FLOATING ISLAND IN DERWENTWATER.
The Floating Island in Derwentwater, in Cumberland, is supposed to be a portion of the peaty bottom of the lake, "which, from some cause not very clearly explained, occasionally rises to the surface. The most probable supposition is, that the mass is swollen and buoyed up by gas, produced by the decomposition of vegetable matter;" as, on piercing it with a boat-hook, carburetted hydrogen gas rises in abundance.
WONDERS OF THE WATERS.
CHAPTER I. THE OCEAN.
The great mass of water which surrounds the land, and which is supposed to cover nearly three-quarters of the whole globe, is generally called the ocean; and it is divided into the Eastern or Pacific Ocean, and the Western or Atlantic Ocean. There are other subdivisions, such as the Frozen Ocean, the Indian Ocean, &c. It is supposed that the bed of the ocean presents the same irregularity of surface as the land, and that it is diversified by mountains, plains, and valleys. No satisfactory reason has yet been given for the saltness of the sea, and its use has not yet been discovered, as it is a singular fact that this saltness does not preserve it from corruption, and it is only the constant rapidity of its motion which prevents it from becoming putrid. It is the same rapidity of motion which prevents it from freezing, as it has been proved that the sun's rays do not penetrate lower than forty-five fathoms, and yet in all countries, the temperature of the surface of the sea is from three to five degrees greater than that of the superincumbent air. The observations of Humboldt shew that both in the Atlantic and the Pacific the waters often retain nearly the same temperature over a great extent, and that between 27° north and 27° south latitude the temperature of the sea is entirely independent of the changes of the atmosphere.
What are called tides are phenomena generally attributed to the action of the moon. The waters of the ocean are observed to flow for six hours from the south towards the north; and during this motion, or flux, the sea gradually swells, and entering the mouths of rivers, drives back the river waters towards their head. After a continued flux of six hours, the sea appears to repose for rather more than a quarter of an hour, and it then begins to ebb, or retire back from north to south for six hours more, in which time, by the subsidence of the waters, the rivers resume their natural course. When the ocean has ebbed there is another pause of about twenty minutes, after which it begins to flow as before. It will thus be seen that each ebb and flow of the tide occupies twelve hours and about three-quarters of an hour, or, to speak more exactly, twelve hours and forty-eight minutes, which is a lunar day; and, as the solar day is only twelve hours long, the moon and tide are forty-eight minutes later every day than they were the day before. Consequently, as it is necessary for all persons interested in the sailing of vessels to know the exact time of the turning of the tide, it has been found convenient, to save the trouble of calculation, to construct tide tables, in which the exact time at which the tide will serve every day is accurately stated.
Another of the phenomena of the ocean relates to what are called the currents, and which appear as yet to be only imperfectly understood. Currents of the ocean are continual movements of its waters in a particular direction, independently of the wind. One current flows from the Azores to the Straits of Gibraltar and the Canaries. Between the Tropics, from Senegal to the Caribbean Sea, flows the equatorial current, which is the longest known, and the course of which is from east to west. Other currents flow to the north or northwest, and others to the east. Whenever a vessel gets into one of these currents flowing in the proper direction, its progress is accelerated in proportion to the rapidity of the current. The current called the Gulf-stream has a rapidity of five nautical miles an hour. The Gulf-stream is known by the elevated temperature of its waters, by their indigo blue colour, by the train of sea-weed which covers their surface, and by the heat of the surrounding atmosphere, which is very perceptible in winter. Humboldt made some curious calculations on the course and rapidity of the Gulf-stream, and found that a drop of water of the current would take two years and ten months to return to the place from which it departed. A boat not acted upon by the wind, and without any artificial means of impulsion, would go from the Canaries to the coast of Caracas in thirteen months; in ten months it would make the tour of the Gulf of Mexico; and in forty or fifty days would go from Florida to the bank of Newfoundland. The Gulf-stream furnished to Christopher Columbus indications of the existence of land to the west, as this current had carried upon the Azores the bodies of two men of an unknown race, and pieces of bamboo of enormous size. An arm of the Gulf-stream, which flows towards the coast of Europe, deposits upon the shores of Ireland and Norway trees and fruits belonging to the torrid zone; and it was from its action that the remains of a vessel (the Tilbury) burnt at Jamaica were found on the coast of Scotland.
