102 Mr. J. LeConte on 



duced by interposed obstacles to the propagation of sound- 

 waves in different media. The following experimental results 

 in relation to acoustical shadows in water may be of interest 

 to physicists. The experiments were executed in 1874*, at 

 my suggestion, by my son, L. J. LeConte, during the engi- 

 neering operations incident to the removal of " Rincon Rock," 

 a sandstone reef in the harbour of San Francisco (near the 

 south-eastern water front of the city), by means of " surface 

 blasting" with "giant powder" or dynamite. The depth of 

 water on the reef was about fifteen feet at low tide, with an 

 extreme tidal range of about six feet. The " cans " or " car- 

 tridges " of " giant powder " used contained each about fifteen 

 pounds of the explosrve compound, comprising about 75 per 

 cent of nitro-glycerine. 



11. Effects of the Explosive Shock. — It was observed that the 

 suddenness of the shock imparted to the water by this explosive 

 agent produced the most remarkable and astonishing effects. 

 At the distance of 300 feet or more from the detonating car- 

 tridge, two distinct shocks were experienced. The first shock 

 came through the intervening water, and was felt as a short 

 concussion or click before there was any sensible elevation of 

 the column of water resting over the point of explosion. The 

 second shock came a little later by the air, and was heard. It 

 was evidently communicated to the air by the water, at the time 

 the elastic pulse transmitted by this liquid (the first shock) 

 emerged, in a direction nearly normal to its surface, over a 

 limited area around a point vertically above the exploding car- 

 tridge. This was obvious from the fact that aerial sound came 

 from this region. The area which was the source of the sound 

 transmitted by the ah', was the same as that from which the 

 small jets of water (noticed hereafter, 14) were projected. The 

 gases generated during the explosion came to the surface much 

 later than this shock, and after elevating the column of water 

 over the position of the cartridge to the height of twenty-five 

 or thirty feet. 



It is the character of the first shock that deserves special no- 

 tice. To a person sitting in a small boat floating on the water 

 at a distance of 300 feet or more from the point of explosion, 

 with his feet resting on its bottom, the shock was felt as a 

 sudden blow applied to the soles of the feet. In fact, it drove 

 out the oakum from the seams in the bottom of the boat. When 

 the observer stood on the top of a vertical wooden pile, this 

 shock was felt as a sudden concussion coming up from the 



* The long delay in writing out the notes of these experiments in form 

 for publication has been due to domestic affliction and the subsequent 

 pressure of perplexing duties. 



