310. The floating masses of ice in the arctic waters are of two kinds, sheet-ice and icebergs, which have quite an independent origin. Sheet-ice is that which is formed by the freezing of the ocean's surface, and is generally level like that of lakes; it rises from two to eight feet out of the water. Vast fields, 20 or 30 miles in diameter, have been found in the Arctic Ocean; sometimes they extend 100 miles, so closely packed together that no opening is left between them. Smaller sheets are called floes. Fields and floes, when much broken up, the fragments crowding together, form what is called packice, which, when much elongated, is called a stream. When the parts of a pack are loose and open, so that a vessel may sail between them, it is called drift-ice. 811. In 1850, Lieut. De Haven, commanding the Grinnell Expedition in search of Sir John Franklin, proceeded into the Arctic Ocean a considerable distance north of Wellington Channel. Here, in the early part of October, while drifting about among large masses of floating ice, his vessels (two in number) were frozen in so firmly that it was impossible, with all the means at command, to disengage them from the ice. In this state they were drifted back through Wellington Channel and Lancaster Sound into Baffin Bay, thence southeasterly through this bay to about latitude 660 north, where, after having been confined in the ice nearly eight months, and having drifted not less probably than 1,500 miles, they were liberated from their icy fetters. 312. Icebergs are fresh-water formations; and, towering like cliffs to a considerable height, they present a very different aspect from ice-fields. They are produced on the shores of arctic lands by the freezing of melted snow, like the glaciers of Switzerland. The frozen masses projecting into the sea, yield to its undermining and wrenching power, by which immense blocks are broken off, constituting icebergs. These huge masses are drifted southward 2,000 miles from the places of their origin to melt in the Atlantic, where they cool the ICEBERGS. water and air to a great distance around. Icebergs vary from a few yards to miles in circumference, and are often 1,000 feet high. 313. It is supposed that the point of the greatest cold is near the northern border of Little Grinnell Land, north of Welling Questions.-810. Of what two kinds are the floating masses of ice? Describe the sheetice. Its extent. Floes. Pack-ice. Streams. Drift-ice. 311. What part of the Arctic Ocean was reached by Lieut. De Haven? What subsequently happened to his vessels? How long were they confined and how far did they drift? 312. What are icebergs? How produced? How far south do they sometimes drift? the Mackenzie River and its neighboring shores to the north; and that, in the highest latitudes yet reached, both animal and vegetable life appear to be more abundant than a few degrees farther south, and the waters exhibit a higher temperature. In further confirmation of this conjecture, Dr. Kane reports discovery of a great body of open water north of Cape Constitution (latitude 81° 22' north), on the coast of Greenland; along the shores of which the exploring party traveled for many miles, and which "was viewed from an elevation of 580 feet, still without a limit, moved by a heavy swell, free of ice, and dashing in surf against a rock-bound shore." 314. The Atlantic Ocean is the best known of any of the divisions of the great deep, it being the highway of the world's commerce, and constantly traversed by hosts of vessels in which millions of property and thousands of lives are embarked. It extends upward of 9,000 miles from north to south, with a width varying from little more than 900 miles between Norway and Greenland, to 1,700 miles between Cape St. Roque, in Brazil, and the coast of Sierra Leone, in Africa. 315. The North Atlantic, though generally very deep, is remarkable for immense shoals occurring in the North Sea, and to the southeast of Newfoundland. It is also noted for the immense portion of its surface occupied by sea-weed (fucus natans), closely matted together, forming what is sometimes known as the "Grassy Sea." A region of this weed extends along the meridian of 40° west longitude, and between the latitudes 20° and 45° north, bearing the name of "Banks of Fucus." It occurs thence in varying quantities to the Bahamas, the area occupied being equal to 1,000,000 geographical square miles and upward,-more than one third the extent of the whole territory of the United States. 316. The Pacific Ocean has about twice the area of the Atlantic, extending upward of 9,000 miles from north to south, and from east to west 12,000 miles. It was so called by the Questions.-813. Supposed point of the greatest cold? Open sea or "Polynia"? Confirmation of this conjecture? 314. The Atlantic Ocean? Its extent? 815. The North Atlantic? For what noted? Where are the Banks of Fucus situated? Where else does it occur? Extent of the sea so occupied? 816. What is said of the Pacific Ocean? Why so called? For what remarkable? early navigators on account of its peaceful aspect, as contrasted with the stormy seas of Cape Horn. It is remarkable for the number of its islands, and for submarine coral formations, which render navigation difficult and dangerous. 317. The Indian Ocean extends upward of 6,000 miles, from the tropic of Cancer, at the head of the Arabian Sea, to the antarctic circle; and has its greatest breadth, amounting to 5,000 miles, between the south point of Africa and Van Diemen's Land. It is remarkable for its hurricanes and monsoons, which will be treated of in another division. where it is of great depth. 