122 IGNEOUS AGENCIES. 



arrow r, would meet the advancing incident waves moving in the di- 

 rection of the arrow £, and concurrence and interference would be in- 

 evitable.* 



Minor Phenomena. — Not only the several kinds of earthquakes, but 

 many of the minor phenomena are explained by the wave-theory. 



1. Sounds. — These are usually described as a hollow rumbling, roll- 

 ing, or grinding ; sometimes as clashing, thundering, or cannonading. 

 They are probably produced by rupture of the earth at the origin, and 

 by the passage of the wave through the imperfectly elastic rocky me- 

 dium breaking the medium and grinding the broken parts together. 

 But what is especially noteworthy is, that these sounds 'precede as well 

 as accompany the shocks. In every earthquake there are transmitted 

 waves of every variety of size. The great waves are sensible as shocks, 

 or jars, or tremors ; the very small waves, too small to be appreciated 

 as tremors, are heard as sounds. But, as already explained, these last 

 run with greater velocity in an imperfectly coherent medium like the 

 earth, and therefore arrive sooner than the great waves, which consti- 

 tute the shock. The same was observed in Mallet's experiments. 



2. Motion. — As to direction, the observed motion is sometimes verti- 

 cally up and down, sometimes horizontally back and forth, and some- 

 times oblique to the horizon. Almost always a rocking motion, i. e., a 

 leaning of tall objects first in one direction and then in the other, is 

 observed. As to violence or velocity of motion, this is sometimes so 

 great that objects are thrown into the air, and whole cities are shaken 

 down as if they were a mere collection of card-houses ; while in other 

 cases only a slow swinging, or heaving, or gentle rocking, is observed. 



If we confine our attention to the principal or normal wave, the 

 difference in direction is wholly due to the position of the observer. At 

 the epicentrum it is of course vertical, and thence it becomes more and 

 more oblique, until at great distances it is usually horizontal. The 

 violence of the shock or velocity of ground-motion depends partly upon 

 the violence of the original concussion, and partly on the distance from 

 the origin or focus. This velocity of the ground-motion must not be 

 confounded with the velocity of the wave already discussed. The lat- 

 ter is the velocity of transit from place to place ; the former is the 

 velocity of oscillatioyi up and down, or back and forth. The velocity 

 of oscillation has no relation to the velocity of transit, but depends 

 only on the height of the wave, which constantly diminishes and be- 

 comes finally very small, though the velocity of transit remains the 

 same, and always enormously great. The rocking -motion is also easily 



* For an excellent discussion of the effects of interference of earth- waves, see a mem- 

 oir by Prof. John Milne, Transactions of the Seismological Society of Japan, vol. i, Part 

 II, p. 82. 



