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Since the meeting the following letter, by Professor- W. 
Thomson, LL.D., F.U.S., Honorary Member of the Society, 
has been communicated by Professor Tait : — 
Kilmichael Brodick, 
Isle of Arran, Oct. 10, 1863. 
My Dear Tait, 
Yesterday evening, when engaged in measuring the 
electrostatic capacities of some specimens of insulated wire 
designed for submarine telegraph cables, I had occasion 
frequently to discharge, through a galvanometer coil, a con- 
denser consisting of two parallel plates of metal, separated 
by a space of air about - 00T inch across, and charged 
to a difference of potentials equal to that of about 800 
Daniell’s elements. I remarked at an instant of discharge 
a sharp sound, with a very slight prolonged resonance, which 
seemed to come from the interior of the case containing the 
condenser, and which struck me as resembling a sound I 
had repeatedly heard before when the condenser had been 
overcharged and a spark passed across its air-space. But 
I ascertained that this sound was distinctly audible when 
there was no spark within the condenser, and the whole 
discharge took place fairly through the 2,000 yards of fine 
wire, constituting the galvanometer coil. I arranged the 
circuit so that the place where the contact was made to 
produce the discharge was so far from my ear that the 
initiating spark was inaudible; but still I heard distinctly 
the same sound as before from within the condenser. 
Usino 1 instead of the galvanometer coil either a short wire 
or my own body (as in taking a shock from a Leyden phial), 
I still heard the sound within the condenser. The shock 
was imperceptible except by a very faint prick on the finger 
in the place of the spark, and (the direct sound of the 
spark being barely if at all sensible) there was still a very 
