In most cases, however, in consequence of having assumed the wave to have originated on a coast line, when the evidence clearly showed it to have originated some distance out at sea, the calculations which have been made are open to criticism. The average depths which I obtained for various lines across the Pacific appear to be somewhat less than the average depth as given by actual soundings. We must, however, remember that the common error in actual soundings is that they are usually too great, it being difficult in deep-sea sounding to determine when the lead actually reaches the bottom. Until oceans have been more thoroughly surveyed with the improved forms of sounding apparatus, we shall not be able to verify the truth of the results which have been given to us by earthquake waves.
Examples of Calculations on Sea Waves.
1. The wave of 1854.—This wave originated near Japan, and it was recorded on tide gauges at San Francisco, San Diego, and Astoria.
On December 23, at 9.15. a.m., a strong shock was felt at Simoda in Japan, which, at 10 o’clock, was followed by a large wave thirty feet in height. The rising and falling of the water continued until noon. Half an hour after, the movement became more violent than before. At 2.15 p.m. this agitation decreased, and at 3 p.m. it was comparatively slow. Altogether there were five large waves.
On December 23 and 25, unusual waves were recorded upon the self-registering tide gauges at San Francisco, San Diego, and Astoria.
At San Francisco three sets of waves were observed. The average time of oscillation of one of the first set was thirty-five minutes, whilst one of the second and third sets was almost thirty-one minutes.
At San Diego three series of waves were also shown, but with average times of oscillation of from four to two minutes shorter than the waves at San Francisco.
The San Francisco waves appear to indicate a recurrence of the same phenomena.
The record at San Diego shows what was probably the effect of a series of impulses, the heights increasing to the third wave, then diminishing, then once more renewed, after which it died away.
The result of calculations based on these data were:—