A. R. Hunt — Nomenclature of Ripple-mark. 413 



Physicists, on the other hand, regard height as from mean water- 

 •level to crest, unless the height from trough to crest is distinctly 

 •stated. In a letter to me in 1884, the late Professor G. G. Stokes 

 refers to " the elevation or depression above or below mean level " ; 

 and again, " Taking it [the wave] as eight feet above or below 

 mean level in the shoal, sixteen feet from crest to trough in all " 

 (Trans. Devon Assoc, vol. xix, pp. 513, 514). Yet in the paper to 

 which Professor Stokes' letter was appended, I, from force of habit, 

 whenever I used the term height, referred to height from trough to 

 -crest. The one measurement is of course treated as being exactly 

 double the other, though it is not necessarily so with breakers in 

 shallow water. 



What is understood by the ' length ' of a wave ? In the case of 

 sea-waves, which are fairly uniform in size at the same time, 

 English writers mean by ' length ' the distance from crest to crest. 

 If, however, we have to regard a wave as isolated, it has then but 

 one crest, and we may treat the total length as the length of the 

 elevated water added to the length of the depi'essed water, both 

 at the level of repose of the water. What, then, is understood by 

 the ' breadth ' of a wave ? Tliis is a term rarely met with, but 

 would probably mean the extent of a wave measured along its crest. 

 We now come to 'amplitude.' What is the amplitude of a wave? 

 So far as I am aware, ' amplitude ' with physicists is alwa'ys connected 

 with the idea of motion, the amplitude of a wave of ether, air, or 

 water being regarded as the extent of the oscillation or vibration 

 ■of the medium, caused by the passage of a wave. 



In the course of my correspondence with Lord Kayleigh and 

 Sir G. G. Stokes on the question of sea- waves, although both those 

 •eminent physicists supplied me with much information, I do not 

 remember a single instance of their use of the term amplitude. 

 If used for ' height ' it would be redundant, and for shore-breakers 

 inaccurate, as in shallow water the crests contract in length, and 

 the height of the wave from trough to crest greatly exceeds the 

 relative height in deep water, where it is equal to the amplitude 

 of vibration of the water itself. The increasing height of the 

 shortening crest came out clearly in tank experiments. 



We note, then, that even in the well-studied case of sea-waves, 

 which are real waves, we have to be careful that we do not confound 

 the height from mean level with the height from trough, and that 

 we do not confound amplitude with either of those terms. 



A difficulty has arisen owing to the indiscriminate application 

 of the simple terms height, length, and amplitude (which in the 

 case of true waves are technical terms with stereotyped meanings) 

 to forms which simulate waves, such as ripple-mark, snowdrifts, and 

 •even mountains and valleys. And even so, the terms are not always 

 used in the same sense by different authors. It is always necessary 

 to ascertain the exact meaning of each author. 



Professor Forel, in describing his ripples and experiments, speaks 

 •of the ' longeur ' of his tank, and of the ' hauteur ' and ' largeur ' of 

 his ripple-mark, but strictly limits * amplitude ' to the extent of the 



