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Before proceeding to the consideration of the wonders connected with and contained in the sea, we shall treat of the composition of the sea itself and of its extent, depth, and bottom.

What is the sea made of?  Salt water, is the ready reply that rises naturally to every lip.  But to this we add the question,-What is salt water? or, as there are many kinds of salt water, of what sort of salt water does the sea consist?  To these queries we give the following reply, which, we doubt not, will rather surprise some of our readers.

Fresh water, as most people are aware, is composed of two gases-oxygen and hydrogen.  Sea water is composed of the same gases, with the addition of muriate of soda, magnesia, iron, lime, sulphur, copper, silex, potash, chlorine, iodine, bromine, ammonia, and silver.  What a dose!  Let bathers think of it next time they swallow a gulp of sea water.

Most of these substances, however, exist in comparatively small quantity in the sea, with the exception of muriate of soda, or common table salt; of which, as all bathers know from bitter experience, there is a very considerable quantity.  The quantity of silver contained in sea water is very small indeed.  Nevertheless, small though it be, the ocean is so immense, that, it has been calculated, if all the silver in it were collected, it would form a mass that would weigh about two hundred million tons!

The salt of the ocean varies considerably in different parts.  Near the equator, the great heat carries up a larger proportion of water by evaporation than in the more temperate regions; and thus, as salt is not removed by evaporation, the ocean in the torrid zone is salter than in the temperate or frigid zones.

The salts of the sea, and other substances contained in it, are conveyed thither by the fresh-water streams that pour into it from all the continent of the world.  Maury, in his delightful work, “The Physical Geography of the Sea,” tells us that “water is Nature’s great carrier.  With its currents it conveys heat away from the torrid zone, and ice from the frigid; or, bottling the caloric away in the vesicle of its vapour, it first makes it impalpable, and then conveys it by unknown paths to the most distant parts of the Earth.  The materials of which the coral builds the island, and the sea-conch its shell, are gathered by this restless leveller from mountains, rocks, and valleys, in all latitudes.  Some it washes down from the Mountains of the Moon in Africa, or out of the gold-fields of Australia, or from the mines of Potosi; others from the battle-fields of Europe, or from the marble quarries of ancient Greece and Rome.  The materials thus collected, and carried over falls and down rapids, are transported to the sea.”

Here, as these substances cannot be evaporated, they would accumulate to such a degree as to render the ocean uninhabitable by living creatures, had not God provided against this by the most beautiful compensation.  He has filled the ocean with innumerable animals and marine plants, whose special duty it is to seize and make use of the substances thus swept from the land, and reconvert them into solids.  We cannot form an adequate conception of the extent of the great work carried on continually in this way; but we see part of it in the chalk cliffs, the marl beds of the sea shore, and the coral islands of the South Seas,-of which last more particular notice shall be taken in a succeeding chapter.

The operations of the ocean are manifold.  Besides forming a great reservoir, into which what may be termed the impurities of the land are conveyed, it is, as has been shown, the great laboratory of Nature, where these are reconverted, and the general balance restored.  But we cannot speak of these things without making passing reference to the operations of water, as that wonder-working agent of which the ocean constitutes but a part.

Nothing in this world is ever lost or annihilated.  As the ocean receives all the water that flows from the land, so it returns that water, fresh and pure, in the shape of vapour, to the skies; where, in the form of clouds, it is conveyed to those parts of the earth where its presence is most needed, and precipitated in the form of rain and dew, fertilising the soil, replenishing rivers and lakes, penetrating the earth’s deep caverns; whence it bubbles up in the shape of springs, and, after having gladdened the heart of man by driving his mills and causing his food to grow, it finds its way again into the sea:  and thus the good work goes on with ceaseless regularity.

Water beats upon the rocks of the sea-shore until it pounds them into sand, or rolls them into pebbles and boulders.  It also sweeps the rich soil from the mountains into the valleys.  In the form of snow it clothes the surface of the temperate and frigid zones with a warm mantle, which preserves vegetable life from the killing frosts of winter.  In the form of ice it splits asunder the granite hills; and in the northern regions it forms great glaciers, or masses of solidified snow, many miles in extent, and many hundred feet thick.  These glaciers descend by slow, imperceptible degrees, to the sea; their edges break off and fall into it, and, floating southward, sometimes in great mountainous masses, are seen by man in the shape of icebergs.  Frequently huge rocks, that have fallen upon these glaciers from cliffs in the arctic regions, are carried by them to other regions, and are deposited on flat beaches, far from their native cliffs.

