The Clauses and Phenomena of Earthquakes. 61 



impulse is upwards, whilst the motion is propagated in every 

 direction. 



These conclusions are derived from a series of lengthened ex- 

 periments on waves, undertaken by a Committee, consisting of 

 Sir John Robinson, K.H., and Mr. Scott Russell, F.R.S., who had 

 to resolve, among other questions, — 1st. The nature of the waves 

 of the sea, and 2nd. Whether the tidal wave is of similar order. 

 Up to 1834, nothing certain was known on the subject, and the 

 true history of the tidal wave dates from 1835. 



The great earthquake sea wave, therefore, arises from the earth 

 wave ; and is one of two that occur in the ocean during violent 

 shocks ; the other called the "forced sea wave," being produced by 

 vibration of the sea bottom in contact with the water. In case 

 the shock occurs near a coast, or in shallow water, the " forced 

 wave" will merge in the " great sea wave," or ride on its back ; and 

 if the latter takes place in a deep ocean it might not be noticed 

 far out at sea, but would break violently upon any obstacle such 

 as a shallow shore, or other impediment. 



The " great sea wave," therefore having arisen from the earth 

 wave, in its propagation, is dependent on the laws that regulate 

 water under such circumstances ; but it may be taken as a 

 measure (not true, perhaps, but to some extent, approximative) 

 of the velocity of transmission. Thus, according to Milne, in 

 1755, the Lisbon shock was transmitted to Madeira in 38 minutes, 

 at the rate of 13 miles per minute, whilst the sea wave reached 

 it at the rate of 3 '7 miles per minute. Its rate at Cadiz was 

 nearly the same. The times of the shock at Lisbon and Madeira 

 differed 29 minutes; in 1761, however, the difference was only 6 

 minutes, the point of disturbance in the latter year being four 

 degrees north of the focus in 1755. The disturbance in 1761 

 was deeper than that of 1755. 



During a severe shock at Lisbon, on 2nd February, 1816, two 

 ships at sea had the earhquake — the one 120 leagues W.S.W. 

 from Lisbon, in two minutes ; and the other, 270 leagues W.S,W., 

 in six minutes from the time it was felt in Lisbon, which gives the 

 respective rates of 180 and 135 miles per minute. 



It is said bhat the disturbance by the sea wave, during the late 

 Sandwiea Island eruption, was transmitted to the coast of Cali- 

 fornia in five hours, nearly 400 miles per hour, and that it 

 affected a continuous coast line of nearly 1000 miles in extent. 

 A similar wave passed from Japan toCalifornia, during the earth- 

 quake of 23rd December, 1854, in 12 hours 38 minutes, at the 

 rate of from 355 to 368 miles per hour. By this a Russian frigate 

 was wrecked at Samoda. The sea wave of translation, which was 

 occasioned by the earthquake of Valdivia on 7th November, 

 1837, made itself felt in the Samoan or Navigator's Islands, a 

 distance of at least 1000 miles more than that from Hawaii to 



