As respects the distribution of seismic energy in space of our earth's surface, it is that of bands of variable and of great breadth, with sensible seismic influence extending to from 5° to 15° transversely, which very generally follow:

  1. The lines of elevated tracts which mark and divide the great oceanic or terra-oceanic basins (or saucers, as I have called them, from their shallowness in relation to surface, in this discussion) of the earth's surface.
  2. And in so far as these are frequently the lines of mountain chains, and these latter those of volcanic vents, so the seismic bands are found to follow these likewise. Isolated Volcanoes are found in these bands also.
  3. While sensible seismic influence is generally limited to the average width of the band, paroxysmal efforts are occasionally propagated to great distances transversely beyond that.
  4. The sensible width of the band depends upon the energy developed at each point of the length, and upon the accidental geologic and topographic conditions along the same.
  5. Seismic energy may become sensible at any point of the earth's surface, its efforts being, however, greater and more frequent as the great lines of elevation and of volcanic activity are approached; yet not in the inverse ratio of distance, for many of the most frequently and terribly shaken regions of the earth, as the east shore of the Adriatic, Syria, Asia Minor, Northern India, etc., are at great distances from active Volcanoes.
  6. The surfaces of minimum or of no known disturbance are the central areas of great oceanic or of terra-oceanic basins or saucers, and the greater islands existing in shallow seas.

Space obliges me to pass unnoticed here many minor but not unimportant deductions. The discussions as to distribution in time and space occupy seventy-two pages of this fourth and last Report, the remainder of which (thirty-one pages) embraces the description and mathematical discussion as to seismometers, to which I may refer, as comprising the most complete account of these instruments that has, I believe, been anywhere given.

The appendix to the Report comprises the entire bibliography of Earthquakes collected during those researches, and a concluding chapter on desiderata, and inquiries as to ill-understood phenomena supposed to be connected with Earthquakes.

In 1849-50, I was honoured by the request to draw up the article "Earthquake Phenomena," which has appeared in the first and subsequent editions of the "Admiralty Manual of Scientific Inquiry." Originally the subject was intended to have formed part of the article on Geology, entrusted to Mr. Darwin, who consulted me upon the subject; and upon my representing how much Earthquakes had, within a short time, become matter for the mathematician and physicist, he, with a singleness of eye to science which it is but just to place on record, took the necessary steps with the Admiralty authorities that Earthquakes should form a separate article, and advised its being placed, as it was, in my hands. To record this will, I believe, be sufficient justification for my reference to this article, in which a good deal of information as to Seismometry is to be found.

By recurring to Mr. Hopkins's Report on Earthquake Theory, before remarked upon ("Report of British Association, 1847"), it will be seen that the solutions of the problems which he there gives for finding the depth of focus of shock are founded upon the velocity of propagation of the wave in the interior of the mass, the apparent horizontal velocity and the horizontal direction of propagation at any proposed point being known (p. 82).

By this it appears plainly that at that time Mr. Hopkins supposed that it was the velocity of translation of the wave of shock that did the mischief, and not the velocity of the wave particle, or wave itself. And, further, that the former might be obtained by reference simply to the modulus of elasticity of the rock of any given formation, as, indeed, was my own earliest view when I produced my "Dynamics of Earthquake" in 1846. From the remarks already made as to the vast difference between the actual transit velocity in more or less discontinuous rocks—such as they occur in Nature—it will be equally obvious that Mr. Hopkins's methods, as above mentioned, are impracticable, even were there no confusion between the velocity of translation of the wave and that of the wave particle or wave itself.

This applies also to the demonstration and diagram (taken from Hopkins) given by Professor Phillips ("Vesuvius," pp. 258-259).