The saltness of the sea rendering it more dense, necessarily renders it more buoyant, than fresh water.  This is obviously a great advantage to man in the matter of commerce.  A ship does not sink so deep in the sea as it does in a fresh-water lake; hence it can carry more cargo with greater facility.  It is easier to swim in salt than in fresh water.

The only disadvantage to commerce in the saltness of the sea is the consequent unfitness of its water for drinking.  Many and harrowing are the accounts of instances in which sailors have been reduced to the most terrible extremities for want of fresh water; and many a time, since navigation began, have men been brought to feel the dread reality of that condition which is so forcibly expressed in the poem of the “Ancient Mariner":-

  “Water, water everywhere,
  And not a drop to drink.”

Science, however, at length enabled us to overcome this disadvantage of saltness.  By the process of distillation, men soon managed to procure enough water at least to save their lives.  One captain of a ship, by accident, lost all his fresh water; and, before he could put into port to replenish, a gale of wind, which lasted three weeks, drove him far out to sea.  He had no distilling apparatus on board, and it seemed as if all hope of the crew escaping the most horrible of deaths were utterly taken away.  In this extremity the captain’s inventive genius came to his aid.  He happened to have on board an old iron pitch-pot, with a wooden cover.  Using this as a boiler, a pipe made of a pewter plate, and a wooden cask as a receiver, he set to work, filled the pot with sea water, put an ounce of soap therein to assist in purifying it, and placed it on the fire.  When the pot began to boil, the steam passed through the pipe into the cask, where it was condensed into water, minus the saline particles, which, not being evaporable, were left behind in the pitch-pot.  In less than an hour a quart of fresh water was thus obtained; which, though not very palatable, was sufficiently good to relieve the thirst of the ship’s crew.  Many ships are now regularly supplied with apparatus for distilling sea water; and on the African coasts and other unhealthy stations, where water is bad, the men of our navy drink no other water than that which is distilled from the sea.

The salts of the ocean have something to do with the creating of oceanic currents; which, in their turn, have a powerful influence on climates.  They also retard evaporation to some extent, and have some effect in giving to the sea its beautiful blue colour.

The ocean covers about two-thirds of the entire surface of the Earth.  Its depth has never been certainly ascertained; but from the numberless experiments and attempts that have been made, we are warranted in coming to the conclusion that it nowhere exceeds five miles in depth, probably does not quite equal that.  Professor Wyville Thompson estimates the average depth of the sea at about two miles.

Of the three great oceans into which the sea is naturally divided-the Atlantic, the Pacific, and the Arctic-the Atlantic is supposed to be the deepest.  There are profundities in its bosom which have never yet been sounded, and probably never will be.

The difficulty of sounding great depths arises from the fact that, after a large quantity of line has been run out, the shock of the lead striking the bottom cannot be felt.  Moreover, there is sufficient force in the deep-sea currents to sweep out the line after the lead has reached the bottom so that, with the ordinary sounding-lines in use among navigators, it is impossible to sound great depths.  Scientific men have, therefore, taxed their brains to invent instruments for sounding the deep sea-for touching the bottom in what sailors call “blue water.”  Some have tried it with a silk thread as a plumb-line, some with spun-yarn threads, and various other materials and contrivances.  It has even been tried by exploding pétards and ringing bells in the deep sea, when it was supposed that an echo or reverberation might be heard, and, from the known rate at which sound travels through water, the depth might thus be ascertained.  Deep-sea leads have been constructed having a column of air in them, which, by compression, would show the aqueous pressure to which they had been subjected; but the trial proved to be more than the instrument could stand.

Captain Maury, of the American Navy-whose interesting book has been already referred to-invented an instrument for sounding the deep sea.  Here is his own description of it:-“To the lead was attached, upon the principle of the screw-propeller, a small piece of clock-work for registering the number of revolutions made by the little screw during the descent; and it having been ascertained by experiment in shoal water that the apparatus, in descending, would cause the propeller to make one revolution for every fathom of perpendicular descent, hands provided with the power of self-registering were attached to a dial, and the instrument was complete.  It worked beautifully in moderate depths, but failed in blue water, from the difficulty of hauling it up if the line used were small, and from the difficulty of getting it down if the line used were large enough to give the requisite strength for hauling it up.”  One eccentric old sea captain proposed to sound the sea with a torpedo, or shell, which should explode the instant it touched the bottom.  Another gentleman proposed to try it by the magnetic telegraph, and designed an instrument which should telegraph to the expectant measurers above how it was getting on in the depths below.  But all these ingenious devices failed, and it is probable that the deepest parts of the ocean-bed still remained untouched by man.

