So long as the velocity of the wind remains constant, the velocity of the current which the wind-waves generate is less than that of the wind, and there is always a differential movement of the water, each layer moving faster than the one beneath. The friction is thus distributed through the whole vertical column of the water in movement, and is even borne in part by the sea-bottom if the movement extends so far down. The greater the depth, the smaller the share of the friction each layer of water is called upon to bear, and the greater the velocity of the current generated by a given wind. But while the wave-motion extends indefinitely downward, the lower limit of agitation effective in erosion is soon reached. Engineering operations have shown that submarine structures are little disturbed at depths of five meters in the Mediterranean and eight meters in the Atlantic.[153] On the other hand, débris as coarse as gravel, which is transported by rolling on the bottom, is not infrequently carried out to depths of 50 feet, and sometimes even to 150 feet. Fine sediment, like silt, is disturbed at still greater depths, for ripple-marks, which indicate agitation of the water, are said to have been found at depths of 100 fathoms.[154] When a wave approaches a shelving shore, its habit is changed. The velocity of the undulation is diminished, while the velocity of the advancing particle of water in the crest is increased; the wave-length, measured from trough to trough, is diminished, and the wave-height is increased; the crest becomes acute, with the front steeper than the back, and these changes culminate in the breaking of the crest, when the undulation proper ceases. Waves of a given height break in about the same depth of water, and the line along which incoming waves break is the line of _breakers_. The line of breakers is in deeper water and farther from shore when the waves are strong than when they are weak. Waves are reported to have broken in 100 fathoms of water,[155] but this must be regarded as very exceptional. The return of the water thrown forward in the crests of waves is accomplished by a current along the bottom called the _undertow_. The undertow is sensibly normal to the coast when uninfluenced by oblique waves, and is efficient in removing the products of erosion.