During the early decades of the 18th century, Newtonian physics was still new. Much effort was expended in testing its validity. One arena in which evidence could be found was the shape of Earth. Was it perfectly spherical? On the observational side there were two hints. In 1671 Jean Richer had measured the rate of ticking of a pendulum clock near the equator and found it to swing more slowly than in Europe: the conclusion was that gravity was weaker towards the equator. And in 1718 Jacques Cassini announced that he had analyzed surveying data over a large region of France and found that they suggested a prolate shape (elongated at the poles). But the precision required to draw such a conclusion with confidence was very great, and there was considerable doubt as to whether it had actually been achieved.

1.
Mary
Terrall
,
The Man Who Flattened the Earth: Maupertuis and the Sciences in the Enlightenment
(
The University of Chicago Press
,
Chicago and London
,
2002
).
2.
Isaac
Newton
,
Philosophae Naturalis Principia Mathematica
, 3rd ed. (1726, reprinted by
University of Glasgow
,
1871
), Liber Tertius, Propositio XIX, Problema III.
3.
Pirooz
Mohazzabi
and
Mark C.
James
, “
Plumb line and the shape of the Earth
,”
Am. J. Phys.
68
,
1038
(
Nov.
2000
).
4.
George
Greenstein
, “
An intuitive approach to Earth’s centrifugal bulge
,”
Phys. Teach.
60
,
369
(
May
2022
).
5.
Larrie D.
Ferreiro
,
Measure of the Earth: The Enlightenment Expedition that Reshaped Our World
(
Basic Books
,
New York
,
2011
).
6.
Larrie D.
Ferreiro
, personal communication.
7.
4eme Discourse en vers sur l’hommme (1737).
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