362 Sir Robert Robertson [May 6, 



It is important also to know the violence of the various explosives 

 used, both by themselves and also when assembled in the various 

 components, and it was in this connection that the principle of the 

 pressure bar, enunciated by the late Prof. Bertram Hopkinson in a 

 discourse to the Royal Institution in January of 1912,* was of the 

 greatest value. This depends on the experimental resolution of the 

 momentum of the blow into pressure and time. When a charge is 

 fired against the end of a cylindrical steel bar ballistically suspended, 

 a wave of compression travels along the bar and is reflected at the 

 far end as a wave of tension. To investigate the properties of the 

 wave, a short length of the end of the bar farthest from the end to 

 which the blow is delivered is cut off and the faces are surfaced, the 

 short piece (known as the time-piece) being caused to adhere closely 

 to the bar, usually by a film of vaseline. The compression wave 

 travels unchanged through the joint into the time-piece, but the 

 reflected tension cannot pass through it. Hence when the amplitude 

 of the reflected tension wave reaching the joint becomes greater than 

 that of the oncoming compression wave, the time-piece is projected 

 from the shaft with a momentum which depends on the pressure 

 exerted by the explosive and the time taken by the wave to traverse 

 the length of the time-piece. This momentum is measured by catch- 

 ing the time-piece in a ballistic pendulum, and, the velocity of the pro- 

 pagation of the wave through steel being known, the mean pressure 

 exerted during an extremely small time interval can be calculated. 



[One of the instruments for determining the pressure developed 

 by a detonator was shown, and a detonator fired, the mark drawn by 

 the swing of the pendulum which caught the time-piece being shown 

 on the screen.] 



The application of this apparatus not only gave important in- 

 formation as to the limitirig quantity of fulminate necessary to 

 bring about complete detonation of the tetryl and as to the effect 

 of the thickness of the wall of the gaine, but it also emphasized the 

 necessity for avoiding gaps in the train of detonation on account of 

 the very rapid falling off in violence of the blow when even a small 

 air-gap is introduced. 



Main Filling. — It was early recognised that the supply of picric 

 acid and T.N.T. by itself would be quite insufficient. It was at this 

 point that the late Lord Moulton took steps to secure supplies of 

 essential explosives and their ingredients, with such success that the 

 supply of explosives shortly came to be ahead of the demand. 

 But even when a method for the production of T.N.T. had been 

 worked out, and its supply on a fairly large scale was in prospect, it 

 was apparent that the demand for high explosives was such that it 

 could not be met by the supplies of nitro-compounds in sight. 



Experiments were then made to test the capabilities of mixtures 

 of ammonium nitrate and trinitrotoluene for shell filling, and these 



* Proc. Royal Inst., Vol. XX. p. 275. 