6. The seasonal changes of the temperature of the air do not affect the ocean beyond the depth of 300 feet. 7. The greatest heat of the surface, 88° 5' of Fahrenheit, is found in the Gulf of Mexico, and in one of the havens of New Guinea. 322. The color of the ocean is generally of a deep bluish green, but it varies with every gleam of sunshine or passing cloud, from the deepest indigo to green, and even to a slaty gray. It is different in different localities, depending upon local causes. It is white in the Gulf of Guinea and black around the Maldives. Between China and Japan it is yellowish, and west of the Canaries and Azores it is green. In some parts, as off California, it has a vermilion hue; in others, as the eastern division of the Mediterranean Sea, a purple tint 318. The Antarctic Ocean presents features similar to the Arctic, but the ice extends farther from the south than from the opposite pole. Its waters are almost always agitated; there is a perpetual swell, and terrific storms are common. Packed ice occurs to an immense extent. Sir James Ross passed through prevails. a belt upward of 800 miles broad. 319. The discoveries, in 1840, of Commodore Wilkes, commander of the United States Exploring Expedition, established the fact that there is a vast antarctic continent near the south pole. Victoria Land, the most southern land known, was discovered in the same year by Sir James Ross, who penetrated to latitude 78° 10' south, the point nearest the south pole ever yet reached. 320. THE temperature of the ocean, though varying in different latitudes, is more uniform than that of the land. Thus, in equatorial regions, the difference in the temperature of the air near the surface of the sea, by day and night, does not appear to exceed three or four degrees. In temperate climates the diurnal range of temperature is rather greater, but it is by no means equal to that on land. THE OCEAN. 321. From numerous observations of the temperature of the ocean the following facts have been deduced: 1. The temperature of the surface of the ocean is generally lower at mid-day than that of the atmosphere in the shade. 2. It is always higher at midnight. 3. Morning and evening the two temperatures usually correspond. 4. The mean temperature of the surface of the ocean far from land is greater than that of the atmosphere with which it is in contact. 5. The water is colder where it is shallow than Questions.-317. The Indian Ocean? For what remarkable? 318. Antarctic Ocean? State of its waters? Packed ice? 319. The discoveries of Com. Wilkes? Of Sir James Ross? 820. What is said of the temperature of the ocean? Illustrate. 821. What is said of the temperature of the surface of the ocean at mid-day? At midnight? Morning and evening? Of the mean temperature of the ocean far from land? Of shallow water? Depths affected by seasonal changes? Where is the greatest heat of the surface found? 323. These various shades are, in most instances, caused by myriads of marine animalcules which pervade the deep; and the magnificent appearance, known as the phosphorescence of the ocean, is owing to the phosphorescent brilliancy of these microscopic tribes. The bed of the ocean, in shallow places, often imparts a tinge to the superincumbent waters, while the gray or turbid appearance, near the mouths of large rivers, arises from the sediment washed in from the land. 324. The depth of the ocean was, until recently, a subject of speculation only. The experiments made during the past few years have added more to our knowledge of the depth of the ocean and the shape of the oceanic basins, particularly that of the Atlantic, than was ever before known. 325. Like the dry land, the bottom of the sea is diversified with slopes, plains, table-lands, eminences abruptly projecting to within a few feet of the surface, or just peering above the waves, and with enormous depressions. It has been generally supposed that the depth of the sea is about equal to the height of the land, the lowest valleys of the ocean's bed corresponding with the summits of the loftiest mountains. 326. The experiments in deep-sea sounding initiated a few years since by Lieut. Maury, formerly of the United States National Observatory, will probably throw some light upon this conjecture. Before a systematic investigation was thus attempted, no well-directed efforts to fathom the lower abysses had been made. Navigators had tied weights to lines and thrown them overboard with the view of measuring the depth; but the lines were often unwieldy, and there was no certain means of knowing whether the plummet had reached the bottom, or, if it had reached the bottom, at what moment. 327. More recent investigations have led to the belief that there is in the ocean, as in the air, a system of circulation, which, by currents and counter currents, upper and under, keeps the waters perpetually in motion. For it has been found that, generally speaking, when a sounding is made in the deep sea, though the vessel from which it is made be perfectly at rest, and though it be known that the plummet has reached the bottom, yet the line will continue to run out, and unless it be suffered to run out, or the plummet be detached from it, a strain Questions. 322. Color of the ocean? Mention its hues in different localities. 328. How are these shades generally caused? What other causes affect its color? 