“Combing” of Waves

Abstract

ALL who have watched waves breaking on the sea-shore must have noticed the furrowed or “combed” appearance of the back of a wave as it curls over. If the water is not much disturbed by wind, it is seen, on attentive watching, that this “combing” appears suddenly, and begins at the advancing edge of the crest, and spreads backwards. With small waves a foot or so in height and of long extended front, such as are seen in shallow water, it may be observed that the crest, which in this case rolls down the front of the wave, is at first smooth and even, while the back of the wave is also smooth and unfurrowed, but the edge of the crest suddenly becomes crenated, and almost simultaneously the combing appears on the back of the wave, travelling rapidly backwards from the crenated edge. Moreover a considerable length of the wave appears to be similarly affected almost at the same instant. With larger waves, whose crest falls rather than rolls upon the concave front, I have observed that the edge is at first smooth and even, but that it suddenly becomes uneven, and often fringed with a row or rows of drops, and that at the same instant the combing appears. In both cases, if there is much wind, the regularity of the phenomenon is disturbed, and observation is in other ways rendered difficult. The action is so exactly parallel to something which takes place in the splash of drops, and which I have described in detail in a paper recently read before the Royal Society (see Proc. Roy. Soc., No. 218), that I think your readers may be interested in a brief statement, with special reference to this more familiar case of waves, of the explanation there put forward. The explanation amounts to this:—It is well known that a long cylinder of liquid is unstable, and will, if left to itself, at once tend to split into a row of equal, equidistant drops; the splitting being effected by a constriction of the cylinder in certain places, and a bulging out in others. Again, if a mass of liquid is bounded by an edge whose surface is approximately a portion of a long cylinder, there is good reason for supposing that this cylindrical edge will be subject to similar laws of stability, and that it will tend to cleave in the same way, the surface being forced in in certain places, and out in others. Now a wave's crest presents such a cylindrical edge. It will, therefore, of itself, cleave in the way described, and the flow of water will thereby be hindered at the constrictions, and aided at the places of bulging out. Thus lines of easiest flow will be set up, which in their turn will determine the furrows on the back of the wave. The fringe of drops is due to the splitting in a similar manner of the cylindrical jets shot out from the places of bulging, where the flow is aided. Indeed, much of the seething at the edge of a wave is, I think, attributable to the breaking up of such jets in this manner. In the case of the minute phenomenon of a drop splash, I have been able, in some degree, to bring this explanation to the test of measurements, which, so far as they go, quite confirm it. The regularly-toothed edge of a spot of candle-wax that has fallen on a cold object, affords in a permanent form a familiar illustration of the same action.