Geoscience Reference
In-Depth Information
In October and November, the expedition measured the latitude difference
between the two places at the extremities of their arc using an instrument called a
zenith sector
. Imagine a 90° frame consisting of a vertical post 12 feet (2.4 meters)
long with an arm of the same length running horizontal from its top. The end of the
horizontal arm and the foot of the vertical post are connected by a brass quarter
circle on which a scale of degrees is engraved. There is an arm that carries the
eyepiece end of a telescope along the scale: it is pivoted at the center of the quarter
circle. The zenith sector is set up with the post vertical, aligned by a plumb bob,
and the astronomer sights a star with the telescope to measure its altitude on the
brass scale, tracks the star in its motion, watching the altitude rise and fall. The star
is at its greatest height above the horizon when it is on the meridian line overhead
and its altitude at that moment tells the latitude of the observing place.
The objective of the expedition was to measure the latitude difference between
two places and the distance between them, and this depended crucially on the
accuracy of the zenith sector. Maupertuis' instrument was made by George
Graham, a leading instrument maker of London. Maupertuis especially admired
the work of the English craftsmen and had sent Celsius to London to commission
several of Graham's instruments for the expedition to Lapland. A calibration test
of the zenith sector instrument by all five members of the expedition on a meas-
ured triangle (with a base 36 toises long at a distance of 380 toises) showed a
measuring error of only 3.75 arc seconds; this was probably due to its thermal
contraction at the low temperature (Chapman 1995). Nevertheless, the measure-
ment of angles by the sector turned out not to be as accurate as the expedition
thought.
People
Réginald Outhier (1694-1774)
Outhier was the canon of the cathedral at Bayeux. He was an amateur scientist and a cor-
respondent of the Cassinis, working with them on the geodesic survey of France in
Normandy. He published an agreeable account of the Lapland expedition, noting the cus-
toms of the people, the geography, etc. (Outhier 1744)
As the winter set in so did the cold, torrential rain and snow; the temperature fell
so low that it froze the liquid in an alcohol thermometer. The result of the Lapland
expedition owes much to the persistence of the astronomers using the instruments.
It cannot have been pleasant, for example, to apply an eye to a frozen brass eyepiece
or to clamp brass screws with bare fingers in the Arctic temperatures. I can speak
of the pain from personal experience, having left a ring of skin from around my
right eye frozen to an eyepiece when I withdrew my head from a telescope in an
upstate New York observatory in a night-time winter temperature of 30 degrees
Fahrenheit of frost (I put salve on the wound, closed down the telescope and went
to bed even though it was clear and the stars were bright). Le Monnier had a similar
experience in Lapland when his tongue froze to a silver cup from which he was
drinking brandy at -20°C.
People
Charles-Etienne-Louis Camus (1699-1768)
A little-known scientist, Camus was a clockmaker, and later an administrator at the
Academy of Sciences. He was the scientist of least stature in the expedition to Lapland.
