EARTHQUAKE PHENOMENA. 429 



its height, its amplitude or length, its velocity, and direction of 

 translation. The height to be taken above the plane of repose of 

 the fluid, and referred to half-tide level. These waves, when on 

 their grandest scale, defy any methods of direct admeasurement; 

 but observations of their results, such as the height to which they 

 have reached on mural faces of roc£, or on such buildings, &c, as 

 may have withstood them, or eye-sight observations made at the 

 moment of transit of the crest of the wave cutting distant objects, 

 should not be omitted. When of a manageable size the height of 

 the crest may be pretty closely obtained by the traces on wharfs, 

 buildings, <fcc, or on posts or piles driven into the littoral bottom. 

 It may be taken from any convenient fixed points of level, and all 

 ultimately referred to half-tide as the datum for all earthquake obser- 

 vations as to level. 



The sextant may be occasionally used to get the elevation of the 

 crest of the passing wave, several observers making a simultaneous 

 observation of an expected wave. The velocity of the wave may be 

 got by noticing from a suitable position, by a seconds watch, the time 

 of its transit inwards between two distant points having water 

 between them whose depth is or may be known. Islands oft" the land 

 are advantageous posts for this purpose. Where tide-gauges can be 

 established they afford the best means of recording all the conditions 

 of these waves when of a manageable height. The state of the tide 

 at the time of their occurrence, and the general nature of the local 

 establishment, with the in and off shore currents, should be ascertained. 



The length of the wave (while entire) should be sought for by 

 a similar method; a knowledge of its length and of the depth of 

 water infers its height. There are two indirect methods by which 

 the dimensions of the great sea-wave may be pretty accurately deter- 

 mined: First. The distance to which solid bodies before at rest are 

 translated by the passage of the wave over them is about equal to 

 its length or amplitude; so that when we can obtain evidence of the 

 distance to which. a large loose rock, for example, whose precise posi- 

 tion was before known, has been carried, we approximate to one 

 dimension of the wave. Secondly. The depth of water at the point 

 where the wave is first observed to break, when capable of being 

 accurately found, gives the height of the wave, which is here equal 

 to the depth of the soundings. This breaking point and depth should 

 always be anxiously tried for. Besides the dimensions of the wave, 

 observations should be made, on the interval of time, after the great 

 earth-wave, or shock, and before the great sea-wave comes in, reck- 

 oning from the commencement of the shock. When more than one 

 great sea- wave comes in, the precise number of successive waves and 

 the intervals in time of their recurrence should be noted; also, what 

 are their relative dimensions; what changes are observable in the direc- 

 tions whence they arrive at the same point of coast, and what are the 

 several in-coming directions at various points along a great stretch of 

 coast, (the latter must be had usually from collected testimony;) what 

 reflux from the beach before or after the coming in of the wave; after 

 the wave has come in and broken, what oscillatory waves are pro- 



