282 Professor Bertram HojiTtinson [Jan, 26, 



illustrates a point of importance in regard to the general effect of 

 blows. Instead of maintaining the pressure during the whole 

 passage of the wave up and down, as in the end-on impact, a pres- 

 sure is suddenly applied to one end, maintained for a short time, 

 and then removed. A corresponding pressure wave travels along 

 the rod. Each portion of the rod is only stressed or in motion 

 during the passage of the wave over it, and after the passage of the 

 wave it is left with a certain forward displacement, but without any 

 velocity or stress. Furthermore, the whole momentum of the blow 

 is concentrated in the short length of the rod covered by the wave. 

 On its arrival at the other end the wave is reflected, but the reflected 

 wave is a wave of tension. As it comes back the head of the tension 

 wave is at first wholly or partially neutralised by the tail of the 

 pressure wave, but after a time it clears this, and the rod is then put 

 into tension of amount equal to the original pressure. If there be 

 a crack or weak place in the rod at a sufficient distance from the 

 free end, the pressure wave will pass over it practically unchanged ; 

 but on the arrival of the reflected 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 unstrained (Fig. 3). 



[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. 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 con- 

 verted into gas at a very high temperature. The time required is 

 probably only 3 or 4 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 expanded, 

 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 impHes 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 sur- 

 rounding air, and the pressure accordingly drops with very great 

 rapidity. It is probable that the pressure is practically gone after 

 BTTauTj- o^ ^ second. The same pressure is, of course, exerted by the 

 gas upon any surface with which the gun-cotton is in contact, and it 



