THE FIGURE OF THE EARTH. 25 



and New York, Dunkirk, and Barcelona on the other, 

 have, however, by their more exact results shown that 

 the contrary is the case, as I have already elsewhere in- 

 dicated. 22 



From the above data, it follows that the pendulum (al- 

 though it is by no means an unimportant instrument in 

 geognostic observations, being as it were a sort of plummet 

 cast into the deep and unseen strata of the earth) does not 

 determine the form of our planet with the same exactitude 

 is the measurement of a degree, or the movements of our 

 satellite. The concentric, elliptical, and individually homo- 

 geneous strata, which increase in density according to certain 

 functions of distance from the surface towards the centre of 

 the earth, may give rise to local fluctuations in the intensity 

 of gravity at individual points of the earth's surface, which 

 differ according to the character, position, and density of the 

 several points. If the conditions which produce these devi- 

 ations are much more recent than the consolidation of the 



22 Cosmos, vol. i, p. 161. Eduard Schmidt (Mathem. und Phys. Geo- 

 graphic, Th. i, s. 394), has separated from a large number of the pen- 

 dulum observations which were made on board the corvettes Descubierta 

 and Atrevida, under the command of Malaspina, those thirteen stations 

 which belong to the southern hemisphere, from which he obtained a 

 mean compression of ^TO-'ST- Mathieu obtained ^g-W fr m a compa- 

 rison of Lacaille's observations at the Cape of Good Hope and the Isle 

 of France with Paris, but the instruments of measurement used at that 

 day did not afford the same certainty as we now obtain by the appli- 

 ances of Borda and Kater, and the more modern methods of observa- 

 tion. The present would seem a fitting place to notice the beautiful 

 experiments of Foucault, which afford so high a proof of the ingenuity 

 of the inventor, and by which we obtain ocular evidence of the rotation 

 of the earth on its axis by means of the pendulum, whose plane of 

 vibration slowly rotates from east to west. (Comptes rendus de I'Acad. 

 des Sc., Seance du 3 Fevrier, 1851, t. xxxii, p. 135). Experiments for 

 noticing the deviation towards the east in observations of falling 

 bodies, dropped from church towers or into mines, as suggested by 

 Benzenberg and Reich, require a very great height, whilst Foucault's 

 apparatus makes the effects of the earth's rotation perceptible with a 

 pendulum only six feet long. We must not confound the phenomena 

 . which may be explained by rotation (as, for instance, Richer's clock 

 experiments at Cayenne, diurnal aberration, the deviation of projectiles, 

 trade winds, etc.), with those that may at any time be produced by 

 Foucault's apparatus, .and of which the members of the Academia del 

 Cimento appear to have had some idea, although they did not farther 

 develope it ^A.utinori, in the Comptes rendus, t. xxxii, p. 635). 



