30 John Le Conte—Sound-Shadows in Water. 
a projecting wall running out from the shore, whose top was 
bove the surface of the lake, there was a very remarkable 
diminution in the intensity of the sound, in comparison 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 air.* 
9. Another fact observed by Colladon during these famous 
experiments is, in this connection, no less significant. He 
0 
hearing at a distance.t Sir John Herschel, in his “Treatise 
on Sound,”t promised to explain this curious difference; but 
has not, as far as I can find, done so. Colladon § explains this 
phenomenon by the nature of the sonorous vibrations in water ; 
Exprrments or L. I. LeConre IN 1874, 
10. The preceding remarks show that comparatively few 
exact observations have been made on the obstruction produced 
wa 
different media. The following experimental results in rela- 
tion to acoustical shadows in water may be of interest to phy- 
sicists. The experiments were executed in 1874, || at my sug- 
gestion, by my son, L. I. LeConte, during the engineering opera- 
_ tions incident to the removal of “Rincon Rock.” 
reef in the harbor of San Francisco (near the southeastern water 
nyse city), by means of “surface blasting” with “ giant 
er 
used contained each about fifteen pounds of the explosive com- 
pound, comprising about seventy-five per cent of nitro-glycerine. 
* Ann. de Chim. et de Phys., 2d series, vol. xxxvi, pp. 256, 257, 1827. 
+ Op. cit. supra, p. 254, ~ - 
Encye. Metrop., art. 101. § Op. cit. 
_ | The long delay in writing out the notes of these experiments in form for pub- 
lication has been due to domestic affliction and the subsequent pressure of per- 
plexing duties, 
, 
See Se 
