JlLY, 1901.] 



KNOWLEDGE 



1 ir. 



IllUSTRATED MAGAZI N E 



lMiEN€EJ-lTERATUMA^^ 



Founded by RICHARD A. PROCTOR. 



Vol. xxn.] 



LONDON: JULY, 1901. 



[No. 189. 



CONTENTS. 



^ PAGK 



The Size of Ocean Waves. — IV. Bv Vait.han ConhMsii, 



D.?C.(VK-T.>, F. '?.>., F.G.?., F.B.G.S. (lUllslrtlted) .. 11") 



The Relative Speeds of some Common Birds. By 



Ch.iklks .V. WrrcnElL ... 1+9 



Foun-Hor'ned Sheep. By R. I.tdekkkk (IlhtslraUd) . ir>0 



The Stars near Nova Persei 152 



Nova Persei and Surrounding Stars. {Plate.) 

 Constellation Studies.— VII. The South Circumpolar 



Stars. By E. W.^lter iI.\rXDKR, f.r.a.s. fltluntntledj 152 

 Prof. Adams' Lectures on the Lunar Theory. By 



1*. H. COWBLL ... l.")4 



Letters : 



Thb Orbit op the Mooy. By Sir Samckl Wiies, 



B.\11T., M.D.. Lt.D.. F R s. ( Tlluxfroteil) ... ... l.'i! 



SrsspOTS AND Winters. By Alex. B. MacDowai.l 



(lllusfratpdj 150 



PETEBlIINATlOy OF Xoos. By Wsi. Davie5 1.57 



CtDCDS ox Mars. Bv T. R. Wabino ... l.'>7 



The Ice Age. By W. H. S. Monck I.j7 



Notes ].")S 



Notices of Books .. ... 15!) 



Books Received . Ifio 



British Ornithological Notes. Conducted hy Harry F. 



WiTBERBV, F.Z.S.. M.B.O.V. ... ... ... ... ... I (>0 



The Insects of the Sea.— IV. Beetles. By aso. II. 



Carpenter, B.ec.(LOSu.) {Illustrated) ... ' 161 



Standard Silver: ItsHistory. Properties and Uses.— III. 



By Kexest A. Smith, Assoc.R.s.M.. F.c..?. . ... ... 163 



Microscopy. Conducted by II. I. Cross (niii.^t,-f,l,dj ... 16.") 

 Notes on Comets and Meteors. By W. F. Denning, 



F.R.A.S. ' 1(50 



The Face of the Sky for July. By A. Fowler, f.e.a.s. ... 167 

 Chess Column. By C. D. Locock, b.a IfH 



THE SIZE OF OCEAN WAVES.-IV. 



{ConcliuUnij article.) 



By Vaughax Cornish, d.sc.^vict.), F.fi..s., f.c.s., f.r.g.s., 



Associate of the Owens CoUeije. 

 The diagram Fig. 1 is composed of two parts. In the 

 upper we have a simple wave of length 300 feet and 

 height 15 feet, and a simple wave of length .5.oO feefc and 

 height 15 feet, which are combined in the thii-d line, 

 giving a slightly irregular wave surface of which the 

 average amplitude for the portion shown is about 16 

 feet, and the avefage distance from ridge to ridge is 

 300 feet. 



Shorter waves, however, flatten out more quickly tlian 

 longer ones, so that after the wind has subsided the 

 shorter will cease to be the more conspicuous wave, and 

 the sea will pi-esent the appearance shown in the fifth 

 line, an irregular swell with an apparent wave length 

 averaging about 500 feet for the portion shown. For the 

 purpose of this illustration I assume that the longer 

 swell has not yet travelled completely beyond the shorter, 

 as. of course, it will when sufficient time has elapsed. 



Fig. "J is a further illustration of the conditions wliicli 

 (Ictoi'miiie the a))parent dominance of one wave, which, 

 1 tliinU, greatly mtliu-nc(^ Ihe record of average observed 

 IcMgl h and height . 



In this diagram, which is dr.iwn ratiier rouglily, but 

 not too roughly for the purpose in hand, the shorter 

 wave, 150 feet long and 7^ feet high, is plainly visible 

 on the third line throughout most of the distance, but is 

 as plainly subordinate to a longer wave. The wave 

 shown in" line 2 is 400 feet long and 20 feet high, it is 

 not only longer than the wave in line 1 but is equally 

 steep, and, therefore, so far dominates the shorter wave 

 that an observer on board ship, who must concentrate 

 his attention if he is to get results at all, measures only 

 the crests A, B, C, D, the average interval between which 

 IS 400 feet. The lower three lines of the figure show 

 tli3 combin.ition of two same waves when of equal amjili- 

 tude. The apparent wave length is now 150 feet. 



For the advancement by observation of our knowledge 

 of ocean waves it is desirable that measurements of indi- 

 vidual heights and lengths should in future be recorded, 

 and the analysis into constituent waves attempted. The 

 want of a fixed datura line will probably render such 

 analysis imperfect, and the absence of fixed relation 

 between the height and length of the components is a 

 difficulty; nevertheless an approximate analysis could be 

 made, and this would be a decided advance on the present 

 condition of ailairs. 



We have here to deal with the measurements at present 

 available ; these record the average size of the most 

 conspicuous ridges seen from on board ship (reckoning 

 amplitude as the vertical height from the trough to the 

 ne.xt succeeding crest without reference to any datum 

 line), and the average does not seem to differ much fioni 

 ihe size of the longest of the steep waves rnniiiiig at 

 Ihe time, which I propose to call the dominant wave. 

 It has been the custom of obsei-vers as far as possible 

 only to take measurements when the waves were fairly 

 regular, and to avoid the very common condition indi- 

 cated in Fig. 3, where the sea is agitated by short groups 

 of higli waves with long " smooths " intervening. 



Let us now proceed to enquire what light the pub- 

 lished observations throw upon the relation of the ob- 

 served size of the waves to the strength of the wind, 

 the distance through which it acts, and the size of the 

 sheet of water. Tables given in the previous articles 

 show the size of the waves in relation to the strength 

 of the wind, the size of the waves being taken when the 

 sea has attained its maximum roughness. In Paris' 

 tabic the lengths of the waves are given. From those 

 of De°bois and Wilson-Barker we can calculate the wave 

 length roughly by multiplying the height by about 18. 

 For small waves the number may be 15, for large waves 

 20. From the length of the waves the velocity is simply 

 calculated by the formula: — 



Speed of Wave = 2j square root of longtli. 

 (in feet ])er second) (■" feet) 



This formula, derived from the theory of regular tro- 

 choidal waves, has been found to apply to the ob- 

 served waves at sea with sufficient accuracy for our 

 present purpose. We find that the speed of waves (i.e. 

 the dominant form) in a storm at sea is much less than 

 th.it of the wind. The long oceanic swells met with 

 after a storm have, however, a velocity approaching that 

 of wind in a strong gale; so much may be said without 

 going into details as to the discrepancy between different 

 records of wind velocity, assuming merely that the con- 

 ventional " numbers ' expressing wind force are roughly 

 comparable in the hands of experienced observers, and 



