Geoscience Reference
In-Depth Information
William Derham (1657-1735) in 1709 in Britain, which resulted in a speed-for-
sound equivalent to 1072 Parisian feet per second.
The first experiments by the Academy of Sciences carried out by Cassini I,
Huygens, Picard, and Romer produced the result that sound passes at a speed of 180
toises, or 1080 Parisian feet per second. The experiments were repeated in March
1738 by Cassini, Maraldi and Lacaille, exploiting the measurements of the survey;
observation points were separated by large, accurately measured distances over
which it took a considerable time for the sound of the cannon to travel. The chosen
stations were the
mire
on Montmartre, the Paris Observatory, the mill at Fontenay
aux Rose and the tower of Montlhéry, 29 kilometers from Paris. Many experiments
were carried out with cannons of different sizes and with different charges of powder,
day and night and in different weather conditions. These measurements gave an
average of 173 toises (1038 Parisian feet) per second. The reduction of 34 Parisian
feet per second between this value and the value by Derham would accumulate to
a difference for the travel time of the sound of the cannon of 3 seconds between
Montlhéry and Montmartre. Cassini estimated that a measurement error of half a
second would be the most there could be, less than 1% of the two minutes it would
take for sound to travel 29 km.
Places
The Tour de Montlhéry
The Tower of Montlhéry is the main structure that remains of a small castle dating to the
14th century. It stands on an isolated hill directly south of Paris and has been occupied as
a defensive position since AD 991. Its prominent high location within sight of and imme-
diately south of Paris meant that it was a very convenient landmark for the trigonometric
surveys of the Paris Meridian. However the castle was damaged during the period of the
Religious Wars, and The Tower has recently been (controversially) restored. It stands high
above the stone foundations of the castle buildings, surrounded by what may have been (on
top of the steep hill) a dry moat; in an attractive grove of trees, some flowering prettily in
the spring, and full of birds. Although it is a pretty woodland setting, it is not easy to imag-
ine that during the eighteenth century measurements of the speed of sound were taken at
this location, and the firing of a cannon north of Paris could be heard. Now, one hears the
continuous sounds of motor vehicles on the motorways criss-crossing the plain below and
the noises of airplanes taking off and landing at Orly airport a few kilometers away.
Cassini discovered that the speed of sound depended on wind direction, with or
against the direction of sound, with the speed increasing or decreasing by the wind
speed. He wanted to find out if the speed of sound depended on the season or the
geographical location and aimed to carry out experiments during the expedition to
southern France which he was about to embark on. Ultimately, though, he did not
find it practical to spend time on the experiments as he had planned, as they were
a distraction to the main purpose of his work, to survey France.
WHEN THE SURVEY of 1739 was completed, France had been surveyed north-
south and east-west, and its extent had been precisely determined. The measure-
ments were used as a series of starting points to construct a map of the whole of
France (Robb 2007) and reached unprecedented accuracy. The surveying expedi-
tions concentrated along river banks, partly for ease of access and partly because
the resources along the river valleys were economically the most important. The
surveying teams erected tall pyramids, or scaffolding, at 10 km intervals on the
