DENSITY OF 1 HE EARTH. 83 



the superficies of that part of the outer crust of the globe 

 which is not covered by water, probably scarcely amounts to 

 between 2*4 and 2'6. If, with Rigaud, we take the ratio of 

 the dry land to the water-covered surface as 10 : 27, and 

 remember that ocean soundings have given a depth or stratum 

 of water of more than 27000 English feet/ it will follow 

 that the mean density of the external portion of our planet, 

 consisting partly of land and partly and more extensively of 

 water, scarcely attains the density of 1*5. It is certainly in 

 correct, as a celebrated geometrician, Plana, has remarked, 

 that the author of the " Mecanique celeste" should have 

 ascribed to the exterior terrestrial crust the density of granite, 

 which Density has itself been rated rather highly by Laplace 

 at =3*0, ( 28 ) whence he obtained the density of the centre 

 of the earth= 10-047. The latter is, according to Plana/ 

 16-27 if we take the upper strata at = l'83 (which differs 

 little from 1*5 or 1'6 as the density of the total crust).' 

 The horizontal pendulum (the torsion balance) may, indeed, ; 

 have been called, as well as the vertical pendulum, a geo- 

 gnostical instrument ; but the geology of the inaccessible 

 internal parts of the earth is, like the astrognosy of " dark 

 heavenly bodies," only to be treated with great caution. In 

 the volcanic section of this work, I shall have to touch 

 on questions which have been propounded by others/ 

 respecting currents in the generally fluid interior of the 

 earth, the probability or improbability of a movement of 

 periodical ebb and flood in detached basins not entirely 

 filled, and on the existence of spaces of less density beneath 

 upheaved mountain-chains. ( 29 ) We ought not to omit in 

 the " Cosmos" any consideration to which actual observa- 

 tions, or not remote analogies, appear to lead. 

 VOL. IY. D 



