Sound-Shadows in Water. 101 



inches on the further side of the disk, the augmentation of the 

 intensity of the sound, in the axis of the acoustical shadow, 

 was obvious both to the ear and to a sensitive flame*. 



Sottkd-Shadows rsr Watek. 



8. It is a significant fact in relation to the phenomenon of 

 acoustical shadows, that they seem to be more perfect or more 

 sharply defined in water than in air. Thus, during the pro- 

 gress of the classical experiments of Daniel Colladon, in 

 November 1826. on the velocity of sound in the waters of the 

 Lake of Geneva, this physicist incidentally observed that when 

 the end of the hearing-tube {cur-net acoustique). plunged into 

 the water, was screened from rectilinear communication with 

 the bell by a projecting wall running out from the shore, 

 whose top was above the surface of the lake, there was a very 

 remarkable diminution in the intensity of tie sound, in com- 

 parison with that observed at a point equally distant from, but 

 in direct communication with, the source of sound, or out of 

 the '"acoustic shadow," thus indicating the relative non- 

 divergence of the rays of sound around obstacles in water as 

 compared with those in airf . 



9. Another fact observed by Colladon during these famous 

 experiments is, in this connection, no less significant. He 

 found that the sound of the bell struck under water, when 

 heard at a distance, has no resemblance to its sound in the 

 air. Instead of a prolonged tone, a short sharp sound is heard, 

 like two knife-blades struck together. It was only within 

 200 metres that the musical tone of the bell was distinguish- 

 able after the blow. In air. it is well known, the contrary 

 takes place — the shock of the first impulse of the hammer 

 being heard only in the immediate neighbourhood of the bell, 

 while the continued musical sound is the only one that affects 

 the hearing at a distance!. Sir John Herschel, in his Treatise 

 on Sound §, promised to explain this curious difference, but 

 has not, so far as I can find, done so. Colladon jj explains 

 this phenomenon by the nature of the sonorous vibrations in 

 water, showing that the duration of the sound will be much 

 less when transmitted by water than when propagated by air. 



Expeeiments of L. J. LeCgnte rs T 1874. 



10. The preceding remarks show that comparatively few 

 exact observations have been made on the obstruction pro- 



* Phil. Mag. oth series, vol. ix. pp. 281, 282 (1850). 



t Ann. de Chim. et de Phys. 2nd series, vol. xxsvi. pp. 256, 2-57 {1627) . 



X Op. cit. supra, p. 254. 



§ Encyc. Metrop. art. 101. Op. cit. p. 2-55. 



