The great forced sea waves, due to earthquakes, and generally to subterranean and volcanic action, have been known to attain the enormous height of 60 feet or more, and sweep to destruction whole towns situated on the shores where they have broken—­as for example Lisbon and places on the west coast of America and in the island of Java.  Though so destructive when they come in toward the land, and begin to feel the shelving sea bottom, it is not probable that, in the open ocean, this wave would do more than appear as a long rolling swell.  It has, however, been observed that “a wave with a gentle front has probably been produced by gentle rise or fall of a part of the sea bottom, while a wave with a steep front has probably been due to a somewhat sudden elevation or depression.  Waves of complicated surface form again would indicate violent oscillations of the bottom.”

The altitude and volume of the great sea wave resulting from an earthquake depend upon the suddenness and extent of the originating disturbance and upon the depth of water at its origin.  Its velocity of translation at the surface of the sea varies with the depth of the sea at any given point, and its form and dimensions depend upon this also, as well as upon the sort of sea room it has to move in.  In deep ocean water, one of these waves may be so long and low as to pass under a ship without being observed, but, as it approaches a sloping shore, its advancing slope becomes steeper, and when the depth of water becomes less than the altitude of the wave, it topples over, and comes ashore as an enormous and overwhelming breaker.

Lastly, there is the storm wave—­the result of the cyclone or hurricane—­and, perhaps, the greatest terror to seamen, for it almost always appears in the character of a heavy cross sea, the period of which is irregular and uncertain.  The disturbance within the area of the cyclone is not confined to the air, but extends also to the ocean, producing first a rolling swell, which eventually culminates in a tremendous pyramidal sea and a series of storm waves, the undulations of which are propagated to an extraordinary distance, behind, before, and on each side of the storm field.

Enough has now been said to show that whatever the character of the waves encountered by the Umbria and Martello in July last, they were in no sense “tidal,” but, if approximating to the dimensions stated, they were either due to storm or earthquake, or, possibly, to a combination of both the last agents.

For those of our readers who may be interested in wave observations, we conclude by introducing Prof.  Stokes’ summary of the method of observing the phenomenon: 

          “For a Ship at Sea.

    “(1.) The apparent periodic time,[2] observed as if the ship
    were at rest.

    “(2.) The true direction from which the waves come, also the
    ship’s true course and speed per hour.