OCEAN WAVES THEIR MAGNITUDE VELOCITY AND FORCE The magnitude of waves is generally greatly exaggerated, the expression "mountains high" being often used. But it is proved that even the largest billows do not disturb the water at a depth of more than 300 feet ; the wave itself being only a sixth of this. As we have already explained, the depth of the trough below the general level is always equal to the height of the crest above it. The " height " of a wave is calculated from its trough to its crest, the " width " being the distance between two successive crests. The magnitude of waves depends, of course, on the force and continuance of the wind, and varies from a mere ripple to huge billows of 30 and 40 feet high. The largest waves known are formed during a strong north west gale off the Cape of Good Hope, when they are often 40 feet from trough to crest, while off Cape Horn waves of 30 feet have been frequently observed. In the North Atlantic the highest do not generally exceed 20 to 25 feet, but some 36 feet high have been seen in the Bay of Biscay. In the shallow North Sea, wind-waves scarcely ever exceed 8 to 12 feet in height, though, during very severe storms, exceptionally high waves have been observed : for instance, in 1870 when the breakwater at Wick was destroyed, the waves must have been at least 40 feet high.
The velocity of waves does not imply the rate at which the water travels, but the rapidity with which the undulations are propagated. This of course depends on the force and
velocity of the wind, modified by the depth of water over which the wave advances. It has been proved that there is a definite relation between the magnitude and velocity of a wave and the depth of water in which it is formed. As will be seen from the following, compiled from that given by Sir G. Airy, the Astronomer-Royal, a wave 100 feet broad, passing over water 100 feet deep, has a velocity of 15 miles an hour.
The force with which a wave strikes is directly pro portional to its magnitude and velocity—the larger and swifter the wave the greater its force. Along the British coasts are many striking proofs of the enormous power of large waves. Huge blocks of 30 or 40 tons have been repeatedly displaced ; strongly-built sea walls have been breached or utterly de stroyed ; even lighthouses—whose builders underrated the power of the waves—have been swept away. The actual force has been experimentally ascertained in several places. The . average force on the west coast of Ireland is about 600 lbs. on the square foot, but occasionally, in severe storms, the pres sure amounts to 7,500 lbs., or nearly 3i tons on the square foot 1 The total pressure to which the seaward faces of light houses, embankments, breakwaters, &c. are thus subject to, is therefore enormous : hence the necessity of compactness and great strength in all such structures.