THE DENSITY OF THE EARTH. 33 



which, moreover, he estimated somewhat highly at 3, which 

 would give him 10.047 for the density of the centre of the 

 earth. 2 * This density would, according to Plana, be 16.27 if 

 we assume that of the upper strata = 1.83, which differs 

 but slightly from the total density of 1 .5 or 1 .6 of the earth's 

 crust. The vertical pendulum, no less than the horizon ;al 

 torsion balance, may certainly be designated as a geognostic 

 instrument; but the geology of the inaccessible parts of the 

 interior of our globe is, like the astrognosy of the unillumi- 

 nated celestial bodies, to be received with considerable cau- 



28 Laplace, Mccanigue Celeste, <k1. de 1846, t. v, p. 57. The mean 

 specific weight of granite cannot be set down at more than 2.7, since 

 the bi-axial white potash-mica, and green uni-axial magnesia-mica range 

 from 2.85 to 3.1, whilst the other constituents of this rock, namely 

 quartz and felspar are 2.56 and 2.65. Even oligoclase is only 2.68. If 

 hornblende rises as high as 3.17, syenite, in which felspar always pre- 

 dominates, never rises above 2.8. As argillaceous schist varies from 

 2.69 to 2.78, while pure dolomite, lying below limestone, equals only 

 2.88, chalk 2.72, and gypsum and rocksalt only 2.3, I consider that 

 the density of those continental parts of the crust of our earth, which 

 are appreciable to us should be placed at 2.6 rather than at 2.4. La- 

 place, on the supposition that the earth's density increases in arith- 

 metical progression from the surface towards the centre, and on the 

 assumption (which is assuredly erroneous) that the density of the 

 upper stratum is equal to 3, has found 4.7647 for the mean density of 

 the- whole earth, which deviates very considerably from the results ob- 

 tained by Reich (5.577) and by Baily (5.660); this deviation being much 

 greater than could be accounted for by the probable error of observa- 

 tion. In a recent discussion on the hypothesis of Laplace, which will 

 soon form a very interesting paper in Schumacher's A str. Nachrichten, 

 Plana has arrived at the result that, by a different method of treating 

 this hypothesis, Reich's mean density of the earth, and the density of 

 the dry and oceanic superficial strata, which I estimated at 1.6, aa 

 well as the ellipticity, within the limits that seem probable for the latter 

 value, may be very closely approximated to. " If the compressibility 

 of the substances of which the earth is formed," writes the Turin geo- 

 metrician, " has given rise to regular strata, nearly elliptical in form, 

 and having a density which increases from the surface towards the 

 centre, we may be allowed to suppose that these strata, in the act of 

 becoming consolidated, have experienced modifications, which, although 

 they are actually very small, are nevertheless large enough to preclude 

 the possibility of our deducing, with all the precision that we could 

 desire, the condition of the solid earth from its prior state of fluidity. 

 This reflection has made me attach the greater weight to the first 

 hypothesis advanced by the author of the Mecanique Celeste, and I 

 have consequently determined upon submitting it to a new investi- 

 gation." 



VOL. V. D 



