PHILOSOPHICAL TRANSACTIONS. 
I. An Experimental Determination of the Velocity of Sound. 
By E. J. Stone, F.B.S., Astronomer Royal at the Cape of Good Hope. 
Received August 21, — Read November 23 , 1871. 
A galvanic current passes from the batteries at the Royal Observatory, Cape of Good 
Hope, at 1 o’clock, Cape mean time. This current discharges a gun at the Castle, and 
through relays drops a time-ball at Port Elizabeth. 
It appeared to me that a valuable determination of the velocity of sound might be 
obtained by measuring upon the chronograph of the Observatory the time between the 
sound reaching some point near the gun and that of its arrival at the Observatory. I 
thought also that it would be a point of interest to check, within the limits of our 
changes of temperature, the variations in the velocity of sound as dependent upon tem- 
perature, and to obtain some test of the applicability of the coefficient of expansion of 
dry air, as determined in cabinet experiments, to the mixture of air and water which 
would be the medium of the propagation of sound in our experiments. 
There is only a single wire between the Observatory and Cape Town ; some little 
difficulty was therefore experienced in making the necessary arrangements, without any 
interference with the 1 o’clock current to Port Elizabeth. I have adopted the following 
plan, which was brought into successful operation on 1871, February 27. It would, 
however, have been quite impossible for me to have had these experiments made, without 
an encroachment upon the time of the Observatory staff which could not have been 
sanctioned, had it not been for the assistance of J. Den, Esq., the acting manager of the 
Cape Telegraph Company. I am indebted to Mr. Den for the preparation of a good 
earth near the gun, for the assistance of one of the gentlemen attached to the telegraph 
office, Mr. Kirby, who has made all the observations at the Cape-Town end, and for a 
general superintendence of the arrangements in Cape Town. Mr. Ivirby stands at a 
distance of 641 feet from the gun, near an earth whose connexion with the single main 
wire is broken at a tapping-piece which Mr. Kirby, at the time of the experiments, holds 
in his hand* A small battery is arranged at the Observatory with one pole to earth 
MDCCCLXXII. B 
