INTONATION OF HIGH EXPLOSIVES OK BY THE IMPACT OF BULLETS. 439 



section B the pressure of the direct wave (which is the moment shown in the 

 figure) the rod, being unable to withstand tension at the joint, parts there and 

 the end flies off'. The end piece has then acquired the quantity of mmm-ntum 

 represented by the shaded area in the figure, equal to the time-integral <>f th- 

 pressure curve from to B, less that of the tension wave during the time for whirl, 

 it has been acting, that is from O' to B. The piece flies off with this umunt ..f 

 momentum trapped, so to speak, within it. If it be caught in a ballistic pendulum 

 and its momentum thus measured we have the time integral of the pressure CIIIA.- 

 between the points B and B' on the pressure-time curve which are such that they 

 correspond to equal pressures on the rising and falling parts of the curve, while th-> 

 time-interval between them is equal to that required for a wave to travel twice the 

 length of the end piece. By taking end pieces of different lengths and measuring tl it- 

 momentum so trapped in each the area of the pressure-time curve over corresponding 

 intervals can be obtained. In general the precise form of the curve itself cannot he 

 deduced tecause the points of commencement of the several intervals are not known. 

 Thus a given set of observations would be consistent with 

 any one of the three forms shown in fig. 3 which can be 

 derived from one another by shearing parallel to the base 

 so that the intercept of any line such as AA' is the same 

 i>n all. But the maximum pressure and the total duration 

 of the impact can always be obtained, and these are the 

 most important elements. The maximum pressure is the limiting value of the 

 average acting on a piece when the piece is very short, and the duration corresponds 

 to twice that length of piece which just catches the whole of the momentum leaving 

 the rod at rest. If the circumstances of the impact are such that the pi-emu re is 

 known to rise or to fall with great suddenness, the curve assumes the form I. or III. 

 and its form may l)e determined completely from the observations. 



This is the basis of the method described in the present paper. A cylindrical rod 

 or shaft of steel is hung up horizontally by four equal threads so that it can swing in 

 a vertical plane remaining parallel to itself. A short piece of rod of the same diameter 

 is l.uttc.l up against one end being held on by magnetic attraction but otherwise free. 

 A rifle bullet is fired at, or gun-cotton is detonated near, the other end-; the short 

 piece flies off and is caught in a box suspended in a similar manner to the long rod. 

 Suitable recording arrangements register the movement both of the long rod and of 

 the box, and the momentum in each is calculated in the usual way as for a Iwllistic 

 pendulum. Sufficient magnetic force to hold the end-piece in position is provided by 

 putting a solenoid round the rod in the neighbourhood of the joint. The slight force 

 required to separate the piece from the rod under these conditions may lie neglected 

 in comparison with the pressures and tensions set up, since these amount to several 

 tons on the square inch, and, practically speaking, the joint will transmit the pressure 

 wave unchanged but will sustain no tension. 



