26 Proceedings of Royal Society of Edinlurgh. [sess, 
case only the leakage has to be supplied. The apparatus appears to 
act electrostatically on the principle of a voltmeter detecting differ- 
ences of potential produced by the charging and discharging of the 
cable, or when the cable is kept charged, by the difference of 
j)otential near and at a distance from a conductor. It is injuriously 
affected by the increase of conductivity of the water between the 
water connections due to increase of salinity, but the law by which 
it varies has not been determined. This loss of efficiency in sea 
water is, of course, counteracted by the insulation of the water con- 
nection, or by introducing an equivalent resistance. 
With the coils separated 10 feet (at the bow and stern of a small 
boat) and an insulated wire 400 feet in length laid through a small 
lake of brackish water 2J fathoms in depth, the alternations pro- 
duced by the bobbins of f ths of one of De Meritens’ magneto-electric 
machines used at the Isle of May Lighthouse, and maintaining 
80 volts at its terminals, was perfectly distinct at the end of the 
loch 340 feet away from the wire, and the limit of audibility could 
not be ascertained. The sound was there quite distinct, although 
the connections were separated only 10 feet, and did not perceptibly 
fall off in the last 50 feet, thus pointing to the conclusion that the 
action is not purely electrostatic. Further trials would be necessary 
to determine the law of the falling off of the intensity of sound 
with the increase of the distance from the cable for a given fixed 
distance between the water connections, as also to determine the 
law of increase of intensity for any increase of distance between 
the connections for a given fixed distance from the cable. The 
experiments in fresh-water shown on fig. 3, made by connecting 
various wires with a small magneto machine, show the difference 
in audibility when the distance between the connections was 
halved. 
The next instrument brought before the Society is simply a coil 
of insulated wire surrounding a core^ i.e. an electro-magnet with 
a telephone in the circuit of the coil (fig. 2). This instrument was 
tried at various distances, and the making and breaking of the 
current produced by 6 dry cells flowing through a wire 
diameter and 200 feet in length could be detected 40 feet off, and the 
current from 12 dry cells can be detected through 60 feet of salt- 
water. When sunk in water the sound seems just as loud, and there 
