« 前へ次へ »
310. The floating masses of ice in the arctic waters are of | ton Channel, and that not far to the north and west of it two kinds, sheet-ice and icebergs, which have quite an inde- there is, in summer, a comparatively open sea, or “Polynia.” pendent origin. Sheet-ice is that which is formed by the The latter opinion is supported by the fact that beasts and freezing of the ocean's surface, and is generally level like that fowls are known to migrate over the ice from the mouth of 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.
311. 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 66° north,
SCENE IN THE ARCTIC OCEAN. 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
Mackenzie River and its neighboring shores to the north ; and their icy fetters.
that, in the highest latitudes yet reached, both animal and veg312. Icebergs are fresh-water formations; and, towering like etable life appear to be more abundant than a few degrees cliffs to a considerable height, they present a very different farther south, and the waters exhibit a higher temperature. aspect from ice-fields. They are produced on the shores of In further confirmation of this conjecture, Dr. Kane reports the arctic lands by the freezing of melted snow, like the glaciers discovery of a great body of open water north of Cape Constiof Switzerland. The frozen masses projecting into the sea, tution (latitude 81° 22' north), on the coast of Greenland ; yield to its undermining and wrenching power, by which im- along the shores of which the exploring party traveled for many mense blocks are broken off, constituting icebergs. These miles, and which was viewed from an elevation of 580 feet, huge masses are drifted southward 2,000 miles from the places still without a limit, moved by a heavy swell, free of ice, and of their origin to melt in the Atlantic, where they cool the 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 BahaICFBERGS.
mas, the area occupied being equal to 1,000,000 geographical water and air to a great distance around. Icebergs vary from square miles and upward,-more than one third the extent a few yards to miles in circumference, and are often 1,000 feet of the whole territory of the United States. high.
316. The Pacific Ocean has about twice the area of the At313. It is supposed that the point of the greatest cold is near lantic, extending upward of 9,000 miles from north to south, the northern border of Little Grinnell Land, north of Welling- and from east to west 12,000 miles. It was so called by the
Questions.—310. 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? 812. What are icebergs? How produced ? How far south do they sometimes drift?
Questions.—813. Supposed point of the greatest cold? Open sea or “Polynia”? Confirmation of this conjecture? 314. The Atlantic Ocean? Its extent? 315. 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 where it is of great depth. 6. The seasonal changes of the with the stormy seas of Cape Horn. It is remarkablo for the temperature of the air do not affect the ocean beyond the depth number of its islands, and for submarine coral formations, which of 300 feet. 7. The greatest heat of the surface, 88° 5' of render navigation difficult and dangerous.
Fahrenheit, is found in the Gulf of Mexico, and in one of the 317. The Indian Ocean extends upward of 6,000 miles, from havens of New Guinea. the tropic of Cancer, at the head of the Arabian Sea, to the 322. The color of the ocean is generally of a deep bluish antarctic circle; and has its greatest breadth, amounting to green, but it varies with every gleam of sunshine or passing 5,000 miles, between the south point of Africa and Van Die cloud, from the deepest indigo to green, and even to a slaty men's Land. It is remarkable for its hurricanes and monsoons, gray. It is different in different localities, depending upon which will be treated of in another division.
local causes. It is white in the Gulf of Guinea and black 318. The Antarctic Ocean presents features similar to the around the Maldives. Between China and Japan it is yellowArctic, but the ice extends farther from the south than from the ish, and west of the Canaries and Azores it is green. opposite pole. Its waters are almost always agitated; there is parts, as off California, it has a vermilion hue; in others, as a perpetual swell, and terrific storms are common. Packed ice the eastern division of the Mediterranean Sea, a purple tint occurs to an immense extent. Sir James Ross passed through prevails. a belt upward of 800 miles broad.
323. These various shades are, in most instances, caused by 319. The discoveries, in 1840, of Commodore Wilkes, com- myriads of marine animalcules which pervade the deep; and mander of the United States Exploring Expedition, estab- the magnificent appearance, known as the phosphorescence lished the fact that there is a vast antarctic continent near of the ocean, is owing to the phosphorescent brilliancy of these the south pole. Victoria Land, the most southern land microscopic tribes. The bed of the ocean, in shallow places, known, was discovered in the same year by Sir James Ross, often imparts a tinge to the superincumbent waters, while the who penetrated to latitude 78° 10' south, the point nearest gray or turbid appearance, near the mouths of large rivers, arises the south pole ever yet reached.
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. LESSON XIV.
