Geoscience Reference
In-Depth Information
In 1727, Languet commissioned an English clockmaker residing in Paris, Henry
Sully (1680-1728), to make the original
meridiana
. Sully had come to France to
establish a watch making industry at Versailles and “Frenchified” his name to Henri
de Sully. Sully's motive for accepting the commission from Languet was different
from Languet's motive for issuing it. The clocks of the churches of Paris were not
synchronized and it was not unusual then to hear the noon bell ringing from different
churches for half an hour or more. As a clockmaker who had been interested in the
problem of determining longitude through the construction of an accurate chro-
nometer, Sully was affronted by this lack of precision. He had in mind to use the
meridiana
to control the bell of St. Sulpice, to which the other bells would be syn-
chronized. He was a member of a benevolent organization founded by the Comte
de Clermont, the Société des Arts, to develop scientific and technical applications
and saw civil advantages in correcting the irregular cacophony of hourly bells in the
French capital. How could the citizens of Paris carry out their business efficiently,
for example gather for a meeting, if they did not know the right time to act? The
Society of Arts supported Sully in his campaign to provide a uniform time for the
whole city. Sully completed plans for the construction of the
meridiana
and started
to build it in 1727 but sadly died the year afterwards, before he could complete it.
The completion of Sully's project fell to Le Monnier. Starting in 1742,
Le Monnier took up the challenge to complete the
meridiana
as a means to deter-
mine subtleties in the mathematics. He directed the instrument maker Claude
Langlois in the engineering work (the same man that made quadrants and sectors for
the expedition to Lapland that Le Monnier had used). Following Sully's footsteps,
Le Monnier installed the entrance aperture for the
meridiana
in the southern window
of the transept, and in fact there seems to have been more than one aperture since at
certain times of the year the Sun cannot be seen from parts of the meridian line. For
a few days around midsummer a ledge blocks the almost vertical beam of sunlight
coming through the principal aperture from reaching the ground immediately below.
Therefore there was a subsidiary aperture higher up above the ledge producing an
image of the Sun on those days. The principal entrance aperture was a three-inch
lens mounted on the right (west) edge of the plain window high (25.98 meters) above
the south entrance, where there is now an empty hole in a mounting plate.
LE MONNIER'S MERIDIAN is marked on the floor of the church as a thin brass
strip about 40 meters long and 7 mm wide with gradations marked alongside at
intervals (
Fig. 27
). The strip is inlaid in white marble stone slabs about 20 cm wide
and lies some 45 cm to the west of Sully's meridian. Sully's meridian was inscribed
directly onto the stone floor but has mostly now has all been rubbed away. Faint
lines associated with it, and the numerals “200” and “10” can be discerned beside
the more identifiable meridian of Le Monnier in the northern transept and near the
porch of the southern transept.
Le Monnier's meridian runs across the transept at an angle of 11° to the geometric
axis of the church. In principle, European churches are built on a common plan and are
shaped like a cross. The head of the cross is also the head of the church, where the altar
and other holiest places are. The long axis of a church lies east-west, perpendicular to
