FORM AND CONSTITUTION OF EARTH—STEWART. 165 
He considered, therefore, the discrepancy a proof of the maccuracy 
of the law of the inverse square, and laid aside for the time his inves- 
tigations in that direction. In January, 1672, at a meeting of the 
Royal Society, the result of Picard’s work was mentioned, giving 69.1 
miles as the length of a degree, and Newton was able to revise his 
calculations, with the result that his hypothesis was amply verified. 
This is generally supposed to have led to the publication of the 
Principia, which laid the foundation of gravitational astronomy, 
though some affirm that Newton delayed the publication of his great 
work until he had proved that a spherical body attracts an outside 
body as if all of its matter were concentrated at its center. 
Up to this time the size of the earth had been investigated on the 
supposition that its form is spherical, and that it is therefore only 
necessary to measure the length of a degree on its surface in order to 
determine its dimensions. A discovery was made, however, by 
Richer (1672) which turned the attention of astronomers to the 
possibility that its form may deviate materially from that of a sphere. 
He had been sent by the Academy of Sciences of Paris to the Island 
of Cayenne to make certain astronomical determinations, and while 
there he found that his clock, which had been regulated in Paris to 
keep correct time, lost about two and one-half minutes daily, so that 
it was necessary to shorten the pendulum by one and one-fourth lines 
to make it beat seconds. His report was received with doubt until 
confirmed by the subsequent observations of Halley, Varin, and 
Deshayes on the coasts of Africa and America. The phenomenon 
was first explained by Newton in the third book of the Principia, where 
he showed that it is the result of a decrease in the force of gravity in 
the neighborhood of the equator due to increased distance from the 
center of the earth combined with the effect of centrifugal force. He 
also investigated the figure of the earth and showed that it must be 
an oblate spheroid. As a consequence of this the lengths of the 
degrees of latitude must increase with the latitude. 
Between the years 1684 and 1718, Picard’s triangulation was 
extended by J. and D. Cassini southward as far as Colidoure and 
northward to Dunkirk, making a total amplitude of 8° 31’. The 
northern portion of the arc, having an amplitude of 2° 12”, gave 
56,960 toises as the length of a degree, while the southern portion 
gave 57,097 toises. These results seemed to negative the theoretical 
conclusions reached by Newton in the Principia, and to point to the 
prolate spheroid as representing the true form of the earth. A heated 
controversy arose in consequence, and in the excitement thus occa- 
sioned, as well as from a desire to know the truth of the matter, the 
Academy of Sciences resolved to apply a crucial test of the rival 
theories by measuring a meridian are at the equator and another at 
the Arctic Cirele. 
