GEOLOGY. 275 



lost. When, on the other hand, the original impulse comes from the 

 bed of the deep ocean, three sorts of waves are formed and propagated 

 simultaneously ; namely, one, or several successively, through the 

 land, which* constitute the true earthquake shock or shocks ; and co- 

 incident with, and answering to every one of these, one or more waves 

 are formed and propagated through the air, which produce the sound 

 like the bellowing of oxen, the rolling of wagons, or of distant thun- 

 der, accompanying the shock ; and a third wave is formed and propa- 

 gated upon the surface of the ocean, which rolls in to land, and reaches 

 it long after the shock or wave through the solid earth has arrived 

 and spent itself." While the impulse is passing under the deep 

 water of the ocean, it gives no trace of its progress at the surface, 

 in all probability; but as it arrives in soundings, and gets into water 

 more and more shallow, the undulation at the bottom, the crest of the 

 earth-wave, brings along with it, carries upon its back, as it were, 

 a corresponding aqueous undulation upon the surface of the water. 

 This, which, adopting Airy's nomenclature, might be called the forced 

 sea-wave of earthquakes, has no proper motion of its own : it is simply 

 a long, low ridge of water, pushed up at the surface by the partial 

 elevation of the bottom immediately below it, this latter elevation 

 travelling with such immense velocity, that the hillock of water 

 pushed up above it has not time to flow away laterally, and reassume 

 its own level. Thus, then, the earth-wave below, when in shallow 

 ' water, is attended by a small forced sea-wave, vertically over it, upon 

 the surface of the sea, and these two reach the inclined beach or shore 

 at the same moment; but as the beach is so inclined, and the forced 

 sea-wave, as well as the earth-wave, are long and flat, and the velo- 

 city of the latter very great, the earth-wave, as it were, slips from 

 under the forced sea-\vave at the moment of reaching the beach, which 

 it for the moment elevates, by a vertical height equal to its own, and 

 as instantly lets drop again to its former level. 



" Besides the surface ocean-wave, a wave of sound will also be 

 propagated through the water, and reach the land long before the sur- 

 face-wave arrives. The sound of the earth-wave, on the contrary, 

 travels with it, and is heard on shore at the same moment that the 

 shock is felt. It seems hard to believe in this literal w r ave-like mo- 

 tion of rigid earth and rock, yet science teaches that the intermobility 

 of particles is not only possible, but actually takes place. The vibra- 

 tions of the air of a draw T ing-room shake the solid \valls of the house, 

 when a tune is played upon a pianoforte, or otherwise the tune could 

 not be heard in an adjoining house. Captain Kater found that he 

 could not perform his experiments upon the length of the seconds 

 pendulum anywhere in London, for the solid ground everywhere vi- 

 brated by the rolling of carriages," &c. In marshy ground resting 

 on sandstone, the vibrations caused by the passage of a railway train 

 have been perceived at a distance of 1,100 feet laterally, but vertically 

 they cannot be detected through sandstone beyond 100 feet. Houses, 

 towers, and tall chimneys rock with the wind. " Salisbury spire moves 

 to and fro in a gale more than three inches from a plumb line." On 

 removing the props of exhausted coal-seams, the superincumbent mass 



