26 



KNOWLEDGE. 



[Fedruary 1, 1896. 



give a clue to the history of plant life under difierent 

 conditions. 



A noteworthy feature in the small flora of South Georgia 

 is the presence of three nortliorn plants, namely, Mantiii 

 fonlana, CaUitficln rerun, and Pliliuiii nljiiiiuw. Two out 

 of three of these plants are also found in the New Zealand 

 region. As previously mentioned, some northern species 

 extend into the southern hemisphere, hut no essentially 

 Antarctic types extend into the northern. The farthest 

 they reach are the Alps of Victoria and the Andes of South 

 America, where a very few outliers occur. 



Another peculiarity of the highest Antarctic flora is the 

 almost total absence of colour in the llowors, which are, 

 moreover, exceedingly small, liutterllies and bees are also 

 absent, whereas they abound in the north, where showy 

 flowers arc found. 



The llora of Macquarie Island, on the opposite side of 

 the world from South Georgia, although very poor, and 

 possessing; no endemic element worth mentioning, presents 

 more striking features. For example, there is one very 

 showy plant allied to the Michaelmas daisies, and another, 

 Stilliocarpa puliiris. belonging to the same family as the ivy, 

 having very large and handsome leaves. Yet, although 

 the climate is much less rigorous than in South Georgia, 

 there is no shrubby plant of larger dimensions than common 

 thyme ; and the whole flora consists of less than thirty 

 species. Comparisons with other islands I must leave for 

 a future article. 



WAVES-II. 

 THE WAVES OF THE SEA SHORE. 



By Vaughan Cornish, M.Sc, 



WHEN the ocean wave reaches the shallowing 

 slopes of the sea shore, the water particles 

 no longer swing freely in the circular orbit 

 which gives rise to the simple form of the 

 deep-sea swell. The circular swing of each 

 particle is exchanged for an elliptical motion, the vertical 

 diameter of the circle being shortened when the depth of 

 the water is no longer great compared with the wave- 

 length, or distance from crest to crest. With the change in 

 the motion of each water particle, which cannot itself be 

 followed by the eye, comes the change in the visible form 



the waves cannot be clearly seen. The cause of these 

 progressive changes in the wave-form is the slower trans- 

 mission of wave-motion through shallow water. When 

 the distance of the sea-bottom is no longer great compared 

 with the dimensions of the wave, the forward movement 

 of the wave is retarded, and, moreover, the drag on the 

 wave's motion is greater in the trough of the wave than at 

 its crest, where the depth of the water is greater. Hence 

 the crest gets more and more ahead of the following 

 trough, until at last we reach the critical moment when it 

 catches up the preceding trough, overhangs the hollow 

 with a trembling cusp, and then breaks. According to 

 Scott Russell's observations, this occurs when the height 

 of the crest above the general level of the water is equal 

 to the depth of the water at that place. 



Everyone who has watched the breakers of a stormy 

 sea has noticed the occasional arrival of succeeding 

 breakers of exceptional size, and the occasional occurrence 

 of nearly smooth water. This is due to the coincidence of 

 two or more sets of waves, the result being a complex wave 

 in which the motion of each particle of water at any 

 position is the result of two or more impulses. The 

 amount of the resulting displacement at any point is either 

 the sum or the difl'ei'ence of the several displacements, 

 according as they are at the moment in the same or in 

 opposite directions. The lowest curve in Fig. 2 repre- 

 sents a wave-form in which rough and comparatively 

 smooth water alternate with one another. This curve is 

 drawn by compounding the two upper curves, each of 

 which represents a simple wave. The length from crest 

 to crest is slightly different in {he two cases. In sections 

 A and C the simultaneous displacements are in the same 

 direction in the two waves ; crest nearly coincides with 

 crest, and we have rough water. In section B the simul- 

 taneous displacements are in opposite directions ; what 

 would be the crest of the first wave (if the wave were 

 travelling by itself) nearly coincides with the position 

 which in the second wave would be a trough. The 

 resulting disturbance in the actual complex wave is there- 

 fore very slight, and there is comparatively smooth water 

 in section B. The wave is supposed to be travelling to 

 the right, so that section C will give large breakers when 

 it reaches the shore, which will be followed by small 

 breakers when section B reaches the shore. It is by 

 availing themselves of the succession of rough water by 

 smooth that skilful boatmen safely beach their small craft 

 in a rough sea. Early training and long practice appear 

 to give them a remarkable power of analysing the com- 



FiG. 1, — The Foniiiug of the Broake 



of the undulating surface oi the sea. When the wave is 

 running freely in deep water each crest is preceded and 

 followed by a trough of the same shape and size. Fig. 1 

 represents the modification of the wave-form in shallowing 

 ■water. The front of the wave becomes steeper, the back 

 more sloping, until the crest of the wave has the cusped 

 form ; when we have reached the breaker, and the crest 

 topples over in a mass of foaming water. This progres- 

 sive change of form can be better observed from a pier 

 than from the beach, where the troughs and backs of 



ponents of a complex wave. In rough weather off a 

 shallow shore there are generally several waves breaking 

 at the same instant at difterent distances from the beach. 

 This would be difficult to understand if the wave-form 

 were a simple one ; but with the more usual complex 

 waves, it is easy to see that there may be a smaller wave 

 breaking near the shore and a larger wave breaking 

 simultaneously in deeper water. On a steep shore the 

 wave phenomena terminate with the breaker ; on a flat 

 shore it is not so. From each succeeding breaker the 