824. What is said of the depth of the ocean? Recent experiments? $25. How is the bottom of tl. sea diversified? What was generally supposed respecting the depth of the sea? 826. What is said of former efforts to ascertain the depth of the sea? 827. To what belief have more recent investigations led? What circumstances favor this belief? so great is brought to bear that it breaks. It is the undertow, | ably a little to the south of the Grand Banks of Newfoundor a system of currents and counter currents below, which it is supposed produces this strain. 328. Most of the vessels of the navy are now furnished with twine made especially for deep-sea soundings; and the results already obtained have enabled the officers at the National Observatory to construct a map of the basin of the North Atlantic Ocean, which shows the depressions of the solid parts of the earth's crust below the sea-level, and which gives us, perhaps, as good an idea of the profile there as geographers have of the contrasts afforded by the elevations of the land in many parts of the earth. 329. The deepest soundings ever reported were made in the North and in the South Atlantic Ocean. Lieut. J. C. Walsh, commander United States schooner Taney, being furnished with a large quantity of iron wire made expressly for the purpose, obtained, on the 15th November, 1849, latitude 31° 59' north, longitude 58° 43′ west, a cast of the plummet, when, after 34,200 feet had run out, the wire parted without reaching bottom, as it was thought. On the 12th of February, 1853, Lieut. Berryman, of the Dolphin, in latitude 32° 55′ north, longitude 47° 58' west, obtained a cast of the lead, using the small twine as a sounding line. At this trial 39,600 feet ran out, when the line parted, and it was consequently thought that the plummet had not reached the bottom. On the 5th of April, 1852, latitude 36° south, longitude 44° 11' west, Lieut. Parker, of the United States frigate Congress, using a 32 lb. cannonball for his plummet, and sounding twine like that of Berryman's, made an experiment at deep-sea soundings, when 49,800 feet of line ran out before it parted. The time occupied for this sounding was eight hours and a quarter. 330. The next great sounding was made by Capt. Denham, of H. M. ship Herald, 30th October, 1852, latitude 36° 49′ south, longitude 37° 06′ west, with 46,236 feet. He had been furnished with sounding twine from the United States frigate Congress, and instead of a 32-lb. shot, his sinker was a 9 lb. lead. By the light which subsequent experience has thrown upon the subject of deep-sea soundings, all four of these immense depths have had their accuracy questioned, and it is believed with reason. 331. An instrument has been invented by Passed Midshipman J. M. Brooke, of the United States navy, which enables the officers who now attempt deep-sea soundings to detach the plummet from the line the moment it strikes the bottom, and then to haul up, attached to the line, specimens of the bottom. In this way specimens have been obtained from the depth of 12,000 feet (about 2 miles). These specimens have been examined with a microscope by Prof. Bailey at West Point, and found to consist entirely of minute sea-shells, not a particle of sand or gravel, or any foreign matter being among them. From this it is inferred that the water at the bottom of the sea is comparatively at rest. land. There is a place there somewhat in the shape of a boot, which none of the officers of the navy have so far been able to fathom. The deepest soundings that have been satisfactorily made show that, in all other parts, the North Atlantic Ocean is not more than 25,000 feet in depth. The soundings which have been made by the navy have established the fact that there is a plateau, or shelf, at the bottom of the ocean between Newfoundland and Ireland, quite shallow enough for the wires of a submarine telegraph, and quite deep enough to keep them beyond the reach of icebergs. LESSON X v. THE OCEAN-(CONTINUED). 333. THE Ocean is subject to a motion of three different kinds: it is agitated by the action of the wind, producing waves; by tides, which result from the attraction of the moon and sun; and by currents, produced under various circumstances, and resulting from a variety of causes. 334. Waves are produced by the action of the winds on the surface of the water, and vary in size from mere ripples to enormous billows. Their height in open sea depends upon the force and duration of the wind, and the angle at which it bears down upon the waters; but in lakes and bays it is affected by the shallowness of the waters and the character of the shores; on which account the shallow waters of Lake Erie are more readily disturbed by winds than the deeper lakes, Ontario and Huron. 335. Waves are not, as appearances would indicate, an onward flow of water. This is proved from the fact that a floating body merely rises and falls with little or no progression. Waves agitate the water but a little way below the surface, and it is supposed that the effect of the strongest gales does 332. The deepest part of the North Atlantic Ocean is prob- not extend deeper than 200 feet. Questions.-828. With what are most of the vessels of the navy provided? Results obtained? 329. When have the deepest soundings been made? Give particulars of the sounding made by Lieut. J. C. Walsh. By Lieut Berryman. By Lieut. Parker. 330. By Capt. Denham. What is thought of these soundings by the light of subsequent experi ments? 881. How have specimens of the bottom of the ocean been obtained? Of what have these specimens been found to consist? Questions.-882. Where is the deepest part of the North Atlantic supposed to be? What do the soundings show with respect to the depth of the North Atlantic in all other parts? What other important fact has been established by these soundings? 