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from Brest to Strasbourg. It was linked to yet another determination of the meridian
carried out by Cassini de Thury (Cassini III), Nicholas Louis Lacaille and Giovanni
Domenico Maraldi.
People
Giovanni Domenico Maraldi (1709-1788)
Giovanni Maraldi (Jean Dominique Maraldi, Maraldi II) was the nephew of Maraldi I,
brought by his uncle to Paris in 1726. He worked with Cassini II and Cassini III on a
number of the large observing programs at the Paris Observatory for over 40 years before
returning to Italy. On the death of Lacaille, Maraldi put his star charts through the press and
wrote a preface on the life of his lifelong colleague and friend. In the opinion of Delambre
(1827) Maraldi was “an industrious and worthy astronomer.” Since Delambre did not have
many enthusiastic things to say about the Cassinis or anyone connected with them, we may
take this faint praise to imply a more positive assessment than at first it seems.
People
Louis-Guillaume Le Monnier (1717-1799)
On the 1739 expedition Cassini also took along the younger brother of the astronomer
Pierre Le Monnier. At the age of 22, Louis-Guillaume Le Monnier was able to make a natu-
ral history survey of the southern regions along the meridian, particularly the botany and
the mines, and wrote a memoir,
Observations d'histoire naturelle faites dans les provinces
méridionales de France, pendant l'année 1739
(1744) “containing the detail of all the
plants and other natural curiosities that he found in his voyage, and which enrich the
Garden of the King.” He noted interesting ecological trends, and in his thirties he worked
as a physicist on fluids and electricity. His work attracted vitriolic criticism from a rival,
Abbé Nollet, which destroyed his self confidence and he wrote little in his later life. He had
an apartment in the Tuileries and was home when a mob of Revolutionaries attacked it on
August 10, 1792; he was saved by an unknown individual. According to historian
L. Plantefol, “Destitute, he retired to Montreuil, where he lived a miserable existence on
the income from an herbalist's shop which he opened.”
THE ACCURATE SURVEY of Paris by triangulation made it possible to make a
precise measurement of the speed of sound, which traveled so fast it was necessary
to make the measurements over large distances. The Academy of Sciences
addressed this question of physics repeatedly with the aid of the Prime Meridian
survey, and the principle of the method was outlined by Marin Mersenne. Mersenne
noted that it was well-known from observing battles that the cannon's flash pre-
cedes the sound of its loud explosion. The light moves so quickly that its speed
cannot be measured, but the sound follows at an interval that Mersenne estimated
by counting his heartbeats or pulse.
In 1640 Mersenne gave a figure of 1380 Parisian feet (
pied du roi
13
) per second
for the speed of sound (450 meters per second). There was a discrepancy between
Mersennes' figure and that of Pierre Gassendi in 1635 (1473 Parisian feet per second)
and Robert Boyle (equivalent to 1125 Parisian feet per second). Boyle explained
this as something arising from a difference in the consistence of air in France and
Britain. The most precise result, however, was achieved by a measurement by
13
The
pied du roi
in Paris was the equivalent of the English foot of 12 inches. It measured 12.8
inches in the English system or 325 mm in modern metric units.
