EARTHQUAKE PHENOMENA. 411 



ternatic form. The greatest shocks are not the most instructive, 

 except as to secondary effects; but every great shock is usually fol- 

 lowed by several smaller; the first should therefore be viewed as a 

 "notice to observe " the latter carefully. Earthquakes must not be 

 confounded, either with the forces producing permanent elevations of 

 the land, or with these elevations themselves. "An earthquake, 

 however great, is incapable of producing any permanent elevation or 

 depression of the land whatever, (unless as secondary effects;) its 

 functions of elevation and depression are limited solely to the sudden 

 rise, and as immediate fall, of that limited portion of the surface 

 through which the great wave is actually passing momentarily." 

 The one class of phenomena must be held as distinct from the other 

 as the rise and fall of the tide is distinct from the momentary and 

 local change of sea-level produced by the waves of its surface. 



The phenomena of every earthquake may be divided into — 1st. 

 Primary, or those which properby belong to the transit of the wave 

 or waves through the solid or watery crust of the earth, the air, &c. ; 

 2d. Secondary, or the effects produced by this transit; and both 

 must be kept distinct from co-existent forces, such as those of 

 volcanic eruption, permanent elevation or depression of land, &c, 

 which, however closely they may be connected with the originating 

 impulse of the earthquake, form no true part of it, though they 

 usually complicate its phenomena. 



The centre of impulse, or origin of earthquakes, is generally con- 

 ceived to be at and due to a sudden volcanic outburst, or sudden 

 upheaval or depression of a limited area, or sudden fracture of bent 

 and strained strata, or probably to the sudden formation of steam 

 from water previously in a state of repulsion from highly heated 

 surfaces, (spheroidal state,) and which may or may not be again sud- 

 denly condensed under pressure of sea-water, or possibly to the 

 evolution of steam through fissures and its irregular and per solium 

 condensation under pressure of sea-water. This origin should be 

 carefully sought for as to its nature and position. 



An earthquake may have its origin either inland or at sea; and as 

 this may be, a different set of phenomena will present themselves. 

 In the former case we may expect, in the following order : 1st. The 

 Great Earth-ivave, or true shod', a real roll or undulation of the sur- 

 face travelling with immense velocity outwards in every direction 

 from the point vertically above the centre of impulse. If this be at 

 a small depth below the surface, the shock will be felt principally 

 horizontally; but if the origin be profound, the shock will be felt 

 more or less vertically; and in this case also we may be able to 

 notice two distinct waves, a greater and a less, following each other 

 very rapidly: the first due to the originating normal wave; the 

 second to the transversal waves vibrating at right angles to it. If 

 we can find the point of the surface vertically over the origin, and 

 the direction of emergence of the shock at a distant point, or the 

 angles of emergence at two distant points, neither of which is 

 vertically over the origin — i. e., in one coseismal line — we can find 

 the depth of the origin from the surface by methods pointed out in 



