sounds of the plates parallel to these planes become equal, and the two modes of division gradually transform themselves into each other, by turning round two fixed points, which, for this reason, I have called nodal centres.
9th. The numbers of vibrations are only indirectly connected with the modes of division, since two similar nodal figures, as in No. 3, fig. 8, and in No. 3, fig. 14, are accompanied by very different sounds; whilst, on the other side, the same sounds are produced on the occurrence of very different figures, as is the case for No. 5 of fig. 8.
10th. Lastly, a more general consequence which may be deduced from the different facts we have just examined is, that when a circular plate does not possess the same properties in every direction, or, in other words, when the parts of which it consists are not symmetrically arranged round its centre, the modes of division of which it is susceptible assume positions determined by the peculiar structure of the body; and that each mode of division, considered separately, may always, subject however to alternations more or less considerable, establish themselves in two positions equally determined, so that it may be said that, in heterogeneous circular plates, all the modes of division are double.
By the aid of these data, which are no doubt still very few and imperfect, a notion may be formed, to a certain point, of the elastic state of crystallized bodies, by submitting them to the same mode of investigation: this is what we have attempted for rock crystal, in a series of experiments which will be the subject of § iii. of this Memoir.
