Page:Proceedings of the Royal Society of London Vol 4.djvu/140

Rh 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