325. Like the dry land, the bottom of the sea is diversified
with slopes, plains, table-lands, eminences abruptly projecting THE OCEAN-(CONTINUED).
to within a few feet of the surface, or just peering above the 320. The temperature of the ocean, though varying in differ- waves, and with enormous depressions. It has been generally
ent latitudes, is more uni- supposed that the depth of the sea is about equal to the height
this conjecture. Before a systematic investigation was thus to exceed three or four de attempted, no well-directed efforts to fathom the lower abysses grees. In temperate cli- had been made. Navigators had tied weights to lines aná mates the diurnal range of thrown them overboard with the view of measuring the depth; temperature is rather great- but the lines were often unwieldy, and there was no certain er, but it is by no means means of knowing whether the plummet had reached the botequal to that on land.
tom, or, if it had reached the bottom, at what moment.
321. From numerous ob- 327. More recent investigations have led to the belief that servations of the temperature of the ocean the following facts there is in the ocean, as in the air, a system of circulation, have been deduced : 1. The temperature of the surface of the which, by currents and counter currents, upper and under, ocean is generally lower at mid-day than that of the atmo- keeps the waters perpetually in motion. For it has been found sphere in the shade. 2. It is always higher at midnight. 3. that, generally speaking, when a sounding is made in the deep Morning and evening the two temperatures usually correspond. sea, though the vessel from which it is made be perfectly at 4. The mean temperature of the surface of the ocean far from rest, and though it be known that the plummet has reached the land is greater than that of the atmosphere with which it is bottom, yet the line will continue to run out, and unless it be in contact. 5. The water is colder where it is shallow than suffered to run out, or the plummet be detached from it, a strain
Questions.-317. The Indian Ocean ? For what remarkable ? 318. Antarctic Ocean ? State of its waters ? Packed ice? 319. The discoveries of Com. Wilkes ? or 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 ?
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? 324. W ba! is said of the depth of the ocean? Recent experiments ? $25. How is the bottom of thJ 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?
80 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 land. There is a place there somewhat in the shape of a boot, supposed produces this strain.
which none of the officers of the navy have so far been able to 328. Most of the vessels of the navy are now furnished with fathom. The deepest soundings that have been satisfactorily twine made especially for deep-sea soundings; and the results made show that, in all other parts, the North Atlantic Ocean already obtained have enabled the officers at the National Ob- is not more than 25,000 feet in depth. The soundings which Bervatory to construct a map of the basin of the North Atlantic have been made by the navy have established the fact that Ocean, which shows the depressions of the solid parts of the there is a plateau, or shelf, at the bottom of the ocean beearth's crust below the sea-level, and which gives us, perhaps, tween Newfoundland and Ireland, quite shallow enough for as good an idea of the profile there as geographers have of the the wires of a submarine telegraph, and quite deep enough to contrasts afforded by the elevations of the land in many parts keep them beyond the reach of icebergs. 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'
LESSON X V. north, longitude 58° 43' west, a cast of the plummet, when,
THE OCEAN-(CONTINUED). after 34,200 feet had run out, the wire parted without reaching bottom, as it was thought. On the 12th of February, 1853,
333. The ocean is subject to a motion of three different Lieut. Berryman, of the Dolphin, in latitude 32° 55' north, kinds : it is agitated by the action of the wind, producing longitude 47° 58' west, obtained a cast of the lead, using the
waves; by tides, which result from the attraction of the moon small twine as a sounding line. At this trial 39,600 feet ran
and sun; and by currents, produced under various circumout, when the line parted, and it was consequently thought that
stances, and resulting from a variety of causes. the plummet had not reached the bottom. On the 5th of April,
334. Waves are produced by the action of the winds on the 1852, latitude 36° south, longitude 44° 11' west, Lieut. Parker, surface of the water, and vary in size from mere ripples to of the United States frigate Congress, using a 32.lb. cannon
enormous billows. Their height in open sea depends upon the ball for his plummet, and sounding twine like that of Berry- force and duration of the wind, and the angle at which it bears man's, made an experiment at deep-sea soundings, when 49,800
down upon the waters; but in lakes and bays it is affected by feet of line ran out before it parted. The time occupied for this
the shallowness of the waters and the character of the shores; 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. on which account the shallow waters of Lake Erie are more In this way specimens have been obtained from the depth readily disturbed by winds than the deeper lakes, Ontario and of 12,000 feet (about 2} miles). These specimens have been Huron. examined with a microscope by Prof. Bailey at West Point, 335. Waves are not, as appearances would indicate, an onand found to consist entirely of minute sea-shells, not a particle ward flow of water. This is proved from the fact that a floatof sand or gravel, or any foreign matter being among them. ing body merely rises and falls with little or no progression. From this it is inferred that the water at the bottom of the sea Waves agitate the water but a little way below the surface, is comparatively at rest.