At last an extremely simple and remarkably successful deep-sea sounding apparatus was invented by Mr Brooke, an American officer.  It consisted of nothing more than thin twine for a sounding-line, and a cannon ball for a sinker.  The twine was made for the purpose, fine but very strong, and was wound on a reel to the extent of ten thousand fathoms.  The cannon ball, which was from thirty-two to sixty-eight pounds’ weight, had a hole quite through it, into which was fixed a sliding rod, the end of which, covered with grease, projected several inches beyond the ball.  By an ingenious and simple contrivance, the cannon ball was detached when it reached the bottom of the sea, and the light rod was drawn up with specimens of the bottom adhering to the grease.

With this instrument the Americans went to work with characteristic energy, and, by always using a line of the same size and make, and a sinker of the same shape and weight, they at last ascertained the law of descent.  This was an important achievement, because, having become familiar with the precise rate of descent at all depths, they were enabled to tell very nearly when the ball ceased to carry out the line, and when it began to go out in obedience to the influence of deep-sea currents.  The greatest depth reached by Brooke’s sounding-line is said to have been a little under five miles in the North Atlantic.

The value of investigations of this kind does not appear at first sight, to unscientific men.  But those who have paid even a little attention to the methods and processes by which grand discoveries have been made, and useful inventions have been perfected, can scarcely have failed to come to the conclusion that the search after truth, pure and simple, of any kind, and of every kind, either with or without reference to a particular end, is one of the most useful as well as elevating pursuits in which man can engage.

All truth is worth knowing and labouring after.  No one can tell to what useful results the discovery of even the smallest portion of truth may lead.  Some of the most serviceable and remarkable inventions of modern times have been the result of discoveries of truths which at first seemed to have no bearing whatever on those inventions.  When James Watt sat with busy reflective mind staring at a boiling kettle, and discovered the expansive power of steam, no one could have for a moment imagined that in the course of years the inventions founded on the truth then discovered would result in the systematic driving of a fleet of floating palaces all round the world at the rate of from twelve to fifteen or twenty miles an hour!  Instances of a similar kind might be multiplied without end.  In like manner, deep-sea sounding may lead to great, as yet unimagined, results.  Although yet in its infancy, it has already resulted in the discovery of a comparatively shallow plateau or ridge in the North Atlantic Ocean, rising between Ireland and Newfoundland; a discovery which has been turned to practical account, inasmuch as the plateau has been chosen to be the bed of our electric telegraph between Europe and America.  The first Atlantic cable was laid on it; and although that cable suffered many vicissitudes at first, as most contrivances do in their beginnings, communication between the two continents was successfully established.  Soundings taken elsewhere showed that somewhat similar plateaus existed in other parts of the Atlantic, and now the whole of Western Europe is being bound more firmly, by additional cables, to the eastern seaboard of America.

This great and glorious achievement has been the result of the discovery of two truths,-of a truth in science on the one hand, and a truth in regard to the structure of the bed of the sea on the other.  The study of electricity and of deep-sea soundings was begun and carried on for the sake of the discovery of truth alone, and without the most distant reference to the Atlantic Telegraph,-yet that telegraph has been one of the results of that study.  Who can tell how many more shall follow?  And even were no other result ever to follow, this one may prove to be of the most stupendous importance to the human race.

Another discovery that has been made by deep-sea sounding is, that the lowest depths of the ocean are always in a state of profound calm.  Oceanic storms do not extend to the bottom.  When the tempest is lashing the surface of the sea into a state of the most violent and tremendous agitation, the caverns of the deep are wrapped in perfect repose.  This has been ascertained from the fact that in many places the bottom of the sea, as shown by the specimens brought up by Brooke’s apparatus, and more recently by Professor Thompson’s deep-sea dredge, is composed of exceedingly minute shells of marine insects.  These shells, when examined by the microscope, are found to be unbroken and perfect, though so fragile that they must certainly have been broken to pieces had they ever been subjected to the influence of currents, or to the pulverising violence of waves.  Hence the conclusion that the bottom of the sea is in a state of perpetual rest and placidity.

Indeed, when we think of it, we are led to conclude that this must necessarily be the case.  There are, as we shall presently show, currents of vast size and enormous power constantly flowing through the ocean; and when we think of the tremendous power of running water to cut through the solid rock, as exemplified in the case of Niagara, and many other rivers, what would be the result of the action of currents in the sea, compared with which Niagara is but a tiny rivulet?  Ocean currents, then, flow on a bed of still water, that protects the bottom of the sea from forces which, by calculation, we know would long ago have torn up the foundations of the deep, and would probably have destroyed the whole economy of nature, had not this beautiful arrangement been provided by the all-wise Creator.