36 cosmos. 



if we assume that of the upper strata ^1*83, which differs 

 but slightly from the total density of 1-5 or 1°8 of the earth's 

 crust. The vertical pendulum, no less than the horizontal 

 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 unillumin- 

 ated celestial bodies, to be received with considerable cau- 

 tion. In a portion of my work, which treats of volcanic 

 phenomena, I can not wholly pass in silence those problems 

 which have been suggested by other inquirers in reference to 

 the currents pervading the general fluid in the interior of 

 our planet, or the probable or improbable periodically ebb- 

 ing and flowing movement in individual and imperfectly filled 

 basins, or the existence of portions of space, having a very 



specific weight of granite can not 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, while the other constituents of this rock, 

 namely, quartz and feldspar, are 2-56 and 2*65. Even oligoclase is 

 only 2-68. If hornblende rises as high as 3 - 17, syenite, in which feld- 

 spar always predominates, never rises above 2*8. As argillaceous 

 schist varies from 2*69 to 2*78, while pure dolomite, lying below lime- 

 stone, equals only 2*88, chalk 2*72, and gypsum and rock-salt 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 24. Laplace, on the supposition that the earth's density in- 

 creases in arithmetical progression from the surface toward the cen- 

 tre, 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 obtained by Keich (5-577) and by Baily (5-660) ; this 

 deviation being much greater than could be accounted for by the prob- 

 able error of observation. In a recent discussion on the hypothesis 

 of Laplace, which will soon form a very interesting paper in Schu- 

 macher's Astr. 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, as well as the ellipticity, within the limits 

 that seem probable for the latter value, may be very closely approxi- 

 mated to. "If the compressibility of the substances of which the 

 earth is formed," writes the Turin geometrician, "has given rise to 

 regular strata nearly elliptical in form, and having a density which 

 increases from the surface toward 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 determ- 

 ined upon submitting it to a new investigation." 



