THE FIGURE OF THE EARTH. 23 



amounting to ^-J^, which is very nearly equal to that yield- 

 ed by the measurements of a degree of latitude (-^17). 



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

 termining the figure of the earth (or, in other words, the re- 

 lation 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 equa- 

 tor toward the pole. The Arabian astronomers, and more 

 especially Ebn-Juuis, at the close of the tenth century, and 

 during the brilliant epoch of the Abbassidian Califs,* 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. t 

 The pendulum, in conjunction with a system of wheels used 

 to regulate the clocks (which were first employed in the im- 

 perfect experiments of Sanctorius at Padau, in 1612, and 

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

 gave the first material proof of the different intensity of gravi- 

 ty at different latitudes in Kicher's comparison of the beats of 

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

 Picard was, indeed, engaged in the equipment of this import- 

 ant voyage, but he does not on that account assume to him- 

 self the merit of its first suggestion. Richer left Paris in 

 October, 1671 ; and Picard, in the description of his meas- 

 urement of a degree of latitude, which appeared in the same 

 year,1: merely refers to " a conjecture which was advanced 



* Cosmos, vol. i., p. 166. Edward Bernard, an Englishman, was 

 the first who recognized the application of the isochronism of pendu- 

 lum-oscillations 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.) 



f Freret de V Etude de la Philosophic Ancienne in the Mem. de VAcad. 

 des Inscr., t. xviii. (1753), p. 100. 



X Picard, Mesure de la Terre, 1671, Art. 4. It is scarcely probable 

 that the conjecture which was advanced in the Paris Academy even 

 before the year 1671, to the effect that the intensity of gravity varies 

 with the latitude (Lalande, Astronomic, t. iii., p. 20 § 2668), should 

 have been made by the illustrious Huygens, who had certainly pre- 

 sented his Discours sur la Cause de la Gravite to the Academy in the 

 course of the year 16G9. There is no mention made in this treatise 

 of the shortening of the seconds-pendulum, which was being observed 

 by Richer at Cayenne, although a reference to it occurs in the supple- 

 ments to this work (one of which must have been completed after the 

 publication of Newton's Principia, and consequently later than 1687). 

 Huygens writes as follows: "Maxima pars hujus libelli scripta est, 

 cum Lutetian degerem (to 1681) ad eum usque locum, ubi de altera- 

 tione, quje pendulis accidit e motu Terra?." Sec also the explanation 



