August 19, 1920] 



NATURE 



777 



Growth of Waves. 



There has always b^en some difficulty in accounting 

 for the growth of waves under the action of wind. 

 Do the individual waves grow in length, or does the 

 wind raise waves of all lengths which separate in 

 virtue of the dependence of wave-velocity on wave- 

 length? The late Lord Rayleigh was in favour of the 

 latter hypothesis, but I believe that the true explana- 

 tion is that the waves do not increase in length unless 

 they are breaking. 



'Ihe excess of energy supplied by the wind to the 

 water beyond that which can be carried in an un- 

 broken wave is expended partly in causing local tur- 

 bulence (ultimately converted into heat) and partly in 

 producing a surface current in the direction of the 

 travel of the wave. In effect, this surface current 

 increases the wave-velocity; and since the addition 

 to the current by each wave depends on the time for 

 which that wave has been in existence, the waves first 

 formed will, after the lapse of time, be travelling 



-ter than the more recently formed waves which 



!low. Thus if waves are set up by wind on a 

 Ijreviously calm water-surface, the wave-length 

 will continuously increase from windward to lee- 

 ward. 



I have made some rough observations on a pond 

 something like looo ft. in length, and found that in 

 a brisk breeze the waves formed at the windward end 

 allowed as ripples of a few 'nches from crest to crest, 

 while at or near the leeward margin the wave-length 

 was about 2 ft. If it is assumed that the wave- 

 length increases regularly, there would be about a 

 thousand crests in the length of the pond, and the 

 gain in length from wave to wave would be about 

 1/ loooth of the mean wave-length. All the waves from 

 the least to the greatest were in a breaking condition. 

 The ripples did not show any foam at their crests, 

 hut it was clear from their shape that they were 



lually breaking. 



There is no satisfactory theory of the shapes 



-umed bv breaking waves. Stokes, in one of his 



rlier papers, showed that the irrotational form of 



i\e cannot have an angle of less than 120° at the 

 I rest (the corresponding limit for the trochoidal wave, 

 i.e. for the cvcloid, is 0°), but he considers that the 

 wave will break before the 120° limit is reached. 



In the problem presented by breaking waves — as, 

 indeed, in most problems relating- to the actual pheno- 

 mena exhibited" by fluids in^ motion — the simple 

 assumptions on which the hydrodynamical theory of 

 text-books rests are insufficient, and experiments are 

 required. 



It would be quite possible to try (say at the Froude 

 tank at the National Phvsical Laboratorv) the effect 

 of a steady artificial wind on a length of several 

 hundred feet of water, and to observe and record the 

 form, length, and velocity of the waves throughout 

 the length of the channel. It would probablv^ be 

 found that the waves were started by the instability 

 due to the discontinuous motion at the boundary of 

 two fluids, and that these waves increased in amplitude 

 only until they began to break, but that after the 

 breaking state was reached the wave-length, as well 

 as the amplitude, increased until there was some ap- 

 proach to equality between the velocity of the wind 

 and the wave. 



I have worked out the results for various assump- 

 tions as to the rate at which the wind can transfer 

 energy to the water, but in the absence of experi- 

 mental data the conclusions are scarcely worth pub- 

 lication. .A. Mai.i.ock. 



q Baring Crescent, Exeter, August 10, 



NO. 2651, VOL. 105] 



The Antarctic Anticyclone. 



In his letter entitled "The Mechanics of the Glacial 

 Anticyclone Illustrated by Experiment " published in 

 Nature for July 22, Prof. Hobbs remarks : '" In all 

 my writings upon the glacial anticyclone I have been 

 at much pains to explain that the domed surface of 

 the ice is essential to the development both of the 

 anticyclone and of the alternating calms and blizzards 

 which record its strophic action." .As, however, one 

 goes on to read the letter one finds that Prof. Hobbs 's 

 explanation demands another "essential," namely, 

 that the domed surface must be cooler than the air 

 in contact with it. Remove this defect of tempera- 

 ture, and the mechanism ceases to act ; reverse it, 

 and the mechanism works in the reverse direction, 

 producing a cyclone instead of an anticyclone. 



Assuming that the Antarctic continent has the 

 domed form postulated by Prof. Hobbs, one might be 

 led to accept his conclusions so far as the winter 

 months are concerned, but what about the summer 

 months? During the summer, with its continuous 

 insolation, the surface of the dome must be at a 

 higher temf>erature than the adjacent air, for there 

 is plenty of evidence that the temperature of a snow 

 surface is very susceptible to solar radiation. The 

 mean amplitude of the daily variation of air-tempeia- 

 ture over the Barrier during November, December, 

 and January was found by Scott's Expedition to be 

 11-5° F., while between November 17-22. 1911, the 

 average amplitude was 20° F., and this with the sun 

 oscillating only between 10° and 35° above the horizon ! 

 If Prof. Hobbs 's theory were correct the Antarctic 

 would have a pronounced monsoon climate, while we 

 know from observations that anticyclonic conditions 

 last throughout the year. G. C. Simpson. 



London. 



Trichodynamics. 



The present writer has had interesting associations 

 since 1915 in various ways with projects for industrial 

 research in the cotton industry and with its actual 

 conduct. In all these the need for a word which 

 would define and describe the field of research peculiar 

 to the textile industries has been intermittently 

 obvious, especially with respect to the processes of 

 spinning and weaving. 



In consequence of this I proposed, in the course 

 of the discussion on industrial research at the 

 tenth International Conference held in Zurich in 

 June last, that the word " trichodvnamics " should 

 be adopted in order to effect this generalisation, 

 together with the related term " trichostatics." The 

 analogy with aerodynamics is obvious, and hence 

 also my justification for suggesting the word. The 

 word itself, is open to question, since, if used in the 

 literal sense, it includes only the hair textiles, e.g. 

 wool and cotton, but the significance. intended is akin 

 to that of the word "capillary," which now conveys a 

 definite meaning independently of actual hairs. 



The chemical and colloidal constitution of textile 

 raw materials, their biology, and the engineering 

 asmcts of tlieir utilisation are fields of study not 

 strictly t>eculiar to the textile industry. On the other 

 hand, the movements and mutual contacts of at- 

 tenunted filaments and the changes which take place 

 in their arrangement as they pass from the tangle 

 of the raw material to their orderly sequence in yarn 

 or cloth, which the proposed names would cover, 

 form a well-defined field of a peculiar kind which 

 awaits phvsical investigation. 



W. Lawrence Balls. 



Edale, Derbyshire, August 11. 



