August i6, 1894] 



NA JURE 



381 



Mr. Buchanan has pointed out why some parts of oceans, 

 deep and vast though they be, are when cut off from communica. 

 tion with others warmer at the bottom. 



Water can only sink through lower layers when it is the 

 heavier, and though a warm surface current becomes from 

 evapor.Uion denser, its heat makes it specifically lighter than 

 the strata below. 



It is only when such a current parts gradually with its heat, 

 as in travelling from tropical to temperate regions, that it sinks 

 and slowly but surely carries its temperature with it, modifying 

 the extreme natural cold of the bottom layers. 



In the North Atlantic and Pacific we have such a condition. 

 The great currents of the Gulf Stream and Japan current as they 

 flow to the north sink, and in the course of ages have succeeded 

 ID raising the bottom temperature three or four degrees. 



In the southern seas this influence is not at work, and, directly 

 connected with the more open water round the South Pole, 

 there is nothing to carry to the abysmal depths any heat to raise 

 them from their normal low temperatures, due to the absence of 

 any heating influence. 



The ice masses round the South Pole have probably little or 

 no effect on bottom temperature, as the fresher, though colder, 

 water will not sink ; and, as a matter of fact, warmer water is 

 found at a few hundred fathoms than at the surface. 



The lowest temperature ever obtained was by Sir John Ross 

 in the Arctic Ocean in Davis Straits at a depth of 6So fathoms, 

 when he recorded a reading of 25° F. This probably requires 

 confirmation, as thermometers of those days were somewhat 

 imperfect. 



In the great oceans the greatest cold is found on the western 

 side of the South Atlantic, where the thermometer stands at 

 32°'3P"., but temperatures of 29 F. have been obtained of recent 

 years east of the Fa^roe L^Iands, north of the ridge which cuts 

 off the deeper waters of the Arctic from the Atlantic. 



Though fcaicely within the limits of my subject, which is the 

 sea itself, I must say a few words on the sea floor. 



The researches carried on in the Challenger revealed that 

 while for a certain distance from the continents the bottom is 

 composed of terrestrial detritus, everywhere in deep water it is 

 mainly composed of the skeletons or remains of skeletons of the 

 minute animals that have lived in the water. 



In comparatively small depths we find remains of many shells. 

 As the depth increases to 500 fathoms or so we get mainly the 

 calcareous shells of the globigerina; 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 radio- 

 laria of countless forms of the greatest beauty and complexity. 

 Deeper still, i.e. in water of — speaking generally — ov,;r 3000 

 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 someday, 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 

 ■ bserved 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 sur- 

 face will be high ; a consideration which observations confirm, 

 but there are many complications which require more observa- 

 tion before they can be resolved. 



In some few places repeated observations permit deductions, 

 hut taking the .sea as a whole we are yet very ignorant of the 

 facts 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 Prof. Darwin have demonstrated that 

 the tidal movement is made up of many waves depending upon 

 different functions of the moon and sun, some being semi- 

 diurnal, some diurnal. The lime of transit over the meridian, the 

 ileclination of both bodies, create great variations ; the chang- 



NO. 1294, VOL. 50] 



ing distance and position of the moon and the position of her 

 node, also have great effect, while the ever-varying direction 

 and force of the winds, and the different pressure of the atmo- 

 sphere pl.ay 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 phenomenon only occurs at particular periods of 

 each lunation, while in the 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, the meteorological tide ex- 

 cepted, by the method of harmonic analysis, perfected by Prof. 

 G. Darwin, 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 very 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 waves arriving from opposite 

 directions, or from the rebound of the tidal waves from other 

 coasts. 



This effect, also, it has been so far found impassible to pre- 

 dict without observation. On our southern coasts, for instance ; 

 in the western part the tide rises about 15 feet, but as it travels 

 eastward the range becomes less and less until, about Poole, it 

 reaches a minimum of 6 feet. Farther east again it increases 

 to Hastings, where the range is 24 feet. Yet farther east it 

 again gradually diminishes. This is due to the reflection from 

 the French coast, which brings another wave which either super- 

 poses itself upon, or reduces the effect of, the main tide advanc- 

 ing up the English Channel ; but the details of such reflection 

 are so complex that no one could forecast them without more 

 knowledge than we possess. 



There can be little doubt that to this cause, reflection, is 

 mainly due the variations in the amount of mean range of tide 

 which are found on many coasts at diflferent parts ; and as these 

 reflected waves may arrive from great distances, and be many in 

 number, we may cease to wonder at the extraordinary difterences in 

 range ol^ tide which prevail, though it will be understood that this 

 is wholly separate from the varying heights of each successive 

 tide, or of the tide at different parts of each lunation, or at dif- 

 ferent times of the year, which depend upon the astronomical 

 influences. 



The actual height of the tide in deep water is small, but on 

 passing into shallow water when appro.aching a shore, and 

 especially when rolling up a gulf of more or less funnel shape, 

 it becomes increased by the retardation caused by friction, and 

 by compression laterally, and hence the height of the tide on a 

 coast affected by other causes is greater than in the open sea. 



The oceanic tide wave is supposed to be from 2 to 3 feet in 

 height, but as this has been assumed from observations made at 

 small oceanic islands, where, although the magnifying influences 

 mentioned are at a minimum, they still exist, we wait for pre- 

 cise information until some means of .actually measuring the 

 tide in deep water is devised. . 



The waves due to wind, though not so far-reaching in their 

 effects as the majestic march of the tide wave, are phenomena 

 which are more apparent to the traveller on the ocean. 



The deep sea in a heavy gale presents, perhaps, the most 

 impressive manifestation of the powers of nature which man can 

 behold, and doubtless many of us have experienced feelings that 

 may vary from awe and wonder to sheer delight, according to 

 the temperament of each individual, at for the first time finding 

 himself face to face with this magnificent sight, though I rather 



