Figure 2. — This drawing, from Richer's Observations astronomiques et physiques jaites en I'isle de 

 Ca'ienne (Paris, 1679), shows most of the astronomical instruments used by Richer, namely, 

 one of the two pendulum clocks made by Thuret, the 20-foot and the 5-foot telescopes and 

 the large quadrant. The figure may be intended as a portrait of Richer. This drawing 

 was done by Sebastian Le Clerc, a young illustrator who made many illustrations of the 

 early work of the Paris Academy. 



Figure of the Earth 



A principal contribution of the pendulum as a 

 physical instrument has been the determination of the 

 figure of the earth. 5 That the earth is spherical in 

 form was accepted doctrine among the ancient 

 Greeks. Pythagoras is said to have been the first to 

 describe the earth as a sphere, and this view was 

 adopted by Eudoxus and Aristotle. 



The Alexandrian scientist Eratosthenes made the 

 first estimate of the diameter and circumference of 

 a supposedly spherical earth by an astronomical- 

 geodetic method. He measured the angle between 

 the directions of the rays of the sun at Alexandria 

 and Syene (Aswan), Egypt, and estimated the dis- 

 tance between these places from the length of time 

 required by a caravan of camels to travel between 

 them. From the central angle corresponding to the 

 arc on the surface, he calculated the radius and hence 

 the circumference of the earth. A second measure- 

 ment was undertaken by Posidonius, who measured 

 the altitudes of stars at Alexandria and Rhodes and 

 estimated the distance between them from the time 

 required to sail from one place to the other. 



With the decline of classical antiquity, the doctrine 

 of the spherical shape of the earth was lost, and only 



5 The historical events reported in the present section are 

 from Airy, "Figure of the Earth." 



one investigation, that by the Arabs under Calif 

 Al-Mamun in A.D. 827, is recorded until the 16th 

 century. In 1525, the French mathematician Fernel 

 measured the length of a degree of latitude between 

 Paris and Amiens by the revolutions of the wheels of 

 his carriage, the circumference of which he had deter- 

 mined. In England, Norwood in 1635 measured 

 the length of an arc between London and York with a 

 chain. An important forward step in geodesy was 

 the measurement of distance by triangulation, first 

 by Tycho Brahe, in Denmark, and later, in 1615, by 

 VVillebrord Snell, in Holland. 



Of historic importance, was the use of telescopes in 

 the triangulation for the measurement of a degree of 

 arc by the Abbe Jean Picard in 1669. 6 He had been 

 commissioned by the newly established Academy of 

 Sciences to measure an arc corresponding to an angle 

 of 1°, 22', 55" of the meridian between Amiens 

 and Malvoisine, near Paris. Picard proposed to the 

 Academy the measurement of the meridian of Paris 

 through all of France, and this project was supported 

 by Colbert, who obtained the approval of the King. 

 In 1684, Giovanni-Domenico Cassini and De la Hire 

 commenced a trigonometrical measure of an arc 

 south of Paris; subsequently, Jacques Cassini, the son 



8 Abbe Jean Picard, La Mesure de la terre (Paris, 1671). 

 John \V. Olmsted, "The 'Application' of Telescopes to Astro- 

 nomical Instruments, 1667-1669," Isis (1949), vol. 40, p. 213. 



306 



BULLETIN 240: CONTRIBUTIONS FROM THE MUSEUM OF HISTORY AND TECHNOLOGY 



