Velocity of Sound. 131 
returned in an echo from the front of the said storehouse. The 
distance from my station to the front of the storehouse, determined 
carefully by a trigonometrical operation, was 1523 feet. 
Of eight rounds fired from the musquet, I failed twice in the 
appreciation of the interval between the sound and the returning 
echo, from a very wrong estimate of its probable duration ; and that 
from an erroneous impression as to the time observed by Dr. Derham 
in a similar experiment.* Of the remaining sia rounds, the musquet 
pointed across the river, the intervals were 2.7, 2".75, 2.74, 2”.72, 
2".75, 2".74; their mean 2”.73. 
Next, three rounds were fired, the musquet being pointed directly 
from the river; the intervals were 2".7, 2”.73, 2".76; mean ‘as before. 
Lastly, four rounds were fired along the bank, at an elevation 
of about 45°; the intervals were 2.75, 2’.7, 2".73, 2.74; mean as 
before. 
Distance occupied by the direct and the reflected sounds 3046 
feet. 
direct, half reflected; therm. 66°. 
The near agreement of this with the former result on the same 
day, serves to confirm the opinion that direct and reflected sounds 
move with the same velocity. 
Thursday, August 21, three o’clock P.M. barom. 29.86 inches, 
Fahr. therm. 64°; clear sunshine; wind scarcely perceptible, 
westerly. 
* He made it 3 seconds, by means of a half-second pendulum. My erroneous recollection 
of his experiment led me to anticipate an interval of between 4 and 5 seconds. I could not 
account for the supposed discrepance, until after my return home, when on examining 
Derham’s paper, and computing the real breadth of the river from my trigonometrical operation, 
I found the correspondence of the two experiments to be quite as great as could be expected, 
considering the different natures of the chronometers employed, and the yarying breadth of 
the river. 
R 2 
