One series of seismic phenomena remains to be explained--the lighter undulations, the tremors, and the remarkable irregularity of the movements of the ground. The indications of the vertical pendulum apparatus which represent these movements form an inextricable tangle of lines running over and crossing one another. The late Japanese professor of seismology, Sekiya, prepared an enlarged model of the tracings of the seismic movements of a point of the earth's surface, which has been much copied. It represents an extremely confusing vibration of the lines. Now we have to confront a very important fact which adds much to the difficulty of seismic research. We never feel and observe the earthquake shocks themselves, never directly in their simplicity or multiplicity, but only the wave movements that are sent out from them in the elastic crust of the earth. These, however multifold their origin, proceed in an immense spherical wave which moves in more or less numerous repetitions through the earth's interior. It is this shaking of the earth by the spherical waves that our instruments represent as earthquakes. We can not include as the earth's crust the surface of the earth on which we live, and which consists of loose materials disintegrated by weathering, breaking, and numerous causes, but the solid crust, often lying at a considerable distance beneath us, which bears these materials, and from which the spherical waves emerge. As the waves of the sea, beating upon the coast, are turned, split up, divided, thrown up, etc., in their surging, so surge, too, the seismic waves upon the disintegrated surface of shingle, pebbles, broken rocks, sand, and earth, in clefts and gorges. We thus never observe the original spherical waves, but only their fragmentary derivative forms, their resolution into numerous single waves which come to us diverted into the most various directions. It is thus most plainly shown that Mallet's effort to determine the center and origin of the earthquake from the direction of the shock was futile. We can only draw scientific conclusions respecting the time of beginning, the duration, and force of the movement. It is thus evident that many of the tremors (not all, by any means) originate in this division; that a fixed point of the earth's surface must describe a very complicated path in so intricate a wave movement; that the division is less marked on firm ground than on loose; that the former, in consequence of the more evenly protracted movement, is less dangerous than the latter; and that multiplied waves interfere, overlay, weaken, or strengthen one another just as water waves do. Thus are explained the earthquake bridges or spots which always remain unmoved through repeated earthquakes, either because they are firmer, or because the progress of the waves is arrested at them by interference.