of the Atlantic, extending from lat. 20° to 10° S, Captain Toynbee
observes that "swells of the sea are not always caused by the
prevailing wind of the neighborhood. For instance, during the northern
winter and spring months, northwesterly swells abound. They are
sometimes long and heavy, and extend to the most southern limit of the
district. Again, during the southern winter and spring months,
southerly and southwesterly swells abound, extending at times to the
most northern limit of the district. They are frequently very heavy
and long."

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