M.A 



1901.] 



KNOWLEDGE. 



99 



crests of the 600 feet undulation and throe crests of the 

 1150 feet undulation. The resulting wave surface of 

 the sea in this case would at any one moment show a 

 series of ridges and hollows differing from one another 

 in wave length and amplitude. This series would be 

 followed by other scries, identical with it. The third 

 (lowest) line of the diagram is a portion of a series. 

 It contains five wave crests, i.e.. the same number as the 

 shorter constituent wave contains. Measiu-ing from 

 the diagram I find that, reckoning from the left, the 

 succeeding waves have — 



Amplitude 38'85 feet. Length 575 feet. Lengtli 62') feet. 



Amplitude 27'5 feet. .Vmplitude 30 feet. 

 Length .575 feet Length 62.") feet. 



Amplitude 27o feet. Amplitude 35 feet. 



The average wave length is 600 feet, precisely that of 

 the shorter constituent, the average amplitude is 

 29.77 feet, which is (within the errors of measurement) 

 identical with the amplitude of the same wave. The 

 record of measurement of waves in such a sea as usually 

 published, that is to say, giving the average 

 dimensions of a series of waves, would give no indication 

 of the existence of the long swell. It would perhaps be 

 just visible if the light were good as something running 

 faster than the waves, not unlike the shadow of a 

 passing cloud upon the water, but I think its magni- 

 tude would be much under-estimated. This swell does 

 not greatly afiFect the amount of surface disturbance, 

 though it renders it more iiTegular ; but at a depth where 

 the effect of the 600 foot wave is no longer felt the 

 heave of the 11.50 foot swell is still strong. Thus, 

 although the existence of the long swell may be barely 

 discernible by the eye in a storm, and although the 

 recorded (average) wave measurement may not reveal 

 Its presence, yet the sea is really in a very different 

 state when such a swell is running from what it would 

 be if affected only by the recorded wave of 600 foot 

 length and 30 foot amplitiide. In drawing figures in 

 which the amplitude of the swell is made equal to that 

 of the storm wave I do not intend to assert that this is 

 the usual condition ; the intention is rather to show 

 how even a great swell is masked by the shorter wave. 



I am trying in these articles to tell what is known 

 about the size of ocean waves, and how that knowledge 

 has been obtained. I have now reached the point, to 

 which one comes sooner or later in almost eve'.-y enquiry, 

 when it is advisable to look more closely into the mean- 

 ing of the word which designates the thing investigated. 

 For research in natural phenomena I prefer this plan to 

 that of beginning with a definition. We find the sur- 

 face of the sea covered with a series of ridges and 

 furrows which are not uniform on the one hand, 

 but have on the other hand only a moderate 

 range of size. There are of course wavelets also 

 present, but these are easily distinguished from 

 the greater waves which we want to measure. The 

 difference in size between succeeding ridges seems 

 to be mainly due to the existence of two or more sets 

 o£ undulations each of which may be regular. An 

 illustration of this may bp obtained by watching one 

 wave crest as it advances. The changes of form which 

 it undergoes are readily understood on the supposition 

 that one billow is catching up and passing another. 

 These changes are the same as those shown in the 

 figures in passing from crest to crest of the combined 

 wave. Xow if we want to know the size of ocean waves 

 it is evident that we must make up our minds whether 

 we mean the size of the ridges and furrows which at 



the time actually constitute the surface of the ocean, 

 or, on the other hand, the sizc^ of the constituent undula- 

 tions the superposition of which constitutes those ridges 

 and furrows. 



The actual ridge or mound of water is not only the 

 chief visible phenomenon but it is also a tangible, often 

 a terrible reality, which the sailor calls " a sea.' On 

 the other hand the constituent undulations are what 

 chiefly receive the attention of the theoretical man, and 

 they have in some respects a greater individuality than 

 the " seas," for they retain each their length and speed, 

 and, sorting themselves as they travel beyond the 

 storm area, they partition out the ocean among them, 

 the longest and swiftest coming to the front, the slower 

 and shoi'ter lagging behind. 



In the systematic records of the size of ocean waves, 

 writers have generally attempted to follow the pro- 

 cedure of the theoretical man in this matter. 

 Thus Monsieur Bertin, in his excellent Memoir oti 

 the E.rperimental Study of Waves.l says, " We are 

 certain in adopting 16 metres (52i feet) as the maximum 

 limit of height to have got beyond all the observed 

 values. I must further remark that I only speak 

 of waves in the open sea, and of those belonging to a 

 single swell. An isolated rock 25 or 30 metres (82 to 

 98 feet) high may be covered by a breaking sea. Waves 

 belonging to different systems of swell may ride one over 

 the other, giving rise to topping seas without speed, 

 and with a short period, or to any other irregular and 

 exceptional agitation " 



In December, 1900, I crossed from Liverpool to 

 Boston in ss. Ivernia, and heavy weather was met 

 with. Quoting from the report sent in to the U. S. 

 Hydrographic Office, we had on December 6th strong 

 westerly winds with high sea; December 7th, strong 

 squally wind, increasing to strong gale, with heavy 

 regular sea; December 8th, strong gale, S.W., W., 

 N.W. ; December 9th, X.W. wind, moderate to force 6, 

 and backing W.S.W., increasing to force 8 ; December 

 10th, fresh gale, with frequent squalls, wind hauling to 

 N.X.E. and X.E., short high sea; December 11th, wind 

 west, having hauled S. to S.S.W., increasing with hard 

 snow squalls and falling thermometer, short high sea ; 

 December 12th, moderate gale to moderate breeze at mid- 

 night; December 13th, wind force 6-7, with occasional 

 snow squalls, sea rough. 



I paid particular attention to the question of the 

 height of the waves, and tried to consider the matter 

 without bias. It seemed to me quite as important to 

 know the height of the larger seas occurring from time 

 to time as the average height of the waves. Thus the 

 average height of the waves on December 8th was less 

 than the elevation of the lower deck, but the lower 

 deck on the weather side was unsafe on account 

 of the occasional big " seas," of which I mea- 

 sured one or two of 40 feet or upwards. It 

 is these larger seas which rivet the attention, 

 and remain in the recollection of a spectator. I 

 do not think anyone could fairly be blamed for saying 

 that on this occasion the ship met with waves about 

 40 feet high, for they were not seen only once but 

 many times, yet, as well as I could guess, the 

 averacje height of the waves was not more than 20-25 

 feet. I think it probable that when we hear from sea- 

 men that they have known the waves in a storm 40 feet, 

 or it may be 50 or 60 feet, in height, we must interpret 



t Institution of Naval Arfhitects. April, 1873. 



