Jl-i.v. 1901.] 



KNOWLEDGE. 



11.7 



tlie lieiglit of wjivcs and the " li 

 wind aooordinsr to T. Stevenson. 



•ngtli of foti-li " of (lie 

 It is taken from the 



be o ^ 



■a 

 5 



a 



13 





I ■% 



article Harljiurs, in the Encyclopoedia Britannica, IXth 

 edition. Column 2 srives tho distance to the nearest 



shore in (lie direction from which the waves come, 

 eolnmn .'J the observed heij^lit, .md columns 4 and 5 tl.e 

 heights calculated from tlie empirical foniiuhc which 

 best satisfy the results of observation. 



It required many years to compile this table, as it gives 

 the observations of " the effects of heavy gales which could 

 only be made at long intervals of time." The author goes 

 <in to say that "the formula h = I'^ya . . . indicates 

 pretty nearly the heights of waves during heavy gales, 

 at least in seas vvfliich do not differ greatly in depth from 

 those where the observations were made. This formula 

 is of course inapplicable where the water is not of sutll- 

 cienb depth to allow the waves to be fully formed, or 

 where it becomes so shallow as to reduce their height 

 after they are formed. It must bo observed that in short 

 fetches, as in narrow lochs or arms of the sea, waves are 

 raised higher during very violent gales than tho formula 

 indicates, though' it does not appear that such waves go 

 on progressing in height in the same ratio for any 

 considerable distance." 



The observed heights agree well with those cah'ulated 

 from the simpler formula (which makes height vary with 

 the square root of the distance) between the limits of a six 

 and a sixty-five mile stretch of water, between which 

 limits, moreover, there is a fairly numerous and uniformly 

 distributed set of observations. Above that distance there 

 are two values given in the table, viz., for distances of 

 114 and 165 miles. The former agi-ces well with the 

 formidDB. the latter docs not. Stevenson obtains the 

 " length of fetch " by measuring on the chart the distance 

 to tiie nearest shore in the direction from which the 

 wind is blowing on the lee shore. When we have to 

 do with greater distances, however, the stretch of water 

 throughout which the wind acts simultaneously in (me 

 pnd the same straight line cannot be thus assumed. If 

 in such cases we restrict the term length of fetch to lln> 

 space over which the wind is acting throughout in one 

 direction, then, since wo have no sure means of ascertain- 

 ing what that distance is, the notion of " length of fetcii " 

 of wind is of little further use to us; if, on the other 

 liand, we continue to note the size of waves in connection 

 with the distance from land in tho direction from which 

 tho waves are coming, we must not expect to find a 

 continuance of such comparatively simple. relations be- 

 tween this distance and the size of wave. Nevertheless 

 iff is desirable to take note of this, which I may call tlu^ 

 hiK/ili' of run, for since ocean waves do not quickly 

 subside, when the wind ceases, tho train of waves may 

 be subjected more than once to wind blowing over them 

 in the direction of their moveinent. St<?venson's values 

 for heights of waves for distances up to 100 miles may 

 lie provisionally accepted as applying to those far from 

 (he lee shore. At 144 miles the height calculated from 

 his (simpler) formula is 18 feet, and this accords fairly 

 with the experience of waves in the English Channel 

 during easterly gales. But in the western Meditenanean 

 (off the Riviera) the average height of waves in a storm 

 does not appear to exceed 18 to 20 feet,* though the 

 length of run may be taken as 500 miles. Stevenson s 

 formula if stretched to apply to this case gives a height 

 of nearly 37 feet, and is, therefore, no loneer applicable 

 to measurement from the windward shore. Whether or not 

 a storm wind blowing in a straight line over 500 miles 

 of water would raise waves averaging 37 feet in height 

 it is nearly certain that such waves would not be normal 

 in the Mediterranean or any other enclosed or nearly en- 

 closed sea. Thus the greatest waves observed by Pans 



• Sec Adm. W. H- Sm.v 



t)i 



■ Tlu,' Mediterranean. 



