5S2 



NATURE 



[August i6, 1894 



fear that discomfort is the prevailing feeling that many carry 

 away. 



The height to which storm waves may rise has never been 

 very satisfactorily determined. Apart from the diificalty of the 

 task and the small number of people who will address them- 

 selves to it when they have the chance, it is but rarely that any 

 individual sees really abnormal waves, even though he may be 

 at sea all his life. 



Different heights for what are called maximum waves have 

 lieen recorded, and they vary from 40 to 90 feet from crest to 

 hollow. 



All we can say is that the most probable figure is about 50 or 

 60 feet. 



These great storm waves travel very far. In some cases they 

 convey a warning, as their velocity always far exceeds that at 

 which the storm is travelling. In others they intimate that a 

 gale of which no more is seen has occurred somewhere — it may 

 be many miles distant. 



When they have travelled beyond the limits of the wind which 

 raised them, they lose the steepness of slope which characterises 

 them when under its influence, and become an undulation which 

 is scarcely noticed when in deep water. 



On approaching shallow water, however, they are again 

 apparent, and the " rollers " that occur unperiodically at various 

 places in latitudes where gales never occur would seem to be 

 caused by such waves, originating in areas many thousands of 

 miles distant. Such appears to be the origin of the well-known 

 rollers at Ascension and St. Helena, where the rocky and 

 exposed nature of the landing has caused this phenomenon to be 

 especially noticed. 



Other rollers are, however, undoubtedly due to earthquakes 

 or volcanic eruptions occurring in the bed of the sea. 



Many of the great and sudden waves which have caused 

 devastation and great loss of life on the shores of western South 

 America are referable to this cause. 



Observations to enable the focus of such a disturbance to be 

 traced have generally been lacking, but it is probable that where 

 the wave has been large the point of origin has not been far 

 distant. 



In one notable instance the conditions were reversed. The 

 point of origin was known, and the distance to which the result- 

 ing wave travelled could be fairly satisfactorily traced. 



This was the great eruption in the .Straits of Sunda, in 

 August 1SS3, which locally resulted in the disappearance of the 

 major part of the island of Krakatoa, and the lo'is of nearly I 

 40,000 lives, on the neighbouring shores of Java and Sumatra, 

 by the huge wave which devastated them. 



The records of automatic tide gauges and the observations of 

 individuals enabled the waves emanating from this disturbance 

 to be followed to great distances. These waves were of great ! 

 length, the crests arriving at intervals of about an hour, and 

 inoving with a velocity of about 350 miles an hour, were about 

 that distance apart. 



The waves recorded at Cape Horn were apparently un- 

 doubtedly due to the eruption, and travelled distances of 7500 

 miles and 7800 miles in their course on either side of the south 

 polar land. 



They were only five inches in height above mean level of the 

 sea, white the waves recorded at places on (he southern part of 

 Africa, at a distance of about 5000 miles from the scene of the 

 eruption, were from one to two feet high, the original long 

 waves heing of an unknown height, but probably did not exceed 

 ten or fifteen feet. 



No other such opportunity of testing the distances to which 

 great waves may travel has ever occurred, and as such a cata- 

 strophe as gave ri«e to them could scarcely be repealed without 

 similar loss of life, it may be hoped we shall not live to see 

 another, interesting though the discussion of the numerous 

 phenomena were. 



The movement of the particles of water due to the tide wave 



extends to the bottom of the deepest water, and doubtless plays 



-. — 'Tnt pirt in keeping up a constant motion in the 



the depth to which the action of the surface waves 



„ „ in wind reach is still hut little known by observa- 

 tion. 



Tf. however, we study the contour of the bollom off the shores 



'5cd to the full influence of the great oceans, we are 



,-; V. ry general rapid increase of slope after a depth 



III .11. 1.1 II) to 100 fathoms (500 to 600 feel) has been reached. 



