370 POPULAR SCIENCE MONTHLY. 



clinals. Such attacks, striking the earth's crust from within, occasion 

 most earthquakes, especially violent, destructive, deep-seated out- 

 bursts like those of Lisbon and Charleston. The relation of the seis- 

 mic and the volcanic phenomena is clearly to be seen. 



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 pen- 

 dulum apparatus which represent these movements form an inex- 

 tricable 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 ex- 

 tremely 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, how- 

 ever 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 spher- 

 ical 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 ma- 

 terials, 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 Mal- 

 let'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 an- 



