at once feel this pressure, but 

 continue to move on as before. The pressure travels with the 

 velocity of sound, which for steel is about 17,000 feet per second, 

 and it takes accordingly ^o Jo'o P'*'^'*^ ^^ ''' second before the pressure 

 has been transmitted throughout the 10 inches length of the rod. 

 The course of the impact is shown in the diagram (Fig. 2). A wave 

 of pressure is initiated at the first contact and travels along the 

 rod. At any instant the part of the rod which has already been 



