In the open ocean, and at a distance from the land, the tide wave is
imperceptible, and the rise and fall of the water is small. Among the
islands of the Pacific four to six feet is the usual spring rise. But
the range is considerably affected by local causes, as by the shoaling
of the water and the narrowing of the channel, or by the channel
opening to the free entrance of the tide wave. In such cases the range
of tide is 40 to 50 feet or more, and the tidal stream is one of great
velocity. It may under such circumstances even present the peculiar
phenomenon called the _bore_--a wave that comes rolling in with the
first of flood, and, with a foaming crest, rushes onward, threatening
destruction to shipping, and sweeping away all impediments lying in
its course.

It is certain that in the open ocean the _great tide wave_ could not
be recognized as a wave, since it is merely a temporary alteration of
the sea level.

_Waves_ which have their origin in the action of the wind striking the
surface of the water commence as a series of small and slow
undulations or wavelets--a mere ripple. As the strength, and
consequently the pressure, of the wind increases, waves are formed;
and a numerical relation exists between the length of a wave, its
velocity of progress, and the depth of the water in which it travels.

The _height_ of a wave is measured from trough to crest; and though
waves as seen from the deck of a small vessel appear to be "enormous"
and "overwhelming," their height, in an ordinary gale, in deep water,
does not exceed 15 to 20 feet. In a very heavy gale of some days'
continuance they will, of course, be much higher.