13fi 



KNOWLEDGE. 



[June 1, 189C. 



momentarily diminished, tlie cuspedwave will roll forward 

 and fall, like tho breaker on the seashore. 



In passinj,', it may bo mentioned that the foam from tho 

 cusp of a standing wave is not ordinarily like the creiiminp; 

 fnith of the sea broiiker ; the bubbles are eohcrent, well 

 formed, and often uf large size. Sometimes they may bo 

 seen rolling like glass marbles on tho upstream slope of the 

 cusped wave. 



During the return of a breaker, when a current of 

 ■water is running down the slope of the beach, one may, 

 under favourable circumstances, see all gradations between 

 the standing waves of running water and the ordinary 

 waves of the sea, which are running waves in standing 

 water. The best conditions are a moderately sloping 

 beach and a long interval between the arrival of succeeding' 

 breakers. The stream made by the return of the breaker 

 is constantly shallowing on the land side and becoming 

 deeper on the seaward side. Hence, if the pebbles which 

 are dragged seawards with the current, or any other 



and the beach. This afifords a good opportunity of 

 watching running waves and standing waves on the same 

 sheet of water ; and tli(> foam on the surface enables one 

 to discriminate nadily the two kinds ol' wave, for the 

 foam in tho current travels with the stream, passing 

 over the standing waves, whereas the running waves 

 pass the foam, leaving it behind them. The waves of 

 running water are to bo seen on the grand scale in the 

 tide races, which run like rivers in the sea — sometimes, as 

 off Portland IJill, with a velocity of six or seven knots. If 

 the water be rather shallow, standing waves are caused by 

 irregularities of the bottom, but this is not the only way 

 in which they are produced. Anything which makes an 

 extra pressure at any point in a stream gives rise to stand- 

 ing waves. They arise, for instance, where a current meets 

 still water, as is the case on either edge of tho rapid 

 current which flows out through the opening in Portland 

 Breakwater. When two currents meet at an angle, standing 

 waves are also raised. 



Fig. 2.— Rapids on tlie Tees. From a Skctcli 



irregularity of the bottom, should corrugate the surface of 

 the stream, we may got a true breaker in running water : 

 the back of the wave is accelerated (i.e., is hastened 

 shorewards) as the water deepens to seaward ; the front 

 of the wave is retarded as the water shallows on the land 

 side ; and at last the little breaker is stranded and falls 

 flat upon the beach. More often than not the next 

 incoming sea wave has arrived before this happens, and 

 this gives a similar but more striking efftct. This is the 

 little breaker, which may be noticed on calm days and 

 when the wind is ofi shore, of which the bottom seems to 

 be tripped up by the heels— if one may be allowed the 

 expression — the whole of the advancing ridge falling "flop'' 

 upon the shore. If the beach be sandy the water in this 

 breaker is always yellow and turbid, even on a calm day 

 when the ordinary breaker is clear and transparent. 



If there happens to be a shoal close to the shore, 

 standing waves may often be seen where a current is set 

 up parallel to the coast in the channel between the shoal 



When the surface of the sea is agitated simultaneously 

 with wind waves and standing waves, the result is a wild 

 turmoil of waters almost impossible to follow with the eye 

 or to analyze with the mind. The races and overfalls of 

 the Channel are described with a certain rough picturesque- 

 ness in the " Sailing Directions for Pilots." The following 

 account of Portland Race is from the twelfth edition of 

 '•The Pilot's Handbook for the English Channel" : — " From 

 about 2h. to llh., F. and C. (or from the time of ^ ebb to 

 nearly the end of flood in Portsmouth Harbour), there is an 

 outset from the West Bay, on the N.W. side of Portland, 

 of nearly !)h. duration, which closely skirts the rocky shore, 

 and gradually increases in strength as it approaches the 

 Bill, where it acquires such velocity as to extend far beyond 

 that point before it turns to the E., leaving a strong eddy 

 between it and the land. Having assumed its E. course, 

 it rushes 6 or 7 knots an hour, during springs, past the 

 pitch of the Bill, leaping and foaming over Portland Ledge 

 with great violence During spring tides the 