338. What are the three different kinds of motion to which the ocean is subject? 334. How are waves pro duced, and how do they vary? Upon what does their height depend? How is the height affected in lakes and bays? 835. How is it proved that waves are not an onward flow of water? 336. The crest of a wave (b, b) is the ridge or highest part, and in strong winds is usually covered with foam; the trough a (c) is the depression between two waves, and is as much below as the crest is above the general level of the ocean. In estimating the elevation of a wave, the perpendicular height from the trough to the crest is taken. 337. Waves are sometimes said to run mountains high, but this is a popular exaggeration. The highest rise noticed in the Mediterranean is 16 feet, and 20 feet off Australia. During a storm in the Bay of Biscay, the highest waves measured scarcely 36 feet from the base to the summit. In the South Atlantic the result of several experiments gave only an entire height of 22 feet, and a velocity for the undulations of 89 miles per hour, the interval between each wave amounting to 1,910 feet. Off the Cape of Good Hope, notoriously the cape of storms, according to its former name, 40 feet is considered the extreme height of waves, or 20 feet above and below the general level of the Off 338. The sea does not regain its placidity immediately after the subsidence of the winds which set it in motion, but continues to heave with mighty undulations for a considerable time afterward. This movement is called the "swell." It frequently occurs, that while the swell is advancing in one direction, the wind rises from an opposite quarter, producing a series of compound waves, and giving to the deep a very complex aspect. 339. Tides are those regular alternate risings and fallings of the waters of the ocean and of bays, rivers, etc., which communicate freely with it. They arise from the attractive influence of the sun and moon, the latter being the more powerful agent. The sea rises, or flows, as it is called, by degrees, about six hours; it remains stationary about a quarter of an hour; it then retires, or ebbs, during another six hours, to flow again after a brief repose. Thus high and low water occur twice every lunar day, or the period elapsing between the successive returns of the moon to the meridian of a place, which is 24 hours 50 minutes. E 340. The theory of the tides may be thus explained: Let E represent the earth surrounded by water in every part, and m the moon. As a solid body draws toward it any other body, by a force which varies with its distance from the point attracted, then the water at a will be more powerfully attracted than those at e and f, and the result will be a bulging out of the water at a, immediately next the lunar body. 341. But while high water is thus produced at a, it is also produced at the same time in the opposite hemisphere at d. Different causes have been assigned for this phenomenon, but it Questions.-336. What is the crest of a wave? The trough? 337. Give instances to show the height of waves. 338. What is the movement of the sea called which continues after the subsidence of the winds? How are compound waves produced? 839. What are tides? From what do they arise? Describe the successive movements of the sea. 340. How may the theory of the tides be explained? Rev. Thomas Milner. is probably likewise owing to the attractive influence of the moon. The earth's center, E, will be more powerfully drawn toward the moon than the waters at d, and recede from them, producing the same effect as though they receded or rose up from the center of the earth. As the waters can not rise in one place without falling in another, they are depressed at e and ƒ when elevated at a and d. 342. If the earth remained stationary upon its axis, there would be two elevations and depressions of the waters in each place in a month, or the time the moon is making her circuit round the earth. But by the diurnal rotation, the moon passes every day the meridian of every part of the globe, producing daily two seasons of high and low water. 343. The time of high tide does not coincide with the time when the moon is on the meridian of the place, a period of seyeral hours ordinarily intervening between the time of the moon's transit and that of high water. The explanation of this is, that the water, having received motion, continues to rise after the moon has passed from its meridian, the impulse continuing for some time after the moon's transit. 344. Besides the attraction of the moon, the influence of the sun is concerned in elevating the waters of the ocean. The solar attraction is not so strong as the lunar, because, though a much larger body, the sun is at a far greater distance from the earth. The tidal influence of the sun has been calculated to be about one third that of the moon. 345. Sometimes the sun and moon act in conjunction, as at the seasons of new and full moon, a and d, and then the tides rise the highest, and are called spring-tides; but when the moon is in quadrature, as at b and c, it acts in opposition to the sun, and then occurs the lowest, or neap-tides. 