Currents are, however, often dangerous to mariners, as "they carry them sometimes insensibly from their intended course, and perhaps irresistibly bear them on to the very rocks which they know must prove their destruction. Along the coast of Guinea, if a vessel overshoots the entrance of a river to which it is bound the current prevents its return; so that it is obliged to steer out to sea, and perform a great circuit to regain the point thus lost." In February, 1847, the ship "Tweed" was wrecked on a coral reef between Havannah and Vera Cruz. There was no storm, but, the weather being cloudy, no observation could be taken of the sun, either on the 11th or 12th of February. However, by the ordinary mode of reckoning, the distance between the coral reef and the proper course of the ship was supposed to be about 124 miles. Unfortunately, however, the ship had got into the current of the gulf-stream, and it struck on a patch of coral reefs, distant about seventy miles from the nearest coast. On this reef the ship was dashed to pieces, and on this the few passengers that were saved remained till they could form a raft, on which they left the reef.
Among the phenomena of the ocean it may be mentioned that springs of fresh water are observed in some places to issue from the sea entirely unaffected by the salt water. The most remarkable of these are a spring in the Persian Gulf, and another on the south coast of Cuba.
The Black Sea is less salt than almost any other; and, indeed, so little so, that carp and other fresh water fish can live in it. The tempests on this sea are tremendous, as the land which confines its agitated waters on three sides gives them a kind of whirling motion. In the winter this sea is so boisterous, particularly from the mouth of the Danube to the Crimea, that it is scarcely navigable. The chief current runs from the shallow sea of Azoph, which is, in fact, only a bay in the Black Sea, to the Hellespont. There is no island in the Black Sea, though there is one in the Bosphorus.
CHAPTER II. LAKES.
The general idea that people have of a lake is an accumulation of fresh water, entirely surrounded by land, and having communication with the ocean or some sea by means of a river. There are, however, some lakes which have no river either flowing into them or out of them; others that have rivers flowing into them but no apparent outlet; and others which do not receive any rivers, but have rivers continually flowing from them. These last are generally seated at the head of a river, and form a kind of reservoir for its waters, and they are often at a great height above the level of the sea. There is a lake of this kind on Mount Rotunda in Corsica, which is about 9000 feet above the level of the sea. The great majority of lakes, however, have a river flowing through them, and this preserves the clearness and sweetness of their waters.
Those lakes which receive streams of water, and often great rivers, without having any apparent outlet, are the most extraordinary; and it was formerly supposed that each of these lakes was furnished with a subterranean channel, by means of which its waters were discharged into the ocean. This, however, is proved not to be the case, as the Dead and Caspian Seas, and the Lake Titicaca in South America, which are the largest known lakes of this description, are all many feet lower than the adjoining ocean; and if there were any communication, however small, between them, the waters of the ocean would flow into the lake, till both were raised to the same level.
Besides these kinds of lakes, there are others which are concealed in cavities covered over by the strata of different kinds of earths, and which consequently are not seen till they are brought to view by the operations of mining, digging of wells, &c. Of this nature are the numerous cavities filled with water, found in the Julian Alps; some of which appear to be the sources of rivers, while others are supposed to receive considerable streams which are known to lose themselves in the recesses of the mountains. Other lakes, which are situated above ground, periodically disappear, and their waters, most probably, flow into similar reservoirs. "That very extensive subterranean cavities exist," observes a writer on the subject, "is sufficiently attested by numerous phenomena. The disappearance of rivers, the water thrown off by volcanoes, the sudden and terrible inundation of mines, the mountains which are sometimes engulfed in the bosom of lakes, and the springs of fresh water which spout up in the midst of the ocean, are all so many evidences of the fact. There is a district in the interior of Algiers, where the inhabitants, after digging to a depth of about 200 fathoms, invariably come to water, which flows up in such abundance that they call it the subterranean sea."
Periodical lakes exhibit some very extraordinary phenomena. Sometimes they are formed by excessive rains, which are evaporated by solar influence and of this nature appears to be the great lake recently discovered in Australia. But there are lakes entirely independent of the rainy season, which appear and disappear at certain intervals; and of this nature is the extraordinary lake of Cirknitz in Illyria, at the bottom of which a crop of corn is sown and reaped at one season of the year, while at another, the water is sufficiently deep to float a vessel of considerable size.
"There are some lakes which present very remarkable phenomena, such as rising and falling like a tide, and boiling, or becoming agitated even during serene weather. Some of the Scottish lakes, and the Wetter in Sweden, often experience violent commotion when the atmosphere is perfectly still. It seems highly probable that these agitations are connected with earthquakes, in distant countries; and a coincidence in dates on certain occasions has given countenance to this belief. In Portugal there is a small lake or pool near Beja in Alemtejo, which emits a loud noise on the approach of a storm. Other lakes appear agitated by the disengagement of subterranean gases, or by winds which blow in some cavern with which the lake communicates. Near Boleslaw, in Bohemia, there is a lake of unfathomable depth, which sometimes in winter emits blasts of wind so strong as to elevate to some height ponderous pieces of ice. In the Marche of Brandenburg, the pool of Krestin often, even in fine weather, boils up in whirlpools, so as to engulf small fishing boats."