TRANSACTIONS OF SECTION E. 707 



Tn comparatively small depths we find remains of many sLells, As the depth 

 increases to 500 fathoms or so we get mainly the calcareous shells of the globi- 

 gerinae which may be said to form by far the greater part of the oceanic floor. 



In deeper water still, where pressure, combined with the action of the carbonic 

 acid, has dissolved all calcareous matter, we find an impalpable mud with skeletons 

 of the silicious radiolaria of countless forms of the greatest beauty and complexity. 

 Deeper still, i.e., in water of — speaking generally — over 3,000 fathoms, we find a 

 reddish-coloured clayey mud, in which the only traces of recognisable organic 

 remains are teeth of sharks and cetacea, many belonging to extinct species. 



What the depths of these deposits may be is a subject of speculation. It may 

 be that some day, as mechanical appliances are improved, we shall find means of 

 boring, but up to the present no such operation has been attempted. 



On the specific gravity of the water of the sea I can say but little except that 

 it varies considerably. 



It is not yet known for certainty how far the specific gravities observed at 

 various points and depths remain appreciably constant. 



In localities where evaporation is great, and other influences do not interfere, 

 it is evident that the specific gravity of the surface will be high — a consideration 

 which observations confirm ; but there are many complications which require more 

 observation before they can be resolved. 



In some few places repeated observations permit deductions, but taking the sea 

 as a whole we are yet very ignorant of the facta bearing on this point. 



The waves which for ever disturb the surface of the sea demand much 

 study. 



The greatest of these, and the most regular, is the tidal wave. On this many 

 powerful intellects have been brought to bear, but it still presents many unsolved 

 anomalies. 



Lord Kelvin and Professor Darwin have demonstrated that the tidal movement 

 is made up of many waves depending upon difl'erent functions of the moon and 

 sun, some being semi-diurnal, some diurnal. The time of transit over the meridian, 

 the declination of both bodies, create great variations ; the changing distance and 

 position of the moon and the position of her node, also have great effect, while the 

 ever-varying direction and foi'ce of the winds, and the difl'erent pressure of the 

 atmosphere play their part, and sometimes a very large part, on what is somewhat 

 loosely known as the meteorological tide. 



The amplitude of the oscillation of the water depending upon each of the 

 astronomical functions varying for every point on the earth, the effect is that, each 

 having a different period, the resulting mean movement of the water has most 

 astonishing variations. 



In some places there is but one apparent tide in the day ; in others this pheno- 

 menon only occurs at particular periods of each lunation, while in fhe majority of 

 cases it is the movements of each alternate tide only that appear to have much to 

 do with one another. 



Though after long observation made of the times and ranges of tides at any 

 one spot they can now be predicted with great accuracy, for that particular place, 

 by the method of harmonic analysis perfected by Professor G. Darwin, the meteoro- 

 logical tide excepted, no one can yet say what the tide will be at any spot where 

 observations have not been made 



Observations all over the world have now shown that there is no part where 

 the tidal movement is so regular and simple as around the British Islands. This is 

 more remarkable when it is found that the tides on the other side of the Atlantic 

 — at Nova Scotia, for instance — are very complicated. 



The minor tides, which in most parts of the world, when combined in one 

 direction, amount to a veiy considerable fraction of the principal lunar and solar 

 tides, and consequently greatly increase or diminish their effects, are in Great 

 Britain so insignificant that their influence is trifling ; but why this should be I 

 have never yet found anyone to explain. 



Nevertheless there are many very curious points about our tides which are 

 plainly caused by interference, or, in other words, by the meeting of two tidal 



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