428 EARTHQUAKE PHENOMENA. 



both. The observation of these should never be neglected, though 

 rather belonging to geology proper. The half-tide level must in all 

 cases be taken as the datum-plane for all questions of level, and 

 opportunities diligently sought for along beaches, quays, wharfs, or 

 inland along mill-streams or irrigating channels, &c, where altera- 

 tions of level may be trustworthily evinced by changes of depth or 

 run of water. Occasionally local, but widely-extended, permanent 

 elevations or depressions accompany earthquakes, which seem to 

 result from lateral compression, and not from direct elevatory forces. 

 These should be distinguished from the preceding. 



Rivers are stated to have sometimes run dry during earthquakes, 

 and again begun to flow after the shock. This is presumed to arise 

 either from the transit of an earth-wave along their courses up stream, 

 thus damming off their sources, or from sudden elevation of the land, 

 and as sudden depression. Where well observed, however, it has 

 nearly always been found due to sudden damming up by falls of rock 

 or earth at narrow points of their courses, the , debris being soon 

 afterwards swept away. 



Observations of the forced sea-wave, whether produced by the 

 earth-wave going out to sea or coming in from it, will be nearly the 

 same. It is desirable to find its height above the surface of repose 

 referred to half-tide level, and its length or amplitude; but from the 

 extreme rapidity of its production and cessation, or conversion into 

 small oscillatory waves lapping on the beach, and its generally small 

 altitude, observations are extremely difficult; they are only possible 

 when the surface of the sea is perfectly calm, and then must be left 

 to the skill of the observer in taking advantage of local circumstances, 

 and of evidence as to the visible circumstances of this wave, which 

 occurs at the instant the shock is felt. 



Observations of the ivaves of sound through the earth, the sea of 

 fresh water, and the air, are indicated pretty fully by the description 

 of these waves already given. The sound-wave through the earth 

 travels probably at the same rate as the shocl^ or earth- wave; it is, 

 in fact, the slioch (or its fractures) heard. Notice if any and what 

 sound is heard before, along with, or after the shock is felt. An 

 observer, putting one ear in close contact with the earth, and closing 

 the other, will hear the sound-wave through the earth separate from 

 that through the air, and thus hear sounds otherwise inaudible. So, 

 also, an observer immersed in the sea will hear the sound-wave 

 through it, sometimes without any complication of that through the 

 earth. 



An exact description of the character and loudness of the sounds 

 heard, and the places in an extensive district where each was heard 

 loudest and faintest, with the nature of the rock formations at these 

 spots, should be noted. The duration of the sound from first to last, 

 through either medium, accompanying each shock, is important. 

 Circumstances of a character analogous to those upon which the 

 rumbling and reverberation of thunder depend, may affect these 

 sounds transmitted through the earth and thence to the air. 



Observations on the great sea-wave should embrace, for each wave, 



