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: Depth of | Length of the Wave in Feet.[1] the Water| | | | | | | in Feet. | 10 | 100 | 1,000 | 10,000 | 100,000 |1,000,000 |10,000,000 ---------+-----+------+-------+--------+---------+----------+---------- | | Corresponding Velocity of Wave per Hour in Nautical Miles. | 1 | 3.2 | 3.4 | 3.4 | 3.4 | 3.4 | 3.4 | 3.4 10 | 4.3 | 10.1 | 10.7 | 10.8 | 10.8 | 10.8 | 10.8 100 | 4.3 | 13.5 | 32.0 | 34.0 | 34.0 | 34.0 | 34.0 1,000 | 4.3 | 13.5 | 42.9 | 101.8 | 107.5 | 107.5 | 107.5 10,000 | 4.3 | 13.5 | 42.9 | 135.7 | 320.3 | 340.0 | 340.3 100,000 | 4.3 | 13.5 | 42.9 | 135.7 | 429.3 | 1013.0 | 1075.3 ---------+-----+------+-------+--------+---------+----------+---------- [Footnote 1: As an example, this table shows that waves 1,000 feet in length travel 43 nautical miles per hour in water 1,000 feet deep. The length is measured from crest to crest.] From these numbers it appears that-- 1. When the length of the wave is not greater than the depth of the water, the velocity of the wave depends (sensibly) only on its length, and is proportional to the square root of its length.