_Mallet's Estimate of the Depth of the Focus._--Mallet measured the angle of emergence at twenty-six places, the mean angle (_i.e._ the mean of the greatest and least observed angles) varying from 72° at Vietri di Potenza and 70° at Pertosa, which are about two miles from the calculated epicentre, to 11-1/2° at Salerno, distant about 40 miles. Fig. 13 reproduces part of the diagram on which he plotted the mean angle of emergence at different places. The horizontal line represents the level of the sea, and the vertical line one passing through the epicentre and focus, called by Mallet the "seismic vertical." The lines on the left-hand side represent the commencing wave-paths (assumed straight) to the observing stations situated to the westward of the meridian through the epicentre, those on the right-hand side corresponding to places to the eastward of the same meridian. Small horizontal marks are added to indicate the depth in miles below the level of the sea. [Illustration: FIG. 13.--Diagram of wave-paths at seismic vertical of Neopolitan earthquake. (_Mallet._)] It will be seen, from this diagram, that all the wave-paths start from the seismic vertical at depths between three and nine miles; but the points of departure are clustered thickly within a portion, the length of which is about 3-1/2 miles and the mean depth about 6-1/2 miles. So great was Mallet's confidence in these calculations that he assigns the diverging origin of the wave-paths to different points of the focus, and thus concludes that, while the mean depth of the focus was about 6-1/2 miles, its dimensions in a vertical direction did not exceed 3-1/2 miles. How far Mallet's results should be accepted as correct, it is difficult to say in our ignorance of the constitution of the earth's interior. There can be no doubt that the focus was of considerable size, and that, in consequence, the wave-paths would diverge from different points of it. But that each wave-path should actually intersect the focus, and so enable its magnitude to be determined, would surely involve an approach to some law connecting the direction of a wave-path with the depth of its own origin, and no such law seems to be ascertainable. Nor can the limitation of these apparent origins between certain depths be held to argue that the focus, or any part of it, was equally confined, for the wave-paths would to a great extent be similarly refracted. I fear that the only conclusions that we can with safety draw from Mallet's admirable work are that his figures indicate the order of magnitude both of the vertical dimensions and of the mean depth of the focus. NATURE OF THE SHOCK.