Everything, therefore, that has been permanently disturbed by an earthquake shock has been thus moved in the direction and with the maximum velocity impressed upon it by the wave particle in the first semiphase of the wave; and thus almost everything that has been so disturbed may, by the application of established dynamical principles, be made to give us more or less information as to the velocity of the wave particle (or as we, for shortness, say, the velocity of shock), the direction of its normal vibration, and the position and depth beneath the earth's surface, from which came the generating impulse. We thus arrive at these as simply and as surely as we can infer from the position taken by a billiard ball, on which certain forces are known to have acted, the forces themselves and their direction; or, from a broken beam, the pressure or the blow which fractured it. It is obvious, then, that nearly every object disturbed, dislocated, fractured or overthrown by an earthquake shock is a sort of natural seismometer, and the best and surest of all seismometers, if we only make a judicious choice of the objects which being found after such a shock, we shall employ for our purpose. This was the principle which I proposed to the Royal Society at once to apply to the effects of the then quite recent great Neapolitan Earthquake of 1857, and which, through the liberality and aid of that body, I was enabled to employ with the result I had pretty confidently anticipated, namely, the ascertainment of the approximate depth of the focus. _Every_ shock-disturbed object in an earthquake-shaken country is capable of giving _some_ information as to the shock that acted upon it; but it needs a careful choice, and some mechanical νους, to select _proper_ and the best objects, so as to avoid the needless perplexity of disturbing forces _not_ proper to the shock, or other complications. When properly chosen, these natural seismometers, or evidences fitted for observation after the shock, are of two great classes, by which the conditions of the earthquake motion are discoverable: 1. Fractures or dislocations (chiefly in the masonry of buildings), which afford two principal sources and sorts of information, namely: _a._ From the observed _directions of fractures or fissures_, by which the _wave path_, and frequently the _angle of emergence_, may be immediately inferred. _b._ Information from the preceding, united with known conditions as to the strength of materials to resist _fracture_, by which the _velocity_ of the fracturing impulse may be calculated. 2. The overthrow or the projection, or both, of bodies large or small, simple or complex. From these we are enabled to infer: _c._ By direct observation, the _direction in azimuth_ of the wave path.