24 COSMOS. 



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

 appreciable influence exerted by surrroimding denser rocks, 

 (basalt, greenstone, diorite, and melaphyre, in opposition to 

 specifically lighter secondary and tertiary formations,) in the 

 same manner as volcanic islands 21 influence gravity and 

 augment its intensity. Many of the anomalies which pre- 

 sented 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, Liifcke, Brisbane and Bumker. 

 These observations have confirmed a fact which had been 

 strikingly demonstrated in the northern hemisphere, namely, 

 that the intensity of gravity is not the same for all places 

 having the same latitude, and that the increase of gravity 

 from the equator towards the poles appears to be subjected 

 to different laws under different meridians. Although the 

 pendulum measurements made by Lacaille at the Cape of 

 Good Hope, and those conducted in the Spanish circumnavi- 

 gating expedition by Malaspina, may have led to the belief 

 that the southern hemisphere is in general much more com- 

 pressed than the northern, comparisons made between the 

 Falkland Islands and New Holland on the one hand, 



led to an error in the calculation, and had rendered a correction neces- 

 sary as early as 1786, (when a somewhat obscure one was given by tha 

 Chevalier de Buat,) 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 Untersuchungen iiber die Lange des einfachen 

 Secundertpendels, 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 important pendulum correction (reduction to a 

 vacuum), see JohnHerschelin the Memoir of Francis Baily, 1845, pp. 

 1721. 



21 Cosmos, vol. i, p. 159. Compare, for the phenomena occurring in 

 islands, Sabine Pend. Exper. 1825, p. 237, and Liitke, Obs. du Pendule 

 invariable, exScuUes 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 Spitsbergen (79 50' N. Lat.) to Valparaiso 

 (33 2' S. Lat.). 



