522 THE POPULAR SCIENCE MONTHLY. 



occur in the following order : Supposing the centre of the disturbance 

 to be beneath the ocean, as at Lisbon, an observer on the shore might 

 expect to experience 



1. The underground rumble, moving at the rate of 8,000 to 10,000 

 feet per second. 



2. The shock, moving from 1,000 to 5,000 _'eet per second. 



3. The sea-wave, moving about 528 feet per second. 



4. Sound, through the air moving at the rate of 1,090 feet per sec- 

 ond. It should be noted, however, that the velocity of the sea-wave 

 depends on depth of water. 



The vibrations of an earthquake, it is evident, differ in no respect 

 from those produced by other causes, excepting in intensity. The jar 

 arising from a discharge of artillery, by a carriage rolling over pave- 

 ments, or slamming of heavy doors, puts in motion a series of moving 

 waves just as truly as does the rending of rocks, or an explosion of 

 steam or gas in a fracture thus produced. But, a question arises, What 

 moves when the earthquake is progressing. The phenomena maybe 

 explained thus : Around the source of disturbance the rock is pressed 

 outward in every direction as air is pressed outward around a vibrating 

 bell, forming what is called a zone or shell of compressed rock. The 

 extent of this compression is the width of the earthquake-wave, and 

 depends on the force exerted and the elasticity of the rock. In each 

 zone or shell there is always a point of maximum density and that is 

 where the energy of compression and the rock's elastic force are equal. 



As the wave passes, another zone is formed, and the particles behind 

 return by their elasticity to their former position. From this it is 

 obvious that, as the wave is passing, the individual particles of the 

 rock have first a forward and then a backward motion a swing or 

 excursion to and fro. The extent of this motion is the amplitude of 

 vibration, and may be very small compared with the breadth of the 

 wave. 



Mallet found by computation that, given a certain depth of fissure, 

 and a certain heat of steam, the expansive force would produce a wave 

 of nine inches amplitude at the surface. His observations of the Nea- 

 politan earthquake of 1857 show that the maximum amplitude at the 

 surface was only 2.5 inches. In his elaborate and beautiful volume on 

 the eruption of Vesuvius, in 1872, just published, Mr. Mallet reaffirms 

 a statement previously made by him, that " it is the vibration of the 

 wave itself, i. e., the motion of the wave-particles, that does the mis- 

 chief, not the transit of the wave from place to place on the surface." 



We understand, then, that there is motion of particles as well as 

 a transit-wave ; that the " travelling zone or shell of vibration " is a 

 zone or shell of "elastic compression." 



Fig. 9 illustrates the " shells " as they move away from B, the 

 focus of disturbance. The transit-wave, with its interstitial vibrations, 

 reaches the surface in the manner shown in the diagram Fig. 10. 



