28 



KNOWLEDGE. 



[Fehiuiary 1, 1896. 



ruddy light stretches from the horizon to the shore. 

 As the sun rises higher tlie band of Hght broadens out, 

 but ceases to be continuous. As day advances wo no 

 longer recognize a band of light, but see .the inter- 

 mittent Hashing of the image of tlie sun from all 

 parts of a broad stretch of sea. Between the observer 

 and the sun the light is caught and rellected, as from a 

 mirror, by the sloping flanks of the waves which are 

 travelling towards 

 him. For a long 

 distance on eitlur 

 hand sideway un- 

 dulations heave the 

 water to such a 

 slope that the he- 

 liographic flash is 

 seen by the same 

 observer. Distance 

 and the quickness 

 of extinction make 

 it diflicult to realize 

 that this appearance, 

 so like the quick 

 flashings from the 

 facets of a diamond, 

 is really an image 

 of the sun. Some- 

 times, however, in 

 slowly heaving 

 water, and when 

 the light is not too 

 strong, the sun's 

 disc may be distin- 

 guished in multitu- 

 dinous repetition. 

 I have noticed this 

 effect very distinctly 

 in the Serpentine 

 when the light of 

 the sun has been 

 suitably reduced by a sonp^on of London fog. I have also 

 noticed a beautiful effect, somewhat of the same class, on 

 looking down from Beachy Head when a light mist concealed 

 the surface of the sea, which, however, was momentarily 

 revealed by heliographic flashes from the waves. 



We have explained the deflection of the wave crests 

 which causes the rollers to travel in opposite directions 

 upon opposite shores of a bay. But why does the sea 

 always break upon the coast, even when the wind is dead 

 off shore •' One would naturally expect that, if the wind 

 continued to blow ofi' shore for a sufficient time, the waves 

 would run frnni the shore ; and in point of fact we know 

 that, if we set out to sea in an off-shore wind, we should find, 

 after going a certain distance, that the waves are travelling 

 away from the shore. Again, if we look at a small sheet 

 of water, such as a pond, when agitated by the wind, we 

 observe that there are breakers on the leeward but not 

 upon the windward shore. The following may serve as a 

 tentative explanation of the anomaly. The motion of a 

 wave depends upon two things : first, upon the action of the 

 wind upon it ; secondly, upon the impulse received from 

 other waves. Quite close to the beach the impulse from 

 other waves cannot act seawards, for the other waves are 

 all upon the seaward side. Again, the power of an off-shore 

 wind to raise the waves at the margin of the sea is no 

 greater than at the edge of a small pond ; and we know 

 that the most powerful wind can raise but a very small 

 wave at the windward edge of a pond. If, therefore, there 

 be present any agitation of the sea other than that due to 



Fio. 3. — Waves breakint; on a Flat Shore. 



the off-shore wind, there exists no agency close to shore 

 adequate to prevent this agitation from sending in waves 

 and forming a breaker. As a matter of observation the 

 turbulence of the sea is so persistent that agitation from 

 past winds or other causes never seems to be wholly 

 absent. This persistent turbulence of the sea we shall be 

 able to explain to some extent in our next article. 



In order to observe the forms and motion of the waves 



the learner should 

 not seek the tumul- 

 tuous surgings on a 

 rocky coast, but 

 should rather go in 

 quiet weather to a 

 flat shore, and walk 

 by the furthest edge 

 of the beach at low 

 tide. The most fa- 

 vourable conditions 

 are when the waves 

 break upon a shoal 

 about a gunshot 

 from the shore, and 

 when a ridge or 

 bank of sand or 

 shingle terminates 

 the beach. Then 

 the incoming wave 

 breaks upon the 

 shoal, and the low 

 foaming ridge of the 

 " solitary " wave 

 traverses the strip of 

 shallow water, at the 

 edge of which the 

 foam disappears as 

 the waves once more 

 assume the usual 

 form. A group of 

 these, of shorter 

 length from crest to crest, makes for the shore, where the 

 second (smaller) breaker discharges itself upon the ridge 

 of sand, from which a reflected wave travels back, meeting 

 the next incoming wave, with which it seems to struggle 

 for a moment ; then each passes through the other, the 

 one hurrying seawards, the other making its way to shore. 

 These and other intricacies of wave motion may here be 

 seen, best of all when the slanting rays of the setting sun 

 mark front and rear of each smallest wave with a double 

 line of light and shadow. 



On the other hand the I'nirc of waves is most strikingly 

 shown where the sea bursts over rocks or breakwaters, as 

 shown in our full-page illustration. We return to the 

 subject of the " Force of Waves " in our next article. 



-*- 



ANTARCTIC EXPLORATION. 



By William S. Bruce, late Naturalist to Antarctic 

 Expedition, 1892-93. 



POLAR exploration, and more especially South Polar 

 exploration, is the first problem to-day among the 

 geographers of the world. At present there are 

 no less than three North Polar expeditions in the 

 field, Norway, America, and Britain each having a 

 representative in the northern world of ice. It is not our 

 intention, however, at present to deal with these northern 

 expeditions, but, in passing, we must wish them success. 



