THE FIGURE OP THE EAKTH. 19 



here again refer to the happy expression of the discoverer of 

 this method " that an astronomer without leaving his obser- 

 vatory may discover tue individual form of the earth in 

 which he dwells, from the motion of one of the heavenly 

 bodies." After his last revision of the inequalities in the 

 longitude and latitude of our satellite, and by the aid of 

 several thousand observations of Burg, Bouvard, and Burck- 

 hardt, 11 Laplace found by means of his lunar method a 

 compression amounting to ^g-, which is very nearly equal to 

 that yielded by the measurements of a degree of latitude 



(2^9)- 



The vibrations of the pendulum yield a third means of de- 

 termining the figure of the earth (or in other words the 

 relation of the major to the minor axis, on the supposition of 

 our planet being of a spheroidal form), by the elucidation of 

 the law according to which gravity increases from the equator 

 towards the pole. The Arabian astronomers, and more es- 

 pecially Ebn-Junis, at the close of the tenth century, and 

 during the brilliant epoch of the Abbassidian Califs 13 , first 

 employed these vibrations for the determination of time, and 

 after a neglect of six hundred years the same method was 

 again adopted by Galileo, and Father Riccioli at Bologna. 13 

 The pendulum in conjunction with a system of wheels used 

 to regulate the clocks (which were first employed in the 

 imperfect experiments of Sanctorius at Padua in 1612, and 

 then in the more perfect observations of Huygens in 1656), 

 gave the first material proof of the different intensity of gravity 

 at different latitudes in Richer's comparison of the beats of 

 the same astronomical clock at Paris and Cayenne, in 1672. 

 Picard was indeed engaged in the equipment of this im- 

 portant voyage, but he does not on that account assume to 

 himself the merit of its first suggestion. Richer left Paris 



give, for the earth's ellipticity, limiting and widely differing values 

 (5f T and -gfa). Astron. Physique, 3eme ed. t. ii, 1844, p. 463. 



11 Laplace, Mecanique Celeste, e"d. de 1846, t. v. pp. 16, 53. 



l - Cosmos, vol. i, p. 158. Edward Bernard, an Englishman, was the first 

 who recognised the application of the isochronism of pendulum-oscil- 

 lations in the writings of the Arabian astronomers. (See his letter, dated 

 Oxford. April, 1683, and addressed to Dr. Robert Huntington, in 

 Dublin. Philos. Transac. vol. xii, p. 567.) 



13 Frgret de V Etude de la Philosophic Ancienne in the Mem. de I'Acad, 

 des Inscr. t. xviii Q753), p. 100. 



C2 



