During these motions there were several maxima.
These few examples of the motions of waters, without any record of the motions of the ground, at the time of the Lisbon earthquake, must be taken as examples of a very large number of similar observations of which we have detailed accounts.
Like agitations, it must also be remembered, were perceived in North America and in Scandinavia, and if the lakes of other distant countries had been provided with sufficiently delicate apparatus, it is not unlikely that similar disturbances would have been recorded.
Besides these movements in the waters of seas and lakes, at or about the time of great earthquakes, we have records of like movements, which take place as independent phenomena.
Thus we read that on October 22, 1755, the waters of Lake Ontario rose and fell five and a half feet several times in the course of half an hour.[149] On March 31, 1761, Loch Ness rose suddenly for the period of three-quarters of an hour.[150]
As another example of the disturbance of water at the time of a great earthquake in districts where the earthquake was not felt, may be mentioned the swelling of the waters of the Marañon, in 1746, on the night when Callao was overwhelmed.
Sudden variations in the level of the water have been many times observed in the North American lakes. The changes in level which sometimes take place in the Genfer and Boden lakes are supposed to have some relation to the condition of the atmosphere. A rising and falling of especial note took place on April 18, 1855.
In Switzerland these sudden changes are known as ‘seiches’ or ‘rhussen.’
From the observations and calculations of Prof. Forel it would seem that the period of the ‘seiches’ depends upon the dimensions of the lakes; the calculated periods dependent on the depths of the lakes being approximately equal to the observed periods.[151]
W. T. Bingham, writing on the volcanoes of the Hawaiian Islands, remarks that it is not unusual for the sea to be agitated by great and unusual tides, and that such sea waves have not been attended with volcanic eruptions or seismic disturbances. Thus in May 1819 the tide rose and fell thirteen times. On November 7, 1837, there was an ebb and flow of eight feet every twenty-eight minutes. Again, on May 17, 1841, like phenomena, unaccompanied by any other unusual occurrences, were recorded.[152]