60 The Causes and Phenomena of Earthquakes. 



but will represent what is called a tidal bore. If the elevation 

 be slow, the wave will represent a breaker. Now, such waves 

 are exhibited daily in our harbour, when a steamer coasts along 

 the shore. The disturbance occasioned by the action of the 

 paddle-wheels causes the water in advance of the vessel to rush 

 towards it, in order to fill up the room of the displaced water ; 

 and immediately the vessel has passed, a wave rushes in from 

 behind, breaking along the beach or dashing against the rocks. I 

 remember once at the time of the Carlist war in Spain (the date 

 I forget), passing in a rapidly moving steamer, where a large body 

 of recruits were assembled on the river bank close by. As the 

 steamer passed, the water rose behind her, rushed up the bank, 

 and dashed over them, thoroughly wetting all, and throwing some 

 over in great confusion. This was, in fact, only a small repre- 

 sentation of a sea earthquake wave, such as we heard of in the 

 account of the eruption at Mauna Loa, in May, and again on 17th 

 August, at New Zealand, the shape of the front of which depends, 

 in all cases, on the depth or shallowness of the water. 



Mr. Darwin long ago explained the sea earthquake in a similar 

 way. 



Mr. Milne (Ed. Ph. Journ., xxxi., p. 269) observes, — That when 

 the sea earthquake wave is studied in examples, it is found that 

 the sea everywhere has first retired. The elevation of the sea 

 level has the effect of drawing the adjoining water towards it, thus 

 lowering the level, and for the same reason the advance of the 

 wave must be heralded by depression. This justifies what I have 

 to say about an earthquake on 15th August, 1868, as deduced from 

 the recession of the sea at Lyttelton in New Zealand, the date of 

 our late marine disturbance in New South "Wales. 



I need not point out that this sea earthquake wave is not what 

 is generally called " the tidal wave" though it has somethiug in 

 common with it. Much misunderstanding appears to prevail, as 

 we learn from recent newspaper correspondence, on this point. 

 The " tidal wave," that from which the ordinary tides result, is 

 different m its character from all other waves, inasmuch as it is 

 not an oscillatory icave. It results from a lifting action of sun and 

 moon, which raises the particles of water, transfers them to a 

 different locality in advance, and leaves them there permanently 

 at rest, without oscillation or retrogression. The motion is all in 

 one direction, and the transference extends through the whole 

 depth. It has, therefore, been called the great primary wave, or 

 wave of translatio?i. The ordinary sea waves of the surface oscil- 

 late, and their displacements do not reach great depths. They 

 are greatest at the surface and diminish rapidly. 



This solitary progressive elevation of the surface of a fluid at 

 rest, is not represented by the sea earthquake wave, inasmuch as 

 the effect produced is by the raising of the sea bottom, and the 



