| ( | 1441 —— 90 | = | ) |
“If we would construct a correct model of our earth, with its seas, continents, and mountains, on a globe 16 inches in diameter, the whole of the land, with the exception of a few prominent points and ridges, must be comprised on it within the thickness of thin writing paper; and the highest hills would be represented by the smallest visible grains of sand.”[17]
Astronomers have measured the distances and weighed the masses of the planets, yet the height of the atmosphere and the depths of the ocean are unsolved problems. The bottom of “blue water” is almost as unknown to us as the interior of the earth. It is a common opinion that the greatest depths of the sea are about equal to the greatest heights of the mountains. Attempts have been repeatedly made to sound out its depths, but no reliance can be placed on any reports of soundings beyond 8000 or 10,000 feet. One ran out his sounding-line 34,000 feet, and did not touch bottom; another 39,000 feet with the same result; one reported bottom at 49,000 feet, another at 50,000 feet. But there are no such depths. There are currents and counter-currents in the ocean, as in the air, which operate upon the bight of the sounding-line, and cause it to run out after the weight has reached the bottom, so that the shock cannot be felt.
The oceanic circulation is as complete as that of the atmosphere, and is possibly subject to, or governed by, the same laws; and there appears to be a law of descent through “blue water,” the same as there is a law of ascent through “blue air.” The one increases in density downwards as the other decreases in density upwards; and the development of this law proves that the sea is not so deep as reports made it.
There is a set of currents in the sea by which its waters are conveyed from place to place through regular and certain channels, traversing from one ocean to the other with the regularity of the machinery of a watch. The chief motive power of marine currents is caused by heat. But an active agency in the system of circulation is derived from the salts of the sea-water, by winds, marine plants, and animals. These give the ocean great dynamical force.
The only reliable deep-sea soundings are those obtained by Brooke’s plummet; and the greatest depths at which the bottom of the sea has been reached with this plummet are in the North Atlantic Ocean, and do not show it to be deeper than 25,000 feet, the deepest place being immediately to the south of the Grand Banks of Newfoundland. Thus, from the top of Mount Everest to the deepest reliable sea bottom reached by sounding, we have a vertical height of nearly 10¼ miles, equal to
| ( | 7912·40 ———— 10·23 | ) | 1 —— 773 |
Chemistry of the Sea.
The specific gravity of Sea-water varies of course with the proportion of salts and the degree of heat it receives from the sun, or by the intermixture of currents of various temperatures; but in our own latitudes it is about 1·028—that is, a given volume of pure distilled water weighing 1000 grains, the same volume of sea-water weighs 1028 grains. Many useful substances are daily extracted from the sea for the use of man, among which we may mention pure water for the use of ships, salt, iodine, bromine, &c. Many attempts have been made to purity sea-water in order to render it potable, not only for supplying ships, but for the use of maritime towns and villages, where pump-water is often brackish, and where the inhabitants are frequently obliged to have recourse to rain-water. Now, when sea-water is submitted to congelation, it abandons its salt almost completely—a fact which appears to have been discovered many years ago by Chevalier Lorgna, who found that a mixture of three parts of pounded ice and two parts of common salt produced a cold of about 4° below the zero of a Fahrenheit thermometer, and that such a mixture caused sea-water to freeze rapidly. A mixture of various chemical salts in proper proportions produces a similar degree of cold. Lately, the cold produced by the evaporation of ether has been proposed for the same purpose. The purification is complete if the ice thus formed be melted and frozen again. In the Polar regions the ice formed from salt-water is more or less opaque, except it be in very small pieces, when it transmits light of a bluish green shade. When melted, it produces sometimes perfectly fresh water, and at other times water slightly brackish. The fresh-water ice resulting from rain or melted snow, as seen floating in the Arctic seas, is distinguished from the salt-water ice by its black appearance, especially when in small pieces, and by its transparency when removed from the water into the air. Its transparency is so great, when compared with sea-ice, that Dr. Scoresby used to amuse his sailors by cutting large lenses out of this fresh-water ice, and using them as burning-glasses to light the men’s pipes. Their astonishment was increased by observing that the ice did not melt, while the solar rays emerging from it were so hot that the hand could not be kept more than a second or two at the focus.—Macmillan’s Magazine.