Geoscience Reference
In-Depth Information
maps. They often chose churches as triangulation stations. As well as being highly
visible, permanent and suitably shaped, churches had particularly practical advantages.
They were likely to be near lodging and other infrastructural support that the scientists
could use during the measurements. They were high, so that the terrain around
could be reconnoitered easily, and each was in the charge of an educated person,
the curé, who could understand requests, provide advice and effect introductions.
The first task of a surveyor on arriving at a new place would be to seek out the curé
or some other local lord and get this person to name the features of the landscape
that could be seen from church tower - the villages, the towers, the woods, the
mountains, the windmills, the abbeys, … This might well have been the first time
that these names had been recorded, and the surveyor was at the mercy of his
informant. This person might have some hidden agenda; for example an egotistical
landowner might want to name features after himself, or give names that tended to
establish his possession of something that was held in common or whose ownership
was disputed, or not give names to features that he wanted to lose (like a ruined
farmhouse, to which occupancy rights might later be claimed).
At a minimum, the surveyors had to carry to the station their surveying instru-
ments to measure the angles of the triangle. These surveying instruments had to be
set up and levelled by means of screw adjustments on their feet against a reading
from a plumb bob. When correctly positioned, this plum bob hung over a fiducial
mark when the rotation axis of the instrument was vertical. To be successful, the
surveyors had to wait for clear weather and make several observations at different
times of the day and at night, especially for distant stations. Night-time observa-
tions were made by sighting onto a lantern at the distant station; someone had to go
there to erect the lantern and its optical reflector and to light the lantern at a pre-
arranged time.
The surveyors also measured the latitude and longitude of some selected stations
using astronomical measurements. Such observations, though, depended on good
weather and there being suitable astronomical phenomena to measure (e.g. eclipses
of Jupiter's satellites). The eclipses had to be timed not only at the station but also at
the Paris Observatory so that the local time interval between them could be deter-
mined in order to measure the difference in longitude, and the observing programs at
the two stations had to be coordinated. Time is measured by a clock so the surveyors
had to carefully carry a clock to the site where it would then have to be set up, leveled,
and regulated. Pendulum clocks had just been invented (see below) and were an enor-
mous advance in accuracy over anything used before, yet they were over a meter high
and extremely delicate. Also, the clock had to be regulated by repeatedly and over
some days observing the transit of the stars in their 24 hour sidereal motion.
All this preparation takes time - days, weeks, even months. If the station was in a
village, Picard could negotiate to buy accommodation at an inn, or he might be fortunate
enough to be given hospitality by a priest of a church or the owner of a château whose
spire or tower had been chosen as a landmark. Picard would need a place to set up
and keep his instruments. If nothing suitable was immediately available then he
would have to build a wooden shed, robust and spacious enough to provide shelter
from the wind for the observers, especially during the cold night-time observations
(
Fig. 11
). Somewhere there would have to be some storage for the instruments, a place
