116 



In this memoir the author investigates the amount of the luni- 

 solar precession and nutation, assuming the earth to consist of a 

 solid spheroidal shell lilled with fluid. For the purpose of present- 

 ing the problem under its most simple form, he first supposes the 

 solid shell to be bounded by a determinate inner spheroidal surface, 

 of which the ellipticity is equal to that of the outer surface ; the 

 change from the solidity of the shell to the fluidity of the included 

 mass being, not gradual, but abrupt. He also here supposes both 

 the shell and the fluid to be homogeneous, and of equal density. 

 The author then gives the statement of the problem which he pro- 

 poses to investigate ; the investigation itself, which occupies the re- 

 mainder of the paper, being wholly analytical, and insusceptible of 

 abridgement. The following, however, are the results to which he 

 is conducted by this laborious process : namely, that, on the hypo- 

 thesis above stated, 1. The Precession will be the same, whatever be 

 the thickness of the shell, as if the whole earth were homogeneous 

 and solid. 2. The Lunar Nutation will be the same as for the homo- 

 geneous spheroid to such a degree of approximation that the dijffer- 

 ence would be inappreciable to observation. 3. The Solar Nutation 

 will be sensibly the same as for the homogeneous spheroid, unless 

 the thickness of the shell be very nearly of a certain value, namely, 

 something less than one quarter of the earth's radius ; in which case 

 this nutation might become much greater than for the solid spheroid. 

 4. In addition to the above motions of precession and nutation, the 

 pole of the earth would have a small circular motion, depending en- 

 tirely on the internal fluidity. The radius of the circle thus de- 

 scribed would be greatest when the thickness of the shell was the 

 least : but the inequality thus produced would not, for the smallest 

 thickness of the shell, exceed a quantity of the same order as the 

 polar nutation, and for any but the most inconsiderable thickness of 

 the shell would be entirely inappreciable to observation. 



In his next communication, the author purposes considering the 

 case in which both the solid shell and the inclosed fluid mass are of 

 variable density. 



" Appercu sur une maniere nouvelle d'envisager la theorie cristal- 

 lographique dans le but d'etablir les rapports de celle-ci avec la forme 

 spherique, on elliptique, des molecules, ainsi qu'avec I'effet des mi- 

 lieux sur la forme cristalline." Par M. L. A. Necker. Communicated 

 by P. M. Roget, M.D., Sec. R.S. 



In this communication, after adverting to Haiiy's theory of cry- 

 stallization, in which the molecules are considered to be polyhedrons, 

 to the views subsequently taken by Wollaston and Davy, and par- 

 ticularly to Brewster's conclusions, that there ought to be difl"erent 

 forms of molecules, some spherical, some elliptical with two 

 equal axes, and a third unequal to these, and others elliptical with 

 three unequal axes ; M. Necker states, that Mr. Dana is the only 

 mineralogist who has attempted to introduce into crystallography 

 the consideration of molecules with curved surfaces. Although, 

 adopting the forms proposed by Brewster, and adding to them those 



