i70 COSMOS. 



last IS the most certain, since it is independent of the difficult 

 determination of the density of the mineral masses of which 

 the spherical segment of the mountain consists near which the 

 observations are made. According to the most recent experi- 

 ments of Reich, the result obtained is 5-44 ; that is to say, the 

 mean density of the whole Earth is 5*44 times greater than 

 that of pure water. As, according to the nature of the min- 

 eralogical strata constituting the dry continental part of the 

 Earth's surface, the mean density of this portion scarcely 

 amounts to 2*7, and the density of the dry and liquid surface 

 conjointly to scarcely 1-6, it follows that the elhptical un- 

 equally compressed layers of the interior must greatly increase 

 m density toward the center, either through pressure or owing 

 to the heterogeneous nature of the substances. Here again 

 we see that the vertical, as well as the horizontally vibrating 

 pendulum, may justly be termed a geognostical instrument. 



The results obtained by the employment of an instrument 

 of this kind have led celebrated physicists, according to the 

 difference of the hypothesis from which they started, to adopt 



tions on mountains, 4-837 (Carlini's observations on Mount Cenis com 

 pared with Biot's observations at Bordeaux, Effemer. Astron. di Milano, 

 1824, p. 184); (3.) by the torsion balance used by Cavendish, vv^ith an 

 apparatus originally devised by Mitchell, 5-48 (according to Button's 

 revision of the calculation, 5"32, and according to that of Eduard 

 Schmidt, 5-52; Lekrbuck der Math. Geographic, bd. i., s. 487); by the 

 torsion balance, according to Reich, 5-44. In the calculation of these 

 experiments of Professor Reich, which have been made with masterly 

 accuracy, the original mean result was 5*43 (with a probable error of 

 only 0-0233), a result which, being increased by the quantity by which 

 the Earth's centrifugal force diminishes the force of gravity for the lati- 

 tude of Freiberg (SO'^ 55'), becomes changed to 5*44. The employ 

 ment of cast iron instead of lead has not presented any sensible differ 

 euce, or none exceeding the limits of errors of observation, hence dis» 

 closing no traces of magnetic influences. (Reich, Versuche uber die mitt- 

 lere Dichtigheit der Erde, 1838, s. 60, 62, and 66.) By the assumptioc 

 of too slight a degree of ellipticity of the Earth, and by the uncertainty 

 of the estimations regarding the density of rocks on its surface, the 

 mean density of the Earth, as deduced from experiments on and near 

 mountains, was found about one sixth smaller than it really is, name- 

 ly, 4-761 (Laplace, Mican. Celeste, t. v., p. 46), or 4-785. (Eduard 

 Schmidt, Lehrb. d€r Math. Geogr., bd. i., $ 387 und 418.) On Hailey'a 

 hypothesis of the Earth being a hollow sphere (noticed in page 171), 

 which was the germ of Franklin's ideas concerning earthquakes, see 

 Philos. Trans, for the year 1693, vol. xvii., p. 563 {On the Structure of 

 the Internal Parts of the Earth, and the concave habited Arch of the 

 Shell). Halley regarded it as more worthy of the Creator " that the 

 Earth, like a house of several stories, should be inhabited both without 

 and within. For light in the hollow sphere (p. 57 * ) provision might in 

 •ome mannor be contrived." 



