Geoscience Reference
In-Depth Information
not believe Denis Papin's view that pendulum rods expand if they are taken from
cooler to warmer climates, in spite of the evidence to the contrary supplied by
Picard and La Hire. Picard observed that an iron rod, which in winter during freezing
weather was 1 foot long, was 1 foot and ¼ ligne
14
long when heated by a fire. La
Hire observed that an iron rod which was 6 feet long in the winter, was 6 feet and
2/3 ligne long in the summer.
In 1671 the Academy of Sciences sent Jean Richer to Cayenne, Guyana, South
America, on the equator, to observe the close approach of Mars to the Earth in 1672.
He was instructed to observe its position to establish its distance and thus the scale of
the solar system, but to do so he needed an accurate clock. He took a pendulum that
had beaten seconds in Paris and in Cayenne re-regulated the clock against the motion
of the Sun and stars. Brought straight from Paris and set up again unaltered, off the
boat, the pendulum ran slow and lost two and a half minutes every day. To make it
beat seconds in Cayenne, Richer had to shorten it by 1¼ gnes (about 3 millimeters).
Richer did a good job to establish the credibility of these measurements by repeating
them weekly over nearly a year throughout an annual temperature cycle. This gave
him evidence that the disparity was not due to the temperature difference between
tropical Guyana and temperate France because the discrepancy was too large.
People
Isaac Newton (1643-1727)
Sir Isaac Newton (
Fig. 17
) was born into a Lincolnshire farming family and when, through
lack of interest, he made a mess of managing the estate, he was sent to Trinity College,
Cambridge University to study, becoming a fellow there in 1667. At first he was interested in
pure mathematics and developed what we now know as the calculus. (He carried on an acri-
monious dispute with Gottfried Leibnitz over who had priority for this discovery, Leibnitz
having discovered the same mathematics independently.) He turned to optics and demon-
strated how white light was made up of rays of light of different colors that refracted differently
through a prism and formed a spectrum, ultimately publishing his theories in
Optiks
. In addi-
tion, he invented and constructed a reflecting telescope and developed laws of motion that
explained the motion of bodies (such as balls that impacted or whirled on a string). Turning
his attention to the motion of a planet around the Sun, he conceived of a force of gravity that
acted like string that tied them together and showed that if the force behaved as a function of
distance according to an inverse square law it would give rise to Kepler's laws. He published
his results in a topic that became hailed as a scientific masterpiece, the
Principia
.
In 1699 he moved to London and became Master of the Mint. He spent his later years in
religious and astrological investigations of little scientific interest.
IN HIS Topic
Principia
(Proposition 20 of Book 3), Isaac Newton quoted Richer's
results on the period of a pendulum on the equator and in France. He did so with
approval, commending the “diligence and caution,” which “seem to have been lacking
in other observers.” Newton offered a solution to Richer's discovery that a pendulum
beat slower at the equator than in France; he interpreted his results as being due to
the shape of the Earth, which bulged at the equator it was rotating and was corre-
spondingly squashed at the poles. As a result, gravity was reduced at the equator
and the pendulum beat more slowly.
14
1 French
ligne
= 0.09 inch, or 2.25 mm.
