278 A Short History of Astronomy [CH. x. 



French arc, about 9 in length, extending from Dunkirk 

 to the Pyrenees, which was measured under the super- 

 intendence 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 Conda- 

 mine (1701-1774), and Louis Godin (1704-1760), with 

 whom two Spanish naval officers also co-operated. 



The expedition started in 1735, ar >d> 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 i 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.* 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 p, 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 miK s 