346. Though high tides occur in open seas soon after the moon has passed the meridian, yet in other places shoals and channels, peninsulas and capes, and the diversified form of shores, so retard the progress S of the undulation, that high water happens at all distances o the moon from the meridian, and consequently at all hours of the day. Differences of local situation cause great differences as to the height to which the tide rises. In the central parts of the ocean the height is comparatively small, seldom exceeding three feet, but in rushing up contracted channels the tide-water frequently causes a difference of twenty, thirty, and sixty feet between high and low water mark. 347. The cradle of tides is supposed to be that part of the Pacific Ocean to the southeast of Australia. Proceeding from this quarter, a wave advances into the Indian Ocean, reaching the island of Sumatra, the southern point of Hindoostan, the Questions.-341. Explain how high water will be produced in the hemisphere opposite the moon. 312. What would happen if the earth remained stationary? What occurs by the diurnal rotation? 843. What is said of the time of high water? How explained? 344. What is said of the influence of the sun on the tides? 345. When do the spring-tides occur? The neap-tides? 346. What serve to retard the progress of the undulation? What is said of the differences of local situation in affecting tides? Illustrate. 347. What is supposed to be the cradle of the tides? Describe the movements of a wave proceeding from this quarter. island of Madagascar, and the Cape of Good Hope about the same time. [See map No. 1.] 348. Entering the Atlantic, the wave proceeds northerly and westerly, bringing high water successively to all parts on the western shores of Africa and eastern shores of America. It moves with much greater rapidity through the central parts of the ocean than along the coast-in consequence of which it reaches the islands of Cuba and Newfoundland, and Cape Blanco, in Africa, simultaneously. The Atlantic coast of the United States receives the wave from the east, while the westcrn coast of Europe receives it from the west, the direction, in the latter case, being nearly the reverse of what it had previously to doubling the Cape of Good IIope. 349. Along the shores of South America, from Rio Janeiro to the Falkland Islands, a wave advances from east to west, bringing high tide later on going southward, as if the wave came from the north. On the western coast of South America the tide travels from north to south, between the Gulf of Panama and the Strait of Magellan. And on the same side of North America, it travels from the Gulf of Panama northward to Queen Charlotte Sound. 350. The height to which tides rise varies greatly in different places. At St. Helena the rise does not exceed three feet; at St. Malo's, on the north coast of France, the spring tides rise 50 feet; at Chepstow, on the British Channel, 60 feet; in the Bay of Fundy, between Nova Scotia and New Brunswick, it is known to rise 70 feet. On some occasions the rapidity of the waters is so great as to overtake animals feeding on the shores. 352. Currents are due to a variety of causes; as the influence of tides and winds,-the evaporating power of the sun,-the expansion and contraction of water by heat and cold,-and the revolution of the earth upon its axis. 353. The effect of the rise and fall of tides in producing an alternate flowing of currents in opposite directions, is perceived in channels between islands, or between islands and the mainland. Thus, in the channel which connects Long Island Sound with the Harbor of New York, known as the East River, strong currents alternately prevail in opposite directions, as the tide ebbs or flows. 354. Evaporation by solar heat is another cause of oceanic currents. Large quantities of water raised from one tract of the ocean are transported to some other, where the vapor is bus test se CC ง ง GIBRALTAR. condensed and falls in the form of rain; this, in flowing back to restore equilibrium, causes sensible currents. A perpetual stream flows into the Mediterranean from the Black Sea through the Bosphorus and the Hellespont, and another from the Atlantic through the Strait of Gibraltar. No counter, lateral, or submarine currents have been discovered sufficient to dispose of the quantity of water flowing inward; hence the inference that the inward current goes to supply the waste caused by an enormous evaporation. 355. The expansion and contraction of water by heat and cold are perhaps the principal causes to which currents are due. Warm water is specifically lighter than cold, and when certain portions become heated, they rise by reason of their buoyancy above the general surface, and are replaced by surrounding colder and heavier fluid flowing in beside or beneath them; while they flow off above. 356. The revolution of the earth upon its axis is still another powerful cause in producing currents, particularly those of the equatorial regions, which have commonly a westerly direction. The winds of tropical climates, which blow continuously, or during long periods in one direction, also lend their influence in affecting this class of oceanic movements. 357. Currents may be classed into constant, periodical, variable, counter, and drift currents. Constant currents are pro Questions.-351. Of what do currents consist? 352. To what are they due? 358. Where is the effect of the rise and fall of tides in producing currents perceived? Example. 854. How does evaporation by solar heat operate to produce currents? Mention what is said of the Mediterranean Sea. 355. What are perhaps the principal causes to which currents are due? Explain how they affect the currents. 356. What other causes are instrumental in producing currents? 857. How may currents be classed? What are constant currents? Periodical currents? Variable currents? Counter currents? Drift currents? |