Gravitational acceleration increases smoothly from the equator to the oblate shape of the Earth, so at any given latitude ( east-west line ), gravity was constant enough that the length of a seconds pendulum was the same within the measurement capability of the 18th century.
32.
While France ultimately based their metre on the Earth's circumference-- something common folk could never hope to measure with the required accuracy-- Jefferson's system used the seconds pendulum to define the metre ( he called it a yard, I think ).
33.
Although not defined by the pendulum, the final length chosen for the metre, 10 " 7 of the pole-to-equator meridian arc, was very close to the length of the seconds pendulum ( 0.9937 m ), within 0.63 %.
34.
Starting on the assumption that the length of a second pendulum in the latitude of London was 39.2 inches, he deduced that the length of one oscillating forty-two times a minute is eighty inches, while that of one oscillating twice as many times is twenty inches.
35.
There was also something about Jefferson being annoyed that the French had chosen to base the metre on the size of the Earth, which was not even known precisely at the time, rather than the distance a seconds pendulum swang, which anyone who cared to could confirm for themselves.
36.
Virtually all proposals were based on the seconds pendulum, in which each swing ( a half period ) takes one second, which is about a meter ( 39 inches ) long, because by the late 17th century it had become a standard for measuring gravity ( see previous section ).
37.
Two branches of Sabine's work are notable : Determination of the length of the seconds pendulum, a simple pendulum whose time period on the surface of the Earth is two seconds, that is, one second in each direction; and his research on the Earth's magnetic field.
38.
In 1816 he was secretary of a commission charged with ascertaining the precise length of the second's or seconds pendulum ( the length of a pendulum whose period is exactly 2 seconds ), and in 1818 he became secretary to the Board of Longitude and superintendent of the HM Nautical Almanac Office.
39.
Talleyrand resurrected the idea of the seconds pendulum before the Constituent Assembly in 1790, suggesting that the new measure be defined at 45�N ( a latitude that, in France, runs just north of Bordeaux and just south of Grenoble ) : despite the support of the Assembly, and of Great Britain and the newly independent United States, nothing came of Talleyrand's proposal.
40.
Pendulums were so universally used to measure gravity that, in Kater's time, the local strength of gravity was usually expressed not by the value of the acceleration " g " now used, but by the length at that location of the " seconds pendulum ", a pendulum with a period of two seconds, so each swing takes one second.
