the velocity of sound, made in Holland . 427 
his command, to give us every assistance in their power, aind 
to take an actual part in these experiments. 
SECTION II. 
As fitted places to make these experiments, two elevated 
spots were selected on the extended heaths of the Province of 
Utrecht. One of these is a small hill between the town of Naar- 
• * 
den and the village of Blaricum, and called the Kooltjesberg ; 
the other is somewhat higher, and situated on the right of the 
road from Utrecht to Amersfoort, and very near the last town. 
Both places were distinctly visible from one another, and the 
distance was between 17000 and 18000 metres (9664 fathoms). 
Our time was kept by two time-keepers, which the Minister 
of Marine had kindly furnished us with; one made by Ar- 
nold, the other by our countryman Mr. Knebel. But the 
exact interval between the observation of light, and the per- 
ception of sound, and consequently the velocity of sound, 
was measured with small clocks with conical pendulums. 
They are made at Wesel by Mr. Wilhelm Pfaffius, and 
proved remarkably well adapted for this purpose. It is well 
known that Hu 1 gens laid down the properties of the conical, 
or centrifugal pendulum, but if we are not mistaken, they 
were employed for similar purposes for the first time by the 
German philosopher Benzenberg.* These clocks with a 
conical pendulum divide the 24 hours of the day in 10,000,000 
parts, and one of the indexes gives -j— part of a decimal se- 
cond. This index or second hand remains quiet, whilst the 
watch work continues moving as long as a certain spring is 
not pressed down with the finger ; and on removing the 
* Some account of these clocks is given in Gilbert’s Annalen d. Physik, 1804, 
B. 16, p. 494 ; and New Series, vol. v. p. 333. 
