SURF AROUND ISLANDS 173 



headland with an offlying islet may shelter a considerable stretch of 

 coast, including the more protected flank of a neighboring beach. And 

 a long, narrow crescentic island with the concavity on the sheltered 

 side, as is true of many sandy islands in regions where the strongest 

 winds are prevailingly from one direction, acts still more efficiently as 

 a natural breakwater. However, if the waves are striking it endwise, 

 no shelter can be expected on either side of it. Sable. Island, off Nova 

 Scotia, which is somewhat crescent shaped, with its main axis easterly 

 and westerly and with its concavity on the northern side, provides 

 shelter in this way from southerly swells or seas ; but there is no shelter 

 anywhere around its shores if the wind is from any other quarter, in 

 spite of its considerable length (21.5 miles), and even though an ex- 

 tensive bar makes out from each end of it. 



The larger an island is, the lower the surf is on its most sheltered 

 side, as a rule, partly because the sidewise expansion of the inner ends 

 of the wave crests is greater around it than in the case of a smaller 

 island, but also because the interference by irregularities of the coast, 

 operating through a longer distance, drains the inshore ends of the 

 waves of their energy more effectively. 



We might also remark, in passing, that the presence of an island, 

 of whatever size or shape, may interfere with the regular wave pattern 

 for a long distance to leeward, in regions where long swells prevail. 

 This is illustrated on a small scale by the interference between the 

 wave trains in the lee of the small islet that is pictured in figure 45. 

 The Polynesian navigators of old were acquainted with this phenom- 

 enon, and made use of it, not only to direct their canoes from island 

 to island over long distances, but also in their search for new islands. 



The preceding account of surf around islands, in general, and 

 around promontories, applies equally to coral atolls, for these are not 

 all circular in shape, as it the common belief, nor even approximately 

 so, but exhibit a wide variety of outlines. Thus, the swell heaves right 

 around Nukuoro Atoll, in the Carolines, during the winter season when 

 the Northeast Trade Winds are at their height, there being no shelter 

 anywhere, except within its entrance, for it is nearly round and only 

 between 3 and 4 miles in diameter (fig. 56). On the other hand, the 

 southerly face of Arno Atoll is so well protected by the long promon- 

 tory known as Northeast Point (fig. 57) that it affords a safe anchor- 

 age and easy landing when the swell is coming from the northeast, 

 though not with swells from any other direction. 



Waves that run in through the passages, by which the lagoons of 

 coral atolls are connected with the open sea, expend their energy in 

 the basins inside, at the expense of their heights, just as happens in 

 harbors of similar shape. And it is well known to everyone who has 

 had experience in coral seas that there is no danger from surf once 



