Geoscience Reference
In-Depth Information
a 36-inch full circular scale to link the Greenwich Meridian across the English
Channel to the Paris Meridian.
This was a well timed project, the American War of Independence having ended
in 1783 and the British government keen to show (in a subtle way) reconciliation
with France. Of course the project required close intergovernmental cooperation,
with the erection of coordinated signal lanterns and the conveyance of messages
etc. across the English Channel that lies between the two countries. Given the unre-
liability of the weather in the Channel, frequent intercommunication was necessary,
and it was agreed that both sides would start at the coasts and link to the capitals
from there. On the English side, Major General William Roy linked across the
Channel from the cliffs above Dover in Kent and near Fairlight in Sussex to church
spires and to signal fires lit on the French coast. The Paris Meridian was linked in
the other direction across the Channel from Dunkerque to the Pas de Calais and to
Dover and Fairlight by the French geodesists including Cassini IV, Pierre Méchain
and Adrien-Marie Legendre.
People
Adrien-Marie Legendre (1752-1833)
Legendre came from a bourgeois family and showed aptitude in mathematics from an early
age. He taught at the École Militaire in Paris and conducted research into mathematics and
mathematical astronomy, most especially celestial mechanics (theory of planetary
motions). He was elected to the Academy of Sciences in 1783 and as a result of his work
in the cross-Channel geodesy as well as a theorem that arose from it on spherical triangles
known as “Legendre's theorem,” he was made a fellow of the Royal Society. He went on
to develop theorems in elliptical functions and the methods of least squares fitting of a
curve to data, in which he disputed priority with Gauss. In the Revolution he lost his “small
fortune” but re-established his place in scientific life by his brilliant mathematical work.
The difference in longitude of the Paris and Greenwich Meridians was measured
by the 1787 expeditions at 2° 19' 51”, about 20” less than the current modern value.
Places
Fairlight
The area near Fairlight, East Sussex, near Hastings, is now a country park and its highest
point is called North's Seat where high spots can be seen up to forty miles away. The near-
est point in France is Cap Gris Nez, 61km due east, behind the trees on the skyline near
Fairlight Church. In October 1787 William Roy, the military surveyor, triangulated from
Fairlight to Wrotham Hill to the north, Dover Castle to the east and across the English
Channel to Cap Blanc-Nez and Montlambert in France. He worked at night using light
signals mounted on a scaffold 10m high. The heritage of Roy's work lies presently in the
bench mark on a concrete pillar although it is not on the original site. Roy's measurement
was repeated in July 1825, when Sir John Herschel, the astronomer, and a French officer
observed from the same spot some rocket firings in France, timing the flashes in local solar
time in order to determine the difference of longitude.
It is curious how in tackling the identical problem of position-finding the British
and the French went about it in quite different ways. The French concentrated on the
administration of France and the problems of the shape of the Earth, using triangula-
tion and the satellites of Jupiter to determine position. The British were driven by the
needs of ships at sea and used the method of lunar distances for the same purpose. In
addition, the French were driven by science and adopted an intellectual, integrated
