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I. §6.] THREE ELEMENTS OF A WAVE. 11
The dotted line is that in which the horizontal plane forming the surface of the sea when at rest cuts the plane of the figure; it is called the level-line. The distance between the two extreme points of the wave, measured along this line, is called the length of the wave. C is the highest point of the crest DCB; E the lowest point of the trough A ED. CF and EG are vertical straight lines through C and E; HCK and LEM are horizontal straight lines through the same two points. The vertical distance between the lines HK and LM is called the amplitude of the wave. Thus AB is the length of the wave, and, if we produce EG and CF to cut the lines HK and LM in N and P respectively, we have, for its amplitude, either of the equal lines EN, PC. Each of these is clearly equal to FC and GE together, that is to say the amplitude of the wave is equal to the height of the crest above the level-line together with the depth of the trough below it. In addition to the length and amplitude of the wave, we have one more element, its form. The wave in the figure has its crest shorter than its trough and higher than its trough is deep. Moreover the part DC of the crest is steeper than the part CB, while in the trough the parts AE and ED are nearly equally steep. Sea-waves have infinitely various shapes dependent on the character of the |
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