THE FIGURE OF THE EARTH. 27 



pendulum results, when compared with those of Sabine, cer- 

 tainly give -j^jy for the compression of the whole northern 

 quadrant ; but when separated into two halves, they yield a 

 still more varying result, giving -^g from the equator to 

 45°, and -jig- from 45° to the pole.* It has been shown in 

 many instances, and in both hemispheres, that there is an 

 appreciable influence exerted by surrounding denser rocks 

 (basalt, green-stone, diorite, and melaphyre, in opposition to 

 specifically lighter secondary and tertiary formations), in the 

 same manner as volcanic islandsf influence gravity and aug- 

 ment its intensity. Many of the anomalies which presented 

 themselves in these observations do not, however, admit of 

 being explained by any visible geological characters of the 

 soil. 



For the southern hemisphere we possess a small number 

 of admirable, but very widely-diffused observations, made by 

 Freycinet, Duperrey, Fallows, Liitke, Brisbane, and Rumker. 



* Op. cit., p. 488. Sabine [Expcr. for determining the Variation in 

 the Length of the Pendulum vibrating Seconds, 1825, p. 352) finds ^-g^.^ 

 from all the thirteen stations of his pendulum expedition, notwith- 

 standing their great distances from one another in the northern hem- 

 isphere ; and from these, increased by all the pendulum stations of the 

 British survey and of the French geodetic measurement from Formen- 

 tera to Dunkirk, comprising, therefore, in all a comparison of twenty- 

 five points of observation, he again found ^-^-.-y • It is still more strik- 

 ing, as was already observed by Admiral "Liitke, that far to the west 

 of the Atlantic region, in the meridians of Petropawlowski and New 

 Ai - changel, the pendulum lengths yield a much greater ellipticity, 

 namely, ^j- As tne previously applied theory of the influence of the 

 air surrounding the pendulum led to an error in the calculation, and 

 had rendered a correction necessary as early as 1786 (when a some- 

 what obscure one was given by the Chevalier de BuatJ, on account of 

 the difference in the loss of weight of solid bodies, when they are either 

 at rest in a fluid, or impelled in a vibratory motion, Bessel, with his 

 usual analytical clearness, laid down the following axiom in his Unter- 

 suchungen iiber die Lange des einfachen Seciindenpendels, s. 32, 63, 126- 

 129 : "When a body is moving in a fluid (the atmosphere), the latter 

 belongs with it to the moved system, and the moving force must be 

 distributed not only over the particles of the solid moved body, but 

 also over all the moved particles of the fluid." On the experiments 

 of Sabine and Baily, which originated in Bessel's practically import- 

 ant pendulum correction (reduction to a vacuum), see John Herschel 

 in the Memoir of Francis Bail//, 1815, p. 17-21. 



t Cosmos, vol. i., p. 167. Compare, for the phenomena occurring 

 in islands, Sabine, Pend. Exper., 1825, p. 237 ; and Liitke, Obs. du Pen- 

 dule invariable, execntees de 1826-1829, p. 241. This work contains a 

 remarkable table, p. 239, on the nature of the rocks occurring at 16 

 pendulum stations, from .Melville Island (79° 50' N. lat.) to Valparai- 

 so (32° 2' S. lat.). 



