9 
MR. E. J. STONE ON THE VELOCITY OF SOUND. 
through the chronograph coil, and the other connected with the Cape-Town wire through 
a tapping-piece similar to that used by Mr. Kirby. At 1 o’clock the observer at the 
Observatory (Mr. Mann) connects the local battery with the main line : this current is 
arranged so that it merely assists the main time-ball current. Mr. Mann holds down his 
tapping-piece until three seconds after 1 o’clock, and thus affords a connexion through 
the chronograph coils to register Mr. Kirby’s signal. When the current has passed the 
telegraph office in Cape Town, the connexion is broken at that office. Mr. Kirby’s 
distance from the gun has been arranged so as to allow of this being done before the 
sound reaches his station. The line after the breaking of the connexion at Cape Town 
is complete except at Mr. Kirby’s tapping-piece. When the sound reaches Mr. Kirby’s 
station he completes the circuit, and his observation is registered on the Observatory 
chronograph. Mr. Kirby then holds down his tapping-piece for half a minute, to make 
earth for the observer at the Observatory station. The connexion at the Observatory 
station is broken, as before stated, at three seconds after 1 o’clock. When the sound 
reaches the Observatory, about 13 s- 2 after Mr. Kirby’s observation, the Observatory 
tapping-piece is again connected, and the time of the sound reaching this station recorded 
on the chronograph. Time-signals are then sent to check the loss of time of gun-fire, 
but not as bearing on the determination of the velocity of sound, the results for which 
are quite independent of any loss of time at the gun, or of any errors of rate except that 
of the chronograph between seconds of the transit-clock and of the transit-clock for 
about 13 s . 
The observations have been made on all the days since February 27 upon which Mr. 
Kirby’s services were available without any interference with his regular duties. The 
observations will be found in Table I. 
The results have been corrected for the effects of the motion of the air upon the dif- 
ference in time between the sound reaching Mr. Kirby’s station and its reaching the 
Observatory, with velocities of the wind found from a set of Robinson’s cups. 
To reduce the equations of condition to a linear form corrections have been applied 
for the second and third terms of the expansion of \/l -\-ol0, where « is the coefficient 
of the elasticity of air under a constant volume for a degree Fahrenheit of temperature, 
and 6 is the excess of the temperature at the time of the experiment above 32°. The 
observed differences have also been diminished by — 0 s- 09 for the effects of personal 
equation between Mr. Mann and Mr. Kirby under the circumstances of these obser- 
vations. 
This personal equation has been found as follows : — A gun was fired at such a distance 
from the Observatory that the sound was heard with about the same degree of distinct- 
ness as the ordinary time-ball gun at the Castle. This was at a distance of 1483 feet 
from the Observatory. Mr. Kirby was placed at a distance of 162 feet from the gun. 
From previous determinations of the velocity of sound, or from the first approximate 
result of the present experiments, we can compute with great accuracy the difference in 
time, at the temperature of the air at the time of observation, of the sound reaching 
