1912] 071 the Fre!<sure of a Blow. 285 



been present, it is to be observed that the shortening of the cylinder 

 in the direction of its axis, which is the immediate effect of the blow, 

 must be accompanied by a corresponding increase in diameter. This 

 increase takes place very rapidly, and implies that at tii-st the metal 

 is moving out in a radial direction with a high velocity. The stop- 

 page of this radial motion requires radial tension, and probably this 

 is greater at points near the axis for nmch the same reason that when 

 a stone is dropped into a pond the circular waves which it causes 

 have their greatest amplitude at points near the centre of disturbance. 

 In the case of the steel cylinder the radial tension wave travels 

 inwards from the surface, and its ampUtude increases as its goes in. 



I have recently been attempting to measure the duration of the 

 pressnres produced by the detonation of gun-cotton. The principle 

 of the metliod may be made clear from the diagram sho^ving the 

 effect of a blow on one end of a rod (Fig. 3). A wave of compression 

 travels along the rod, the length of the wave corresponding to the 

 time during which the pressure has acted ; that is, it is equal to the 

 velocity of sound, multiplied by that time. We may assume that the 

 time was -^^^qq of a second, which would give a wave just 10 inches 

 long. This wave travels to the end of the rod, is there reflected as a 

 wave of tension, and comes back. If the rod be cut across, the sur- 

 faces of the junction being accurately faced and in firm contact, the 

 pressure wave will pass the joint without change, but on the arrival 

 of the head of the tension wave at the joint, the parts will separate 

 and the end piece will fly off. If the tail of the pressure wave has 

 then cleared the joint, the separated end-piece will have trapped within 

 it the whole momentum of the blow, and the part left behind will 

 remain at rest and unstrained. In the case supposed things will 

 happen in this way if the end-piece is more than 5 inches long. If 

 it be less than 5 inches long, say 4 inches, there will, on the arrival 

 of the reflected wave at the joint, be still 2 inches of pressure wave 

 in the other part of the rod, and the corresponding quantity of mo- 

 mentum. In this case, therefore, only a portion of the whole momen- 

 tum is trapped in the piece, the balance being left in the other part 

 of the rod, which moves forward with the corresponding velocity. In 

 order to discover how long the pressure lasts it is only necessary to 

 try a series of experiments with the joint at different distances from 

 the free end. It will be found that if that distance exceeds a certain 

 amount, the rod which was originally struck remains at rest, the whole 

 momentum being transferred to the free end-piece. If the distance 

 be less, only a fraction of the momentum is so transferred, and the 

 balance remains in the struck rod, which accordingly moves forward. 



I have applied this method to investigating the curve of pressure 

 developed by the detonation of gun-cotton. I will not weary you 

 with the details, but the results, so far as I have obtained them, are 

 shown in the curve (Fig. 7). This exhibits the pressure produced by 

 the detonation of one ounce of dry gun-cotton on the end of a steel 



