98 Mr. J. Le Conte on 



With the coils mentioned above and with one Lechanche* 

 in the primary, 6 was 13 scale-divisions (the galvanometer 

 being shunted), r was 1\ seconds, or about 2ir. When the 

 cylinder was making about 4 revolutions per second (giving 

 20 breaks per second), </> was found to be a little over 500; 

 that is, the light was off the scale. 



Now equation (3) gives 



= 2nd = 520. 

 This shows that a value of $ enormously in excess of 6 may 

 be obtained ; and though the experiment is only a rough proof 

 of equation (3), it serves to show the greatly increased sen- 

 sitiveness obtained. 



Other experiments of a similar nature were made ; but, 

 being merely confirmatory of this, they need no further men- 

 tion. It is possible that a powerful vibrator might with ad- 

 vantage be substituted for the rotating cylinder; and this 

 would be completely self-acting; but it is doubtful whether it 

 would be easier of manipulation than the cylinder, which can 

 be very conveniently worked by a treadle, thus leaving the 

 hands quite free to adjust the resistances. 



In using the rotating cylinder with the bridge, E Gr would 

 be inserted in the " bridge wire ; " and since that is thereby 

 broken whenever the battery is on, the effect due to the in- 

 duced current is separated from that due to any very slight 

 derangement of the balance — a great advantage when it is 

 remembered how troublesome the presence of this latter effect 

 is liable to be with the ordinary arrangement. 



Derby, December 30, 1881. 



XII. On Sound- Shadows in Water. By John LeConte*. 

 1. "A/TORE or less perfect sound-shad ows thrown by hills, 

 -L'-L buildings, piers, and other obstacles to the trans- 

 mission of aerial vibrations, must be within the experience of 

 all. Nevertheless the boundaries of such shadows are so 

 imperfectly defined, that they can hardly be compared, except 

 in a general way, with those of light. Moreover, in some 

 cases the obstacles placed in the route of the sound-waves, 

 being elastic, propagate more or less perfectly the sonorous 

 vibrations of the air through their thickness; so that, under 

 these conditions, it is similar to producing a light-shadow by 

 means of a transparent or translucent body. 



2. But even in cases in which the sound-vibrations in air 

 are not sensibly transmitted through the intervening obstacle 



* From advance sheets of the 'American Journal of Science,' vol. xxiii. 

 January 1882. Communicated by the Author. 



