TRANSACTIONS OF SECTION E. 709 



Other Tollers are, however, undoubtedly due to earthquakes or volcanic erup- 

 tions 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 sucb 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 resulting wave travelled could be fairly 

 satisfactorily traced. 



This was the great eruption in the Straits of Sunda, in August 1883, which 

 locally resulted in the disappearance of the major part of the island of Krakatoa, 

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

 by tlie 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 dis- 

 tances. These waves were of great length, the crests arriving at intervals of about 

 an hour, and moving with a velocity of about 350 miles an hour, were about that 

 distance apart. 



The waves recorded at Cape Horn were apparently undoubtedly due to the 

 eruption, and travelled distances of 7,500 miles and 7,800 miles in their course on 

 either side of the south polar land. 



They were only 5 inches in height above mean level of the sea, while the waves 

 recorded at places on the southern part of Africa, at a distance of about 5,000 

 miles from the scene of the eruption, were from 1 to 2 feet high, the original long 

 waves being of an unknown height, but probably did not exceed 10 or 15 feet. 



No other such opportunity of testing the distances to which great waves may 

 travel has ever occurred, and as such a catastrophe as gave rise to them could 

 scarcely be repeated 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 an important part in keeping up 

 a constant motion in the abysses, but the depth to which the action of the surface 

 waves originating in wind reach is still but little known by observation. 



If, however, we study the contour of the bottom off the shores of land exposed to 

 the full influence of the great oceans, we are struck by the very general rapid 

 increase of slope after a depth of about 80 to 100 fathoms (500 to 600 feet) has 

 been reached. 



It appears probable that this is connected with the depth to 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 bases of a suitable 

 depth and material upon which the coral animal can commence operations. 



This question also awaits more light, and I merely offer this 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 constantly varies, in 

 some places more than others. 



This subject has not yet been worked out. 



