410 



EARTHQUAKE PHENOMENA. 



homogeneous) with one normal and two transversal vibrations. Ne- 

 glecting the latter for the present, the wave may be imagined trans- 



ferred outwards, in all directions in coneentric spherical shells, whose 

 volume at the same phase of the wave is constant. The interval be- 

 tween any two such shells, therefore, diminishes as r 2 , r being the 

 mean radius, and the overthrowing energy of the shock in the direc- 

 tion of r varies inversely as the square of the distance from the origin. 



The shock reaches the surface at B directly above the origin verti- 

 cally, but for all points around that it emerges with angles getting 

 more and more nearly horizontal as the distance measured on the sur- 

 face increases. The intersecting circle of any one shell with the sur- 

 face, which is that of simultaneous shock, is the coseismal line, or crest 

 of the earth wave, circular (like the circles on a pond into which a 

 stone has been dropped) if in a homogeneous medium, more or less 

 distorted if in a heterogeneous one, such as constitute the various 

 formations of the earth, but always a closed curve. The transversal 

 vibration is transmitted outwards in the normal direction (Ac) more 

 slowly than the normal one, which is one cause of the small jarring 

 impulses often felt after the great shock. (For more complete infor- 

 mation as to the physical and mathematical conditions, see " Jamin, 

 Cour de Physique, 1858," 9th and 10th chapters ; Rankine's "Ap- 

 plied Mechanics," chap. 3, sec. 1, and chap. 5, sec. 4, and passim; 

 Dr. Young's Lectures, "Nat. Phil.," passim, but especially lectures 

 8, 13, 31, 49, and 50; Herschcl, Art. Sound, " Encyc. Metrop. ;" 

 Hopkins, Report, "Brit. Ass. Trans.," 1847-' 48 ; Mallet's Fourth 

 Report. Facts and Theory of Earthquakes, "Brit. Ass. Trans." 

 1857-'58.) 



Observers in earthquake countries should make themselves fa- 

 miliar with the usual features, succession of events, and concomitants 

 which, with a certain sort of regularity, apply to all earthquakes. 

 Mr. R. Mallet's "First Report upon the Facts of Earthquakes," 

 Trans. Brit. Ass. for 1850, gives these in a condensed and sys- 



