Sound- Shad oils in Water. 109 



correspondingly small. Hence, when the time of the blow or 

 explosive impulse is exceedingly brief, the wave-length must 

 be proportionately short. 



17. Application to Sound-Shadows. — In the experiments of 

 Colladon, if we assume that the brief shock of the hammer on 

 a limited portion of the bell was alone transmitted to the dis- 

 tant observer, it is clear that only the short sound-waves thus 

 generated would reach the distant obstructing wall or screen 

 in the water: and consequently, the greater definiteness of 

 the acoustical shadows in water as compared with those in 

 air would be the necessary result of the greater shortness of 

 the sound-waves in water. Under the foregoing assumptions, 

 the theory of undulations appears to afford a satisfactory ex- 

 planation of the phenomenon observed by Colladon. Never- 

 theless it would have been extremely interesting and in- 

 structive, as a very severe test: of theory, had this distinguished 

 physicist made observations on the relative distinctness of the 

 sound-shadows in the water within the musical range of the 

 submerged bell, as compared with those observed at points 

 so remote that only the sharp blow of the hammer was audible 

 in the water. 



In like manner, the application of the principle of briefness 

 of elastic-wave-genesis to the explanation of the phenomena 

 observed by my son in his i; Dynamite " experiments, is suffi- 

 ciently obvious. In fact, all the phenomena incident to the 

 explosion or detonation of the nitro-glycerine compounds indi- 

 cate that the impulse generated is of indefinitely brief dura- 

 tion ; indeed its suddenness is almost beyond conception. 

 Thus, a dynamite cartridge placed upon a log of wood, uncon- 

 fined and free, with nothing above it except the atmosphere, 

 will, when exploding, shiver the portion of timber under it to 

 atoms — the detonation being so instantaneous that the super- 

 incumbent air. as well as the gases generated, having no time 

 to be displaced, become a veritable tampion. 



The efficiency of surface-bias ting under water by means of 

 these explosive compounds, depends upon this extraordinary 

 suddenness of detonation, which renders the effect akin to 

 that of the sudden blow of an enormous unyielding mass. It 

 is evident, that the wave generated in an elastic medium like 

 water by an explosion of this character must be very intense 

 and very short. Hence the acoustical shadow produced by 

 an obstacle placed in its path of propagation must, as in the 

 case of light, be sharply defined and definite in its boundaries. 

 Thus the striking fact, that the protecting influence of the 

 piles on the glass vessels plunged in the water was narrowly 

 circumscribed within the limits of the geometrical shadow 



Phil. Man. S. 5. Vol. 13. No. 79. Feb. 1882. K 



