It is certain, however, that earthy impregnations will not always account for the hue of lakes and rivers, for the Jumna and the Ganges both take their rise in the snow, flow almost in parallel lines, run through nearly a similar soil, and yet the water of the one is pure as crystal, and that of the other yellow and thick. The Bala-Pool, or Pimble-Mere, is an exception to the general aspect of the lakes of Wales, its water being so pure, that the nicest chemical tests can detect scarcely any quantity of foreign admixture. It refuses to mingle with the tan-coloured Dee, whose waters run through it in a streak of almost inky blackness.

CHAPTER VIII.

THE OCEAN.

ROM the land-enclosed waters of the globe, we now pass to consider those which enclose the land, occupying the greater part of the surface of the earth, in various places to an unknown depth, and presenting a thousand interesting and astonishing features. However occasionally disastrous by its tempests to human life and property, the ocean is essential to the existence of man and of

all vegetation, purifying the atmosphere he breathes by its constant motions, and sending off from its immense reservoir a perpetual supply of vapours, which condense into clouds, and are the sources of moisture and fertility to the soil. Nor is the facility afforded by the great deep for the intercourse of distant nations an unimportant circumstance, while numerous marine productions, in the hands of civilisation, minister to the comfort and improvement of society. Owing to the enterprise of scientific individuals, to commercial adventure, and to costly expeditions fitted out by different governments, the surface of the ocean has been largely traversed, and the sinuosities of its coasts explored, though the line of its circumference has not yet been fully traced, and various parts of it which have been visited have not been accurately surveyed. The passage of the Polar Sea to the north of America has hitherto baffled the skill and hardihood of our countrymen, though the accomplishment of it is probably nigh at hand; and the configuration of its shores to the north of Asia is a very recent geographical achievement, due

to the efforts of the Russian authorities. But enough is known of the great world of waters, of its extent, utility, and varying phenomena,

"Calm or convuls'd, in breeze, or gale, or storm,

Icing the pole, or in the torrid clime
Dark heaving,"-

to invite the eye of admiring contemplation, and enrich the mind with conceptions of grandeur, beauty, and beneficence. Unstable as it appears, so as to have become a common emblem of inconstancy with the nautical races, it has far more permanent stability than the solid earth; nor is the language of the modern poet

"Time writes no wrinkle on thy azure brow;

Such as creation's dawn beheld, thou rollest now,"

any violation of philosophical truth, for the level of the ocean, however temporarily fluctuating, appears to experience no enduring change.

Similar inequalities to those which mark the surface of the dry land-abrupt eminences, gentle slopes, and deep depressions seem to characterise the bed of the ocean. Hence the depth of its waters is very various, from the thin stratum which scarcely conceals the sand-bank from the eye of the navigator, to the enormous mass which no plummet has ever sounded. In the North Sea, Lord Mulgrave let down a heavy sounding lead to the depth of 4,700 feet without reaching the bottom; and Captain Scoresby, off the coast of Greenland, sounded to the depth of 7,200 feet, with the same result. But this of course does not prove the ocean to be a bottomless abyss; it only overreaches the limited extent of our sounding lines. Nor perhaps do such experiments show the approximate depth in those places, for an under-current may have carried the lead far away from a perpendicular direction. Along a low, level, and sandy shore, the sea is generally shallow, but the reverse in the neighbourhood of a bold and towering coast. The recession of the tide off the flats of Lincolnshire and Holland, converts large tracts into dry land, while the Mediterranean, where mount Athos rises abruptly from it to the height of 6000 feet, has a depth of from 500 to 600 feet close in shore. Around low islands, except those of coral formation, shoals and shallows are common, often at a considerable distance from the beach; but around those which project from the bosom of the ocean to a great elevation, as St. Helena, the depth frequently cannot be sounded.

On approaching Aurora Island, one of the Panmato group of South Sea Islands, the officers of the American expedition sounded at one hundred and fifty feet from its perpendicular cliff, and found no bottom at a hundred and fifty fathoms.

