NATURAL PHILOSOPHY. 110 



stant of discharge a sharp sound, with a very slight prolonged resonance, 

 which seemed to come from the interior of the ca.se containing the conden- 

 ser, 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 aud- 

 ible when there was no spark within the condenser, and the whole dis- 

 charge took place fairly through the 2,000 yards of fine wire, constitut- 

 ing the galvanometer coil. 1 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. 



Using 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 th^ spark, and (the direct sound 

 of the spark being barely, if at all sensible) there was still a very audible 

 sound, always of the same character, within the condenser, which 1 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, but could see none. I have since 

 ascertained that suddenly charging the condenser out of one of the spec- 

 imens of cable charged ibr the purpose produces the same sound within 

 the condenser; also that it is produced 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 re- 

 garding the part played by air or other dielectric in manifestations of elec- 

 tric 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 considerable distances from any part of the 

 line of discharge, and which by some h ive been supposed to demonstrate 

 an err>r in the common theory of sound. I may add that Mr. Macfarlane 

 tells me he believes he has heard, at the instant ot 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. 



INTERESTING ELECTRICAL PHENOMENA. 



Prof. Piazzi Smyth recently sent to the British Journal of Photog- 

 raphv a photographic picture, accompanied by the following note. 

 Oa the 21st of July, I was trying the qualities of some newly-pre- 

 pared drv plates by taking a window view of house-tops, and was 

 surprised to find every chimney top surmounted by a black streak or 

 brush; i. e. black in the negative, and therefore indicating light. 

 Nothing of the kind was visible to the naked eye in the scene itself, as 

 a really existent fact, nor was any similar appearance visible on the 

 ground-glass of the camera. The appearance, therefore, did not 

 result from auv bad action of the lens, which is a very good one. 



