the Superficial Crust of the Earth. 153 



in which a denotes the radius of the stratum (the mean 

 radius of the superficial stratum being supposed equal to 

 unity), and e the constant quantity by which the depth of 

 each successive stratum ; 1 — a is to be multiplied conform- 

 ably to the assumed law of density. Admitting the ellipti- 

 city of the earth to be equal to 000326, Laplace found the 

 value of e to be 2-349, and hence determined the mean den- 

 sity to be 4*764. This value differs considerably from the 

 results which Reich and Baily have deduced from their ex- 

 periments with the batance of torsion ; the former having 

 obtained 5*44, and the latter 5*6604, for the mean density of 

 the terrestrial spheroid, the density of pure water being sup- 

 posed equal to unity. 



The remarks of Humboldt on the density of the superficial 

 stratum of the earth, contained in the first volume of his 

 Kosmos, would seem to imply that the value of this element 

 assumed by Laplace is erroneous. He states, that from the 

 nature of the rocks which constitute the superficial strata of 

 the solid parts of the globe, the density of continents is 

 hardly 2*7 ; and he hence infers, that the mean density of 

 continents and seas taken together does not amount to 1/6. 

 The researches of Plana, contained in the note above referred 

 to, serve to confirm this conclusion. Supposing the ellipti- 

 city of the earth to be represented by 000326 (1 - 0*008479), 

 he has found that the mean density 5*44, and the initial den- 

 sity 1*6, may be satisfactorily accounted for. The ellipticity 

 derived either from actual measurement or from researches 

 on the lunar theory, cannot be regarded as sufficiently trust- 

 worthy to render the value here assumed inadmissible. On 

 the other hand, if the ellipticity be supposed equal to 0*00326, 

 the mean density deducible is 4*76 ; a result which is incom- 

 patible with the precision of the experiments made for the 

 purpose of determining this element. — Proceedings Astron. 

 Soc, Dec. 1852. 