It appears probable that this is connected with the depth to 



NO. I 294, VOL. .Sd 



which wave action may extend, the fine particles brought down 

 by rivers or washed from the land by the attrition of the 

 breakers being distributed and gradually moved down the slope. 



When we examine banks in the open sea we find, however, 

 that there are a great many with a general depth of from 30 to 

 40 fathoms, and the question arises whether this may not be the 

 general limit of the power of oceanic waves to cut down the 

 mass acted upon when it is fairly friable. 



The question has an interesting bearing on the subject of the 

 ever-debated origin of coral atolls, for this is the general depth 

 of many large lagoons ; and granted that the sea can cut down 

 land to this depth, we have at once an approach to the solution 

 of the problem of the formation of b.oses of a suitable depth and 

 material upon which the coral animal can commence operations. 



This question also awaits more light, and I merely oflerthis 

 remark as a suggestion. 



It is, however, somewhat remarkable that in recent cases of 

 volcanic islands piled up by submarine eruptions, they have all 

 been more or less rapidly washed away, and are in process of 

 further diminution under the surface. 



Observations on the mean level of the sea show that it con- 

 stantly varies, in some places more than others. 



This subject has not yet been worked out. 



In some localities it is plainly due to wind, as in the Red Sea, 

 where the summer level is some two feet below that of winter, 

 owing to the fact that in summer the wind blows down the 

 whole length of the sea, and drives the water out. 



In many places, as in the great estuary of the Rio de la 

 Plata, the level is constantly varying with the direction of the 

 winds, and the fluctuation due to this cause is greatly in excess 

 of the tidal action. 



In others the cause is not so clear. 



.•\t Sydney, New South Wales, Mr. Russell found ihat during 

 eleven years the level was constantly falling at about an inch a 

 ye.ir, but by the last accounts received it w.is again stationary. 



The variations in the pressure of the atmosphere play aa 

 important part in changes of sea level. 



A difference of one inch in the barometer has been shown to 

 be followed by a difference of a foot in the mean level of the 

 sea, and in parts of the world where the mean height of the 

 barometer varies much with the seasons, and the tidal range is 

 small, this effect is very marked. 



Of any secular change in the level of the sea little is known. 

 This can only be measured by comparison with the land, and it 

 is a question which is the more unstable, the land or the water 

 — probably the land, as it has been shown that the mass of the 

 land is so trifling, compared with that of the ocean, that it 

 would lake a great deal to alter the general mean level of the 

 latter. 



All the points connected with the sea lhat I have had the 

 honour of bringing before you form part of the daily observa- 

 tion of the marine surveyor when he has the chance, but I can- 

 not refrain from also mentioning other duties, which are indeed 

 in the present state of our knowledge and of the practical 

 requirements of navigation the principal points to which he has 

 to pay attention, as it may explain why our knowledge on so 

 many interesting details still remains very imperfect. 



Working as we do in the interests of the vast marine of Great 

 liritain, the paramount necessity of good navigational charts 

 requires that the production of such charts should be our 

 principal aim. 



It is difficult for a landsman and difficult even for a sailor who 

 has never done such work to realise the time that is necessary 

 to make a really complete marine survey. The most important 

 part, the ascertainment of the depth, is done, so to speak, in the 

 dark — that is to say, it is by touch and not by sight that we have 

 to find the different elevations and depressions of the bottom of 

 the sea. 



In making a map of the land, an isolated rock or hill stands 

 up like a beacon above the surrounding land, and is at once 

 localised and marked, but a similar object under the sea can 

 only be found by patient and long-continued sounding, and may 

 very easily he missed. 



When it is considered that marine surveying h.as only been 

 seriously undertaken for almut 100 years, with a very limited 

 number of vessels, we shall, 1 think, understand how in the vast 

 area of the waters, taking only those bordering the shores, many 

 unsuspected dangers are yearly discovered. 



Very, very few coasts have been minutely surveyed, and 

 setting aside for a moment the great changes lhat take place off 