Analogy thus leads us to infer a general correspondence between the height of the land and the depth of the sea. If this be so, then the greatest depth of the ocean will be nearly 30,000 feet, equal to the elevation of the loftiest peaks of the Himalaya mountains. Perhaps, without much inaccuracy, its mean depth may be taken at a quarter of a mile; according to which its solid contents, allowing the ocean to occupy three fourths of the superficies of the earth, will be about 32,058,9393 cubic miles. The level of the ocean, however, is not the same in all places, nor at all times in the same place, for astronomical and atmospherical causes, producing tides and winds, operate to effect a change. Apart also from these disturbing causes, it is found that the level of the water in some gulfs and inland seas has in general a greater elevation than that of the main deep. Thus the waters of the Red Sea, separated by the Isthmus of Suez from the Mediterranean, were found by the French engineers 32 feet higher than those of the latter. A fact of a similar kind was demonstrated by Humboldt, with reference to the waters of the Gulf of Mexico, which he estimated to be from 20 to 23 feet higher than those of the Pacific Ocean, on the opposite shore of the Isthmus of Panama. This effect appears to be occasioned by the tropical current of the ocean from east to west, caused by the earth's rotation upon its axis from west to east, which accumulates the water in those gulfs opening eastward.

The saltness of the ocean is one of its prime characteristics; but, as yet, we have nothing but hypothesis, with reference to its cause and design. In addition to pure water, it has been ascertained, by the experiments of different chemists, to hold in solution muriate of soda, or common salt, muriatic and sulphuric acid, fixed mineral alkali, magnesia, and sulphate of lime, besides the animal and vegetable matter, in a state of decomposition, with which it is impregnated. The great specific gravity of the sea, resulting from these ingredients, explains its buoyancy. The proportional specific gravity of different kinds of water is stated to be as follows:

The quantity of salts in the water of the ocean varies in different places. Lord Mulgrave found the proportion at the back of Yarmouth sands to be 3·125 per cent of the weight of the water; and Captain Scoresby, in N. lat. 77° 40′ and E. long. 2° 30′, found the proportion to be 3.56 per cent in a quantity of water taken from the surface. Different observers have given the following results:—

The sea was formerly supposed by physical enquirers to be the saltest under the equator; but Humboldt has deduced from good experiments conclusions as follows:

Proportion of salt between 0° and 14° lat. =0·0374

From this table it appears that the saltness of the ocean is greater towards the tropics than at the equator, and least towards the poles; for which a reason may be found in the

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immense amount of the equatorial rains, and the neighbourhood of the polar snows, which diminish its intensity. In the water of the Firth of Forth, an analysis of Dr. Thomson gives th of saline contents, and in the neighbourhood of Great Britain th of the whole weight is salt. The origin and object of this peculiar constitution of sea-water is one of the mysteries of physics. Why so great a difference between the waters of the ocean and of the land? Why are the former hateful alike to man and beast, covering such a mighty expanse as they do, and feeding by evaporation, and in some cases by filtration, the pure springs and streams which keep both man and beast alive? The saline quality of the ocean water alone does not preserve it from corruption. The water of the Nile never becomes putrid, although kept for any length of time in small vessels in a house, or in large cisterns out of doors. But that of the ocean soon becomes offensive in the hold of a ship, and the state of the equatorial seas, after a long calm, answers, to some extent, to the strong phrase of Coleridge, "the very deep doth rot!" With reference to the cause of its saline quality, we only know the fact, that different salts are constituents of the terraqueous system; that beds of rock-salt of enormous thickness, as in Cheshire and Poland, form part of the crust of the globe; and analogy leads to the conclusion, that immense banks of the mineral exist in the bed of the deep. Local causes operate in various parts of the ocean to lessen its saltness; as at the mouths of rivers, where large volumes of fresh water are constantly mingling with its waves. The singular circumstance also has previously been referred to of freshwater springs rising up in the midst of the sea. Humboldt was informed by Don Francisco le Maur, that in the Bay of Xagua, to the south-east of the island of Cuba, springs of this kind gush up from the bottom with such force as to prove dangerous to small canoes; and that vessels sometimes take in supplies from them, while the lamartin, or freshwater cetacea, abound in the vicinity. There are similar fountains in the Persian Gulf, which furnish the inhabitants of Aradus with their ordinary drink; and in several places in the volcanic regions of the Mediterranean, the sea is fresher at great depths than at the surface, owing to the presence of these springs; a phenomenon which is not uncommon near the islands of the Pacific.

