Geoscience Reference
In-Depth Information
of the King
with a superposition of the line of the accurate survey of the French
coast on Sanson's map showing the startling differences. This was the first map to
show the Paris Meridian.
Having mapped France, Cassini then mapped the world using the same tech-
niques. He trained French explorers and Jesuit missionary priests in the techniques
of determining latitude and longitude. When they reached their assigned destina-
tions, they measured where they were and sent the results back to Paris. Cassini used
the measurements to position places on a huge world map laid out on the third floor
of the Observatoire de Paris in a circle about 10 meters in diameter, drawn on the
floor with the North Pole at the center and with a concentric and radiating system of
lines of latitude and longitude at ten-degree intervals. Using the measurements of
latitude and longitude from around the world, Cassini added far-flung cities to the
map like Québec, Santiago, Lisbon, Venice, Cairo, Siam, India, Canton, and Peking.
“When he visited the work in progress, Louis XIV could stamp on the world and
pinpoint places with toecap accuracy.” (Fernández-Armesto 2006) In 1696 Cassini
published the result of all this work and it became the first accurate world map.
It had taken 20 years for the astronomers and geodesists at the Paris Observatory
to confirm Colbert's original suspicions about the maps of France and to show the
scale of the problem. It was indeed substantial! But while the work had not resulted
in an accurate map of the whole of France, it had provided evidence of the extent
of the solution to the problem- the equipment, the numbers of people that were
needed, the training that would provide their skills, and the time-consuming
expeditions.
BASED ON THE experience of the trial surveys, in 1681 Picard defined a plan to
triangulate along the whole of the meridian across France towards the north and the
south and measure arcs in lines of latitude to the east and the west. This framework
(or
chassis général
) would serve as the secure basis of smaller maps of the interior,
linking them securely together. However Picard died in 1682, and in 1683 his plan
was taken up by Cassini I with Colbert's backing. In the summer of 1683, the plan
was approved by the Academy and assigned resources by Colbert. Cassini I led the
southern expedition to Perpignan in the south and La Hire went to the north.
In 1683 the plan suffered a grievous set back. Colbert died, and his successor in
charge of the Academy, François Le Tellier, Marquis de Louvois, was not so enthu-
siastic. By the end of 1683 the meridian had been surveyed from Mont Cassel in the
north, near Calais, to Montluçon near the Massif Central, but field work was sus-
pended for the winter because the snows in the Auvergne and in Limousin rendered
several key stations inaccessible. Louvois recalled the expeditions to wait for more
favorable weather in the spring. He never reactivated them - there were always
greater priorities for the resources at his disposal. The survey was suspended,
although some preliminary reconnaissance for its completion was carried out by an
engineer, M. Loire, in order to identify a set of mountain tops from Berry southwards
that could form triangulation stations, visible from one another. His reconnaissance
was not entirely successful, though, since the mountains that he identified centered
along a line that drifted east of the Paris Meridian towards Bezier.
