French arc, about 9° in length, extending from Dunkirk to the Pyrenees, which was measured under the superintendence of the Cassinis in continuation of Picard's arc, the result being published by J. Cassini in 1720. In neither case, however, were the data sufficiently accurate to justify the conclusion; and the first decisive evidence was obtained by measurement of arcs in places differing far more widely in latitude than any that had hitherto been available. The French Academy organised an expedition to Peru, under the management of three Academicians, Pierre Bouguer (1698-1758), Charles Marie de La Condamine (1701-1774), and Louis Godin (1704-1760), with whom two Spanish naval officers also co-operated.
The expedition started in 1735, and, owing to various difficulties, the work was spread out over nearly ten years. The most important result was the measurement, with very fair accuracy, of an arc of about 3° in length, close to the equator; but a number of pendulum experiments of value were also performed, and a good many miscellaneous additions to knowledge were made.
But while the Peruvian party were still at their work a similar expedition to Lapland, under the Academician Pierre Louis Moreau de Maupertuis (1698-1759), had much more rapidly (1736-7), if somewhat carelessly, effected the measurement of an arc of nearly 1° close to the arctic circle.
From these measurements it resulted that the lengths of a degree of a meridian about latitude 2° S. (Peru), about latitude 47° N. (France) and about latitude 66° N. (Lapland) were respectively 362,800 feet, 364,900 feet, and 367,100 feet.[1] There was therefore clear evidence, from a comparison of any two of these arcs, of an increase of the length of a degree of a meridian as the latitude increases; and the general correctness of Newton's views as against Cassini's was thus definitely established.
The extent to which the earth deviates from a sphere is usually expressed by a fraction known as the ellipticity, which is the difference between the lines c a, c b of fig. 78 divided by the greater of them. From comparison of the three arcs just mentioned several very different values of the
- ↑ 69 miles is 364,320 feet, so that the two northern degrees were a little more and the Peruvian are a little less than 69 miles.
