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.

Scoresby has observed them 30 ft. high in the North Atlantic; and Ross measured waves of 22 ft. in the South Atlantic.  Wilkes records 32 ft. in the Pacific.  But the highest waves have been reported off the Cape of Good Hope and Cape Horn, where they have been observed, on rare occasions, from 30 to 40 ft high; and 36 ft. has been given as the admeasurement in the Bay of Biscay, under very exceptional circumstances.  In the voyage round the world the Venus and Bonite record a maximum of 27 ft., while the Novara found the maximum to be 35 ft.  But waves of 12 to 14 ft. in shallow seas are often more trying than those of larger dimensions in deeper water.  It is generally assumed that a distance from crest to crest of 150 to 350 ft. in the storm wave gives a velocity (in the change of form) of from 17 to 28 miles per hour.  But what is required in the computation of the velocity is the period of passage between two crests.  Thus a distance of 500 to 600 ft. between two crests, and a period of 10 to 11 seconds, indicates a velocity of 34 miles per hour.

The following table, by Sir G.B.  Airy (late Astronomer Royal), shows the velocities with which waves of given lengths travel in water of certain depth: