THE PHYSICAL CONDITION OF THE OCEAN. 357 



1 to 2 feet high, the original long waves being of an unknown height, 

 but probably did not exceed 10 or 15 feet, l^o other such opportunity 

 of testing the distances to which great waves may travel lias 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, Ave study the contour of 

 the bottom oft' 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 GOO feet) has been 

 reached. It appears probable that this is connected with the depth to 

 which wave action may extend, the line 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 tlie 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, audi 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. In some localities it is plainly due to wind, as in the Ked 

 Sea, where the summer level is some 2 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 E.io 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. At Sydney, 

 New South Wales, Mr. Kussell found tbat during eleven years the level 

 was constantly falling at about an inch a year, but by the last accounts 

 received it was again stationary. 



The variations in the pressure of the atmosphere play an important 

 part in changes of sea level. A difference of 1 inch in the barometer 



