22 COSMOS. 



increase or decrease of gravity (intensity of local attraction), 

 assumes that gravity at the surface of our rotating spheroid 

 must have remained the same as it was at the time of our 

 earth's consolidation from a fluid state, and that no later 

 alterations can have taken place in its density. 17 Not- 

 withstanding the great improvements which have been made 

 in the instruments and methods of measurement by Borda, 

 Kater, and Bessel, there are at present in both hemispheres, 

 from Spitzbergen in 79 50' N.L., to the Falkland Islands, in 

 51 35' S.L., where Freycinet, Duperrey, and Sir James Ross 

 successively made their observations, only from 65 to 70 

 irregularly scattered points, 18 at which the length of the 

 simple pendulum has been determined with as much accu- 

 racy as the position of the place in respect to its latitude, 

 longitude, and elevation above the level of the sea. 



The pendulum experiments made by the French astrono- 

 mers on the measured part of a meridian arc, and the obser- 

 vations of Captain Kater in the trigonometrical survey of 

 Great Britain concurred, in showing that the results do not 

 individually admit of being referred to a variation of gravity 

 proportional to the square of the sine of the latitude. On this 

 account the English Government determined, at the sugges- 

 tion of the Vice-President of the Royal Society, Davies 

 Gilbert, to fit out a scientific expedition, which was en- 

 trusted to my friend Edward Sabine, who had accompanied 

 Captain Parry on his first polar voyage in the capacity of 

 astronomer. In the course of this voyage, which was con- 

 tinued through the years 1822 and 1823, he coasted along 



17 Compare Biot, Astronomic Physique, t. ii, 1844, p. 464, with Cosmos, 

 vol. i, p. 160, and vol. iv, p. 427, where I have considered the difficulties 

 presented by a comparison of the periods of rotation of planets, and 

 their observed compression. Schubert (Astron. Th. iii, s. 316) has 

 also drawn attention to this difficulty, and Bessel in his treatise On 

 Mass and Weight says expressly, that the supposition of the invariability 

 of gravity at any one point of observation has been rendered somewhat 

 uncertain by the recent experiments made on the slow upheaval of large 

 portions of the earth's surface. 



18 Airy in his admirable treatise on the Figure of the Earth (Encycl. 

 Metropol. 1849, p. 229) reckoned fifty different stations where trust- 

 worthy results had been obtained up to the year 1830, and fourteen 

 others, (those of Bouguer, Legentil, Lacaille, Maupertuis and La 

 Croyere), which however do not bear comparison with the former iu 

 point of accuracy. 