The waters of inland seas are commonly less saline than those of the main ocean, especially where they communicate with it by very narrow channels, and receive numerous and extensive rivers. This is strikingly the case with the Baltic. Analysis shows that three pounds of its water will yield about 390 grains of salt, while the same quantity taken from the German Ocean, with which it is connected, contains 747 grains. This small degree of saltness is to be attributed to the narrowness of its outlet, and to the numerous rivers that flow into it, which drain more than one fifth of the surface of Europe, and are fed by a larger amount of snow than falls in any other inhabited country of the world. The average weight of the Baltic water, taken from the centre, is to that of fresh water as 1.038 or 1.041 to 1·000, while that of the Atlantic is as 1.288. There are some variations in the quality of the waters of this inland sea, which seem to depend upon their locale, with reference to the ocean and the rivers. Thus, those of the Gulf of Bothnia contain less salt than other portions of the Baltic, and here the in-flowing streams are the most numerous, and the out-lying ocean at the greatest distance. The quality of the water also changes according to the seasons; for while at midwinter 50 tons of water taken from the Gulf of Bothnia will yield a ton of salt, it will require 300 tons at midsummer to produce the same quantity. It is the larger amount of fresh water poured into the gulf, through the melting of the snows at the commencement of the summer, that contributes to this result. The direction of the wind likewise largely influences the character of the Baltic water. Its specific gravity, as ascertained by the experiments of Wilcke, under the circumstances stated, is as follows:

Specific Gravity.

1.0030 Wind at E.

1.0047 Wind at W.

Specific Gravity. 1.0118 Storm at W.

10098 Wind at N. W.

It appears from this table, that the proportion of salt in the waters of the Baltic is least when the wind is east, greater when it is west, and greatest during the prevalence of a westerly storm. This is readily explained. An east wind co-operates with the natural current of the Baltic to keep out the waters of the open sea, while a west wind checks the current, changes its direction, and causes an influx from the ocean. Sometimes, during a strong easterly gale, the Baltic water is sufficiently fresh to be fit for domestic use. It is owing to its inferior saltness and scanty depth, that its shores are ice-bound, and large portions of its surface are frozen over, during a severe season. In the year 1333, the sea presented a surface of solid ice from the Danish islands to the coast of Prussia, over which for some time communication was uninterruptedly maintained, and public-houses were erected along the road. The Swedish monarch Charles X. marched his army in 1559 over both Belts to the conquest of Zealand, and in 1809 the Russian soldiers travelled across the ice from Finland to Sweden. The water of the Mediterranean exhibits a striking difference to that of the Baltic, containing a somewhat larger proportion of salt than the ocean. The specific gravity of the Atlantic west of the Straits of Gibraltar has been found to be 10294, while that of the Mediterranean to the east of the Straits is 10338. This is perhaps the combined effect of a variety of causes, and may be due to the mineral character of its bed, to the strong current which sets into it from the Atlantic, and to the extensive evaporation to which the water of this close sea is subject, produced by a temperature which is five or six degrees higher than that of the ocean under the same latitude.

From a series of experiments made some years ago by Dr. Marcet, the following general conclusions were deduced: 1. That the Southern Ocean contains more salt than the Northern Ocean, in the ratio of 1.02919 to 1.02757. 2. That the mean specific gravity

of sea water near the equator is 1.0277. 3. That there is no notable difference between sea water under different meridians. 4. That there is no satisfactory evidence that the sea at great depths is more salt than at the surface. 5. That the sea in general contains more salt where it is deepest, and that its saltness is always diminished in the vicinity of large masses of ice. 6. That small inland seas, though communicating with the ocean, are much less salt than the ocean. 7. That the Mediterranean contains rather larger proportions of salt than the ocean.

Sea water taken from the surface has a bitter as well as a saline taste, which does not belong to it when taken from a considerable depth. This is supposed to arise from animal and vegetable matter, in a state of decomposition, impregnating the surface fluid. To the same cause, the extraordinary presence of sulphuretted hydrogen in various parts of the ocean is attributed. The evolution of this gas has been observed in water brought by Captain Hall from the Yellow Sea in the Chinese Ocean; in a specimen brought by Mr. Schmidtmeyer from N. L. 10° 50′ and W. L. 24° 26', which had an hepatic smell, and blackened the bottle in which it was contained; and it exists in large quantities in the waters along the north-west coast of Africa. Vessels going to the latter region were observed to have their copper sheathing speedily injured; a fact which attracted attention to the composition of the water, of which eight bottles, taken up in different places, were submitted to Professor Daniell for analysis. He found the saline contents in the proportions usually appertaining to sea-water, but analysis disclosed a strong impregnation with sulphuretted hydrogen, which in the case of a portion taken from Lopez Bay amounted to almost as much per gallon as in the Harrowgate waters. It was shown by subsequent investigations that this gas impregnated the seas and rivers along shore, in