429 
the velocity of sound, made in Holland. 
served in the open air; Mr. Daniell’s hygrometers were also 
placed in the open air ; and the light of a candle reflected 
from the surface of the ball of the hygrometer, gave the 
means of observing the deposition of dew with great ac- 
curacy. ■ ' - .. 7 , 
It was deemed of the utmost importance that the shots on 
both stations should be fired at as nearly the same moment 
as possible. To obtain this, the following plan was adopted. 
At 7 h 55' P. M. by the chronometer of Zevenboompjes, a 
rocket was fired at Zevenboompjes, which being observed 
at the other station of Kooltjesberg, was immediately answered 
by another rocket from the latter place. This was the signal 
that on both stations every thing was ready for observation. 
At 8 h o' o" by the chronometer of Zevenboompjes, a cannon 
shot was fired on that station, whilst the observers at Koolt- 
jesberg took as exactly as possible the time on their chrono- 
meter when the light was observed. A second shot was fired 
at Zevenboompjes at 8 h s' P- M. by the chronometer of 
that station, and the time at which the light was seen was 
carefully taken down by the chronometer of Kooltjesberg. 
By these means the difference of the two chronometers at 
both stations, in a distance of about 9 miles, was ascertained 
with great accuracy ; and in order to show that this prepara- 
tory investigation was made with due care, a cannon shot 
was fired on both places at the moment when the chrono- 
meter of Zevenboompjes marked 8 h io' o". If the lights of 
both shots were seen exactly at the same time, it was a proof 
that the difference of both time-keepers was known, and that 
experiments might be safely made. 
We own that we did not suppose before hand, that it could 
