encroached upon the land at the rate of nine feet in a year; and at Sullivan's island, near Charleston, South Carolina, four hundred and forty feet in a year. The waves of inland seas and lakes produce similar effects. The indentations of the shores of Lake Erie were caused by this agency. The mode in which it operates at Cleveland, has been already illustrated in § 33. The ordinary effects of the ocean's agency in wearing and transporting rocks, are greatly enhanced by storms. Masses of rock of from ten to thirty tons weight, have been forced by them up an inclined shore. During the erection of the Bell Rock lighthouse, six granite blocks were thrown over a rising ledge twelve or fifteen paces, and an anchor, weighing two thousand two hundred pounds, was thrown up upon the rock from a depth of at least sixteen feet.

38. The periodical elevations of the ocean waters, called tides, varying from two and a half feet to seventy feet in height, extend the limits of the ocean's power, and produce very marked effects in narrow channels, bays and estuaries. The action of the tide wave is alternate, advancing and receding, destroying as it advances, and bearing away the debris as it recedes; differing in this respect from currents, which carry the fragments which they produce or meet with, only in the direction of their course.

"The bore" is a term applied to a sudden influx of the tide into a river or strait, which, resisted by the descending water, and forced into a narrow channel, rises suddenly, and exhibits the phenomena of breakers on a shelving shore. It is most conspicuous at the time of spring or highest tide. The bore in the Severn is sometimes ninc feet high; in the Bay of Fundy seventy feet, producing inundations, sometimes sweeping off trees and animals.

39. Another kind of movement in the waters of the ocean is exhibited by marine currents, which perform a most important part in the economy of nature. They are of two kinds, drift, and stream currents. Drift currents are the result of a constant or prevailing wind on the surface of the ocean, penetrating to no great depth, rarely exceeding in velocity a half mile per hour, and are easily turned from their course. They are produced chiefly in the regions of the trade winds, and sometimes originate stream currents, which are immense oceanic rivers, covering a space of one to three hundred miles in breadth, and reaching to a very great depth. They are caused by the tendency of water that is displaced to restore the equilibrium of the surface of the ocean. The origin of the displacement is not satisfactorily ascertained. It has been ascribed to winds, unequal evaporation, difference of rapidity of diurnal revolution in different latitudes, &c.

The velocity of the currents varies in different parts of their course-in some the average is sixty miles, with a maximum of one hundred and twenty miles in a day. Their temperature is either higher or lower than that of the surrounding sea, according to the temperature of the region in which they have their origin. This difference amounts to from 10° to 30° Fahrenheit. Some of these currents extend their course through many thousand miles. The same current has different names applied to it in successive parts of its course.

40. The chart, Figure 18, presents a general view of ocean currents, especially of our Gulf stream, which is a continuation of the Mozambique current running between the eastern coast of Africa and Madagascar: doubling the Cape of Good Hope, it enters the Atlantic ocean as the

Cape or Agulhas current. Thence, under the name of southern Atlantic current, it flows north-easterly until its course is turned to the westward by the coast of Africa, and the opposition of the Guinea current from the north. It now forms the Equatorial current, and stretches across the Atlantic, on both sides of the Equator; about midway between Africa and South America it divides, sending a branch southward along the eastern coast of South America, forming the Brazil current, while the main stream continues its course by the coast of Guiana, crossing the waters of the Amazon and receiving those of the Oronoco, and enters the Caribbean Sea, as the Guiana current. From the Gulf of Mexico, it issues as the Gulf stream, running north-easterly by the coast of Florida and Cape Hatteras to the St. George and Nantucket Banks, and thence eastward by the Azores to the coast of Europe. The length of the Gulf stream from Florida to the Azores, is three thousand five hundred

miles, which is traversed in seventy-eight days-at an average rate of thirty-eight miles per day. The amount of water conveyed in it is more than three thousand times the amount discharged by the Mississippi river, many times greater than all the fresh water in the rivers of the globe. In a part of its course near the Florida Gulf, its velocity is that of a torrent-five miles per hour.*

41. The temperature of the Gulf stream at the Florida coast is 86° Fahrenheit, declining, as it advances northward, to 73° at the Azores. This vast expanse of water, whose temperature is 10° above that of the ocean, must have a great effect upon the climate of adjacent countries. "A simple calculation," says Lieutenant Maury, "will show that the quantity of heat discharged over the Atlantic from the waters of the Gulf stream in a winter day, would be sufficient to raise the whole column of atmosphere that rests upon France and the British islands, from the freezing point to summer heat. It is the influence of this stream upon climate, that makes Ireland the Emerald isle of the sea, and clothes the shores of England with evergreen robes; while in the same latitude on the other side, the shores of Labrador are fast bound in fetters of ice. In A. D. 1831, the harbor of St. Johns, Newfoundland, was closed with ice in the month of June, although it is 2o farther south than Liverpool; and the influence of the Gulf stream is felt in Norway, and on the shores of Spitzbergen."

42. On issuing from the straits of Florida, the waters of the Gulf stream are of a deep indigo blue color, and the line of separation between it and the green waters of the Atlantic, is plainly visible for hundreds of miles. The

*Johnston's Physical Atlas.

great eddy in the middle of the Atlantic, caused by these currents, extending from 30° west longitude to the Bahamas, and between the parallels of 20° and 45° north latitude embracing two hundred and sixty thousand square miles-is called the Sargazo sea, because its surface is covered with the gulf weed, (in Spanish, Sargazo.) In many places it is so thickly matted, as to retard the progress of vessels through it.

43. The Arctic current, originating within the polar circle, runs by the shores of Greenland and Labrador forming the Hudson Bay current; comes in collision with the gulf stream at Newfoundland, where it divides, and sends a branch southward by the coast of the United States, constituting the counter current, between the gulf stream and the coast. It enters the Caribbean sea as an under current, replacing the warm water sent through the gulf stream, and mitigating the climate of Mexico and Central America. The temperature of the Caribbean sea at the depth of 240 fathoms, has been found as low as 48° while that of the surface was 85°. This current conveys icebergs from the polar seas. Captain Scoresby counted five hundred icebergs in it at one time. Meeting the gulf stream at Newfoundland, they deposit enormous loads of rocks and earth, having thus greatly extended, and probably originated the Banks. The depth of the great oceanic stream currents has not been generally ascertained, but is in some cases seventy fathoms and probably more. Their mechanical effects, especially when they move rapidly, must be great at considerable depths, and they transport materials hundreds of miles. 44. As tides and currents powerfully co-operate with waves in destroying rocks on the shore of the ocean, so also do they conspire in reproducing land, forming banks, and