10. 11. 12. Bear'st thou not from distant strands Was telling of a mournful prison, While the brave and fair were dying. Wreathed with sapphire green and roses? Hast thou been a tribute flinging, Up that bold and jutty steep, Pearls upon the south wind stringing? Faded wave! a joy to thee, Now thy flight and toil are over! Oh may my departure be Calm as thine, thou ocean rover! When this soul's last joy or mirth On the shore of time is driven, Be its lot like thine on earth, To be lost away in heaven!-Anonymous. LESSON XIX.-TIDES AND CURRENTS. 1. THE alternate elevation and depression of the waters of the ocean twice every twenty-four hours, was formerly considered one of the greatest mysteries of nature. The first man who clearly explained the cause and phenomena of tides was Sir Isaac Newton. Their true cause he demonstrated to be the attraction of the sun and moon, particularly the latter on account of her proximity to the earth. 2. The average height of the tides will be increased by a very small amount for ages to come, on account of the decrease of the mean distance of the moon from the earth; but after they have reached their greatest height, a reverse movement will take place. Thus there are great tides of tides, or oscillations between fixed limits, requiring immense periods of time for their accomplishment. The tidal wave extends to the very bottom of the ocean, and moves with great velocity. 3. "Currents of various extent, magnitude, and velocity," says Mrs. Somerville, "disturb the tranquillity of the ocean; some of them depend upon circumstances permanent as the globe itself, others on ever-varying causes. Constant currents are produced by the combined action of the rotation of the earth, the heat of the sun, and the trade-winds; periodical currents are occasioned by tides, monsoons, and other long-continued winds; temporary currents arise from the tides, melting ice, and from every gale of some duration. A perpetual circulation is kept up in the waters of the main by these vast marine streams; they are sometimes superficial and sometimes submarine, according as their density is greater or less than that of the surrounding sea." 4. The most constant and most important of all these currents, and one which exerts a modifying influence on all the others, is that produced by the rotation of the earth on its axis. As the waters descend from the poles, where they have no rotatory motion, the earth's surface revoltes more and more rapidly, until, at the equator, it has acquired an easterly motion of a thousand miles an hour; and as the waters do not fully partake of this motion, they are left behind, and consequently seem to flow westward in a vast stream nearly four thousand miles broad. This stream, being broken, and its parts changed in various directions by the islands and continents which it meets in its course, gives rise to numerous smaller currents, which in their turn are again modified by the general westerly flow, and by winds, rivers, and melting ice. 5. Among these smaller currents is the "Gulf Stream," occasioned chiefly by the constant flow of the waters of the tropics westward across the Atlantic Ocean. A part of this vast heated current is directed into the Gulf of Mexico; issuing thence, it proceeds in a northeasterly direction along the coast of the United States, and being deflected still farther eastward by the great island of Newfoundland, it crosses the Atlantic, and spreads its warm waters around the coasts of the British Isles. "It is the influence of this stream upon climates," says Lieutenant Maury, "that makes Erin the Emerald Isle, and clothes the shores of Albion with evergreen robes; while, in the same latitude on the other side, the shores of Labrador are fast bound in fetters of ice." Any convulsion of the globe that should open a broad channel through the isthmus of Panama would direct this stream into the Pacific, and change the British Isles into a scene of sterility and desolation. 6. It is very important for navigators to study the course and velocity of the ocean currents, as the length and safety of the voyage depend upon them. So much does this circulation of the ocean resemble the circulation of fluids in the human system, that our distinguished countryman, Captain Maury, who has so successfully studied and described them, has been appropriately called the "Harvey of the seas." 1. THE depressions on the surface of the earth, caused by earthquakes, volcanoes, or other means, are frequently filled with water, and constitute what are termed lakes. Rivers meeting with obstructions of hills and rocky ridges often form a lake, or chains of lakes, which serve the purposes of navigation, and give variety to the inland landscape. 2. Many lakes are fed by springs, and sometimes they are the sources of large rivers. It is estimated that more than half the fresh water on the globe is contained in the great American lakes, the largest of which is nearly as large in area as England. Lakes are most numerous in high latitudes, where there is abundant rain and but little evaporation. 3. The five great American lakes, Superior, Huron, Erie, Michigan, and Ontario, are much higher than the level of the ocean. Lake Superior has an elevation of more than six hundred feet, and Lake Ontario two hundred and thirty-four feet. The Great Salt Lake, situated in the elevated tableland east of the Rocky Mountains, is about forty-two hundred feet above the level of the sea. Yet it is a curious fact that those great salt-water lakes of Asia, the Caspian Sea, Lake of Tiberias, and the Dead Sea, are each below the sea level, the first eighty-four feet, the second six hundred feet, and the third more than thirteen hundred feet. The poet Percival, in the following ode, has painted the witching charms of hundreds of our small interior lakes: 4. 5. 6. 7. 8. 9. TO SENECA LAKE. "On thy fair bosom, silver lake, The wild swan spreads his snowy sail, As blows the north wind, heave their foam, As late the boatman hies him home. Thy golden mirror spreading wide, And see the mist of mantling blue Float round the distant mountain's side. A sheet of silver spreads below, And swift she cuts, at highest noon, Light clouds, like wreaths of purest snow. Oh! I could ever sweep the oar, When early birds at morning wake, LESSON XXI.-SPRINGS AND RIVERS. Great Geyser of Iceland. 1. In addition to the common springs, with whose origin every one is familiar, mineral springs of great variety abound in different countries, the waters of some of which merely present a sparkling appearance, owing to the presence of carbonic acid gas, while others are variously impregnated with mineral substances, the chief of which are iron, sulphur, and salt. 2. Besides these, Iceland presents us a remarkable group of hot springs, called geysers, which burst forth with subterranean noises, and frequently at regular intervals, throwing up water and steam, sometimes to the height of one or two hundred feet. The supposed cause of this peculiar action is the heating of some internal fountain of water by volcanic agency, until a sufficient quantity of steam is formed forcibly to expel the wa Section of a Geyser. ter through a channel which ficient to refer Intermitting Spring. 3. The excess of water precipitated as rain and snow, over what is evaporated from the surface, runs in streams, called rivers, to lakes, or to the ocean. The position of mountains and elevated ridges determines the course and length of rivers. Few physical causes have had more influence in the location and fortunes of men, than rivers. Capitals of states and countries are generally on rivers, and large cities either on navigable rivers or bays. 4. Rivers are associated with the earliest efforts of mankind to emerge from a state of barbarism; but they are no less serviceable to nations which have reached the acme of civilization. In the earliest ages they were regarded with veneration, and became the objects of a grateful adoration, surpassed only by that paid to the sun and the host of heaven. 5. Nor is this suprising; for in countries where the labors of the husbandman and shepherd depended, for a successful issue, on the falling of periodical rains, or the melting of the collected snows in a far-distant country, such rivers as the Nile, the Ganges, and the Indus were the visible agents of nature in bestowing on the inhabitants of their banks all the blessings of a rich and spontaneous fertility; and hence their waters were held sacred, and they received, and to this day retain, the adoration of the countries through which they flow. The See p. 347. Intermitting springs sometimes flow only during the dry season. cut above will explain this. Suppose the internal fountain to be empty. When the earth has become fully saturated with water during the wet season, the water begins to penetrate to the fountain, which gets filled as high as the upper bend of the siphon about the time when the dry season commences, and it is just then that the siphon begins to empty it; and it is evident that it will continue to act until the fountain is exhausted. After stopping, the water can not flow again until the fountain has been again filled, which prob. ably will not be until near the end of the wet season. |