THE FIGURE OF THE EARTH. 25 



Heard' S measurement of a degree. In an age bo remarkable 

 for the successful emulation that distinguished the cultivators 

 of science, and when theoretical views led to the prosecution 

 of observations which, by their results, reacted in their turn 

 upon theory, it is of great interest to the history of the math- 

 ematical establishment of physical astronomy that individual 

 epochs should be determined with accuracy. 



Although direct measurements of meridian and parallel 

 degrees (the former especially in the case of the French meas- 

 urement of a degree* between the latitudes 44° 42' and 47° 

 30', and the latter by the comparison of points lying to the 

 east and west of the Italian and Maritime Alps)f exhibit 

 great deviations from the mean ellipsoidal figure of the earth, 

 the variations in the amount of ellipticity given by pendulum 

 lengths (taken at different geographical points and in differ- 

 ent groups) are very much more striking. The determina- 

 tion of the figure of the earth obtained from the 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 con- 

 solidation from a fluid state, and that no later alterations can 

 have taken place in its density. $ Notwithstanding 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 Spitsbergen in 79° 

 50' north latitude, to the Falkland Islands, in 51° 35' south 

 latitude, where Freycinet, Duperrey, and Sir James Ross 

 successively made their observations, only from Q5 to 70 ir- 

 regularly scattered points § at which the length of the simple 



* Delambre, Base du Syst. Metrique, t. hi., p. 548. 



f Cosmos, vol. i., p. 167. Plana, Operations Geodesiques et Astrono- 

 miques pour la Mesure cVun Arc du Parallele Moyen, t. ii., p. 847 ; 

 Carlini in the Effemeridi Astronomiche di Milano per I anno 1842, p. 57. 



X Compare Biot, Astronomic Physique, t. ii., 1844, p. 464, with Cos- 

 mos, vol. i., p. 168, and vol. iv., p. 105, 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., 

 § 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. 



§ 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 



Vol. V.— B 



