110 Mr. J. LeConte on 



may be rationally traced to the extreme shortness of the 

 elastic wave, due to the inconceivably brief duration of action 

 of the generative detonations. This view seems to afford a 

 satisfactory explanation of the remarkable results revealed by 

 the experiments in the harbour of San Francisco. 



18. Gunpowder Explosions. — If the foregoing is the true 

 explanation of the definiteness of the sound-shadows cast in 

 the preceding experiments, then the waves generated by the 

 explosion of ordinary gunpowder, being less sudden, should 

 not produce as sharply-defined shadows as those clue to the 

 detonation of dynamite. We have, so far as known, no 

 specific experiments testing this point; but it seems to be quite 

 reasonable that such will be found to be the case whenever 

 the test of experiment is applied; for it is well known that the 

 subaqueous explosions of ordinary powder do not give rise to 

 the remarkable concussions so characteristic of the detonations 

 of the nitro-glycerine mixtures. 



19. Dynamite Explosions in Air. — Moreover, if my explana- 

 tion is correct, the acoustical shadows produced by nitro- 

 glycerine detonation in air ought also to be more sharply 

 defined than those due to sounds less suddenly generated. In 

 other words, if the distinctness of sound-shadows depends 

 upon the duration of the impulse which produces the accom- 

 panying sound-wave, then the definiteness of the shadows cast 

 by sounds propagated through the air should vary with the 

 suddenness of the action of the generating cause. 



Inasmuch as the variations in the duration of the genesis 

 of audible sounds in the atmosphere must, in ordinary ex- 

 perience, be very great, it may, at first sight, appear incredible 

 that the corresponding differences in the perfection of sound- 

 shadows cast by obstacles in the paths of different kinds of 

 sounds should have escaped the most casual observation. But 

 it must be recollected that, for the reasons already assigned 

 (1), (2), (3) and (4), aerial acoustical shadows are not readily 

 appreciated by the ear. Moreover, in the case of sounds 

 transmitted by the air, the distinctness of such shadows is most 

 seriously impaired by the numerous reflected waves which 

 come from circumjacent objects. It should be borne in mind 

 that it is only very recently (5) that the influence of acute- 

 ness of sounds on the distinctness of the resulting shadows 

 has been satisfactorily verified by experiment. In like 

 manner I venture to predict that careful experiments will 

 verify the deduction that the shadows clue to sounds generated 

 by the extraordinarily brief detonations of dynamite are more 

 sharply defined than those owing their origin to sounds less 

 suddenly produced. 



