534 



NATURE 



[February 15, 1912 



tension wave the rod will part, because the crack cannot 

 sustain the tension, and the forward part will move on, 

 having trapped within it the whole momentum of the 

 blow. The rest of the rod will remain at rest and un- 

 strained. 



(The propagation of waves in rods was illustrated by 

 means of a model, consisting of horizontal wooden bars 

 fixed at equal intervals to a vertical wire.) 



The fact that a blow involving only pressure may, by 

 the effects of wave action and reflection, give rise to 

 tensions equal to or greater than the pressure applied, 

 often produces curious effects which may be illustrated in 

 many ways. I shall choose by way of illustration some 

 observations which I have been making recently, and 

 which I think are new. I have here a small cylinder of 

 gun-cotton. By the use of a small quantity of fulminate 

 in the hole provided for the purpose it is possible to 

 detonate the gun-cotton, which means that in an excessively 

 short time it is converted into gas at a very high tempera- 

 ture. The time required is probably only three or four 

 millionths of a second, and is so excessively short that the 

 gas does not during the process expand appreciably into 

 the surrounding atmosphere. 



Thus the gas generated, which, when completely ex- 

 panded, will fill a space several thousand times as great, 

 is for a minute fraction of time confined within the volume 

 of this small fragment of gun-cotton. This confinement 

 implies great pressure — how much is at present a matter 

 of doubt. I understand that Sir Andrew Noble estimates 

 it at 120 tons per square inch. The only thing which 

 restrains the expansion of the gas is the inertia of the 

 surrounding air, and the pressure accordingly drops with 

 very great rapidity. It is probable that the pressure is 

 practically gone after 1/25,000 of a second. The same 

 pressure is, of course, exerted by the gas upon any surface 

 with which gun-cotton is in contact, and it will be seen 

 that the force so produced has the characteristics of a 

 blow, namely, great intensity and short duration. If such 

 a cylinder of gun-cotton weighing one or two ounces be 

 placed in contact with a mild steel plate, the effect, if the 

 plate be half an inch thick or less, will be simply to 

 punch out a hole of approximately the same diameter as 

 the gun-cotton, just as though it had been struck by a 

 projectile of that diameter. But if the plate be three- 

 quarters of an inch thick, the curious result which I 

 exhibit here is obtained. Instead of a complete hole being 

 made, a depression is formed on the gun-cotton side of 

 the plate, while on the other a scab of metal of corre- 

 sponding diameter is torn off and projected away with a 

 velocity sufficient to enable it to penetrate a thick wooden 

 plank, or to kill anyone who stands in its path. The 

 velocity, in fact, corresponds to a large fraction of the 

 whole momentum of the blow. The scab behaves much in 

 the same way as the piece which we saw would be shot 

 off the end of a rod struck at the other end if the rod 

 were divided or weakened, so as to be unable to sustain 

 the reflected tension wave. The separation of the metal 

 implies, of course, a very large tension, which can only 

 result from some kind of reflection of the original applied 

 pressure ; but the high velocity shows that this tension 

 must have been preceded by pressure over the same 

 surface, acting for a time sufficient to give its momentum 

 to the scab. 



Wishing to ascertain how and where the separation 

 originates, I caused a two-ounce cylinder of gun-cotton to 

 be detonated in contact with a somewhat thicker plate. 

 In this case no separation of metal was visible, the only 

 apparent effects being a dint on one side and a correspond- 

 ing bulge on the other. On sawing the plate in half, how- 

 ever, I was gratified to find an internal crack, obviously 

 the beginning of that separation which in the thinner plate 

 was completed. 



The pressure exerted by the gun-cotton in the experi- 

 ments which I have just described is practically confined 

 to the circular area of contact between it and the metal, 

 ■ as is shown by the accurate agreement of the print on the 

 plate with that circle. The effects of that pressure must, 

 however, be largely conditioned by the fact that the metal 

 upon which it acts is attached to the surrounding portions 

 of the plate, and is by them held back. In order to get 

 an idea of the effect of this factor, I have tried the experi- 

 ment of removing this outside metal, leaving the steel 

 NO. 2207, VOL. 88] 



cylinder opposed to the gun-cotton. If such a short 

 cylinder of steel be placed in contact with a gun-cottoa 

 cylinder of equal diameter, the result of detonation was at 

 first sight merely to flatten it out slightly, and to produce 

 a depression on one side with something of a bulge on the 

 other. No external crack was visible. But on sawing the 

 piece in half a remarkable system of cracks was disclosed ; 

 the cracks spread in all directions, as though tension ha<t 

 been acting in every direction ; in fact, it appeared a» 

 though the steel cylinder had begun to burst. The tension 

 necessary to produce these cracks, which, as you will see, 

 must have radial as well as axial components, must 

 originate in some kind of wave action which follows the 

 blow. The problem is very complicated, and I have not 

 yet succeeded in finding a full explanation of the pheno- 

 menon ; but there cannot be much doubt that the longi- 

 tudinal tensions are due to a wave generally similar to 

 that which we have been discussing in connection with the 

 rod. To account for the radial tensions which the cracks 

 show also to have 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 accom- 

 panied by a corresponding increase in diameter. This 

 increase takes place very rapidly, and implies that at first 

 the metal is moving out in a radial direction with a high 

 velocity. The stoppage of this radial motion requires 

 radial tension, and this probably is greater at points near 

 the axis, for much 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 

 amplitude increases as it goes in. 



I have recently been attempting to measure the duration 

 of the pressures produced by the detonation of gun-cotton. 

 The method depends on the reflect-on of a tension-wave 

 at the free end of the rod. A wave of compression 

 travels along the rod, the length of the wave corre- 

 sponding 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 1/20,000 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 surfaces of the junc- 

 tion 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 momentum. In this case, 

 therefore, only a portion of the whole momentum 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. By trying a series of 

 experiments with end-pieces of different lengths, the rate at 

 which the pressure disappears can be determined. In this 

 way I have shown that the pressure developed by the 

 detonation of o-i ounce of gun-cotton is practically all gone 

 in 1/30,000 of a second. 



I have on the table some specimens to show the effects 

 of detonating larger quantities of gun-cotton. Here is a 

 steel plate which has been broken by firing a charge of 

 about I lb. in contact with it. It is interesting to note 

 the character the fracture produced. This plate is a good 

 qualit>' of mild steel, such as is used for msdcing boilers. 

 ' It would be possible by a steadily applied pressure to bend 



