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audible sound, always of the same character, within the 
condenser, which I heard at the same instant as I felt the 
spark on my finger. Mr. Macfarlane could hear it distinctly 
standing at a distance of several yards. We watched for light 
within the condenser, hut could see none. I have since 
ascertained that suddenly charging the condenser out of one 
of the specimens of cable charged for the purpose produces 
the same sound within the condenser ; also that it is pro- 
duced by suddenly reversing the charge of the condenser. 
Thus it is distinctly proved that a plate of air emits a 
sound on being suddenly subjected to electric force, or on 
experiencing a sudden change of electric force through it. 
This seems a most natural result when viewed in connection 
with the new theory put forward by Faraday in his series 
regarding the part played by air or other dielectric in mani- 
festations of electric force. It also tends to confirm the 
hypothesis I suggested to account for the remarkable 
observation made regarding lightning, when you told me of 
it about a year ago. and other similar observations which I 
believe have been reported, proving a sound to be heard at 
the instant of a flash of lightning in localities at consider- 
able distances from any part of the line of discharge, and 
which by some have been supposed to demonstrate an error 
in the common theory of sound. I may add that Mr. 
Macfarlane tells me he believes he has heard, at the instant 
of a flash of lightning, a sound as of a heavy body striking 
the earth, and imagined at first that something close to him 
had been struck, but heard the ordinary thunder at a sensible 
time later. 
Yours truly, 
WILLIAM THOMSON. 