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.
Question 8.–328. With what are most of the vessels of the navy provided ? Results obtained ? 329. When have the deepest soundings been made ? Give par iculars 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 experiments? 881. How have specimens of the bottom of the ocean been obtained? Or 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 soundingg? 338. What are the three different kinds of motion to which the ocean is subject? 334. How are waves proJuced, and how do they vary? Upon what does their height depend ? How is the height affected in lakes and bays # 335. How is it proved that waves are not an onward dow of water?
336. The crest of a wave (6,5) is the ridge or highest part, is probably likewise owing to the attractive influence of the
and in strong winds is usually moon. The earth's center, E, will be more powerfully drawn 7
covered with foam; the trough toward the moon than the waters at d, and recede from them, (c) is the depression between producing the same effect as though they receded or rose up
two waves, and is as much be- from the center of the earth. As the waters can not rise in one low as the crest is above the general level of the ocean. In place without falling in another, they are depressed at e and f estimating the elevation of a wave, the perpendicular height when elevated at a and d. from the trough to the crest is taken.
342. If the earth remained stationary upon its axis, there 337. Waves are sometimes said to run mountains high, but would be two elevations and depressions of the waters in each this is a popular exaggeration. The highest rise noticed in the place in a month, or the time the moon is making her circuit Mediterranean is 16 feet, and 20 feet off Australia. During a round the earth. But by the diurnal rotation, the moon passes storm in the Bay of Biscay, the highest waves measured scarcely every day the meridian of every part of the globe, producing 36 feet from the base to the summit. In the South Atlantic daily two seasons of high and low water. the result of several experiments gave only an entire height of 343. The time of high tide does not coincide with the time 22 feet, and a velocity for the undulations of 89 miles per hour, when the moon is on the meridian of the place, a period of sey. the interval between each wave amounting to 1,910 feet. Off eral hours ordinarily intervening between the time of the moon's the Cape of Good Hope, notoriously the cape of storms, accord- transit and that of high water. The explanation of this is, that ing to its former name, 40 feet is considered the extreme height the water, having received motion, continues to rise after the of waves, or 20 feet above and below the general level of the moon has passed from its meridian, the impulse continuing for ocean.
some time after the moon's' transit. 338. The sea does not regain its placidity immediately after 344. Besides the attraction of the moon, the influence of the the subsidence of the winds which set it in motion, but con- sun is concerned in elevating the waters of the ocean. The tinues to heave with mighty undulations for a considerable solar attraction is not so strong as the lunar, because, though a time afterward. This movement is called the “swell.” It much larger body, the sun is at a far greater distance from the frequently occurs, that while the swell is advancing in one di- earth. The tidal influence of the sun has been calculated to be rection, the wind rises from an opposite quarter, producing a
about one third that of the moon. series of compound waves, and giving to the deep a very com- 345. Sometimes the sun and moon act in conjunction, as at plex aspect.
the seasons of new and full moon, a and d, and then the tides 339. Tides are those regular alternate risings and fallings rise the highəst, and are called spring-tides; but when the of the waters of the ocean and of bays, rivers, etc., which moon is in quadrature, as at b communicate freely with it. They arise from the attractive and c, it acts in opposition to S influence of the sun and moon, the latter being the more pow- the sun, and then occurs the erful agent. The sea rises, or flows, as it is called, by degrees, lowest, or neap-tides. about six hours; it remains stationary about a quarter of an
346. Though high tides ochour; it then retires, or ebbs, during another six hours, to flow cur in open seas soon after the again after a brief repose. Thus high and low water occur moon has passed the meridian, twice every lunar day, or the period elapsing between the yet in other places shoals and successive returns of the moon to the meridian of a place, channels, peninsulas and capes, Do nich is 24 hours 501 minutes.
and the diversified form of 340. The theory of the tides may be thus explained: Let shores, so retard the progress · E represent ih2 earth surrounded by water in every part, and of the undulation, that high water happens at all distances o
m the moon. As a solid body the moon from the meridian, and consequently at all hours draws toward it any othe: of the day. Differences of local situation cause great differbody, by a force which varies ences as to the height to which the tide rises. In the central with its distance from the parts of the ocean the height is comparatively small, seldom point attracted, then the wa-' exceeding three feet, but in rushing up contracted channels the
ter at a will be more power- tide-water frequently causes a difference of twenty, thirty, and fully attracted than those at e and f, and the result will be a sixty feet between high and low water mark. bulging out of the water at a, immediately next the lunar body. 347. The cradle of tides is supposed to be that part of the
341. But while high water is thus produced at a, it is also Pacific Ocean to the southeast of Australia. Proceeding from produced at the same time in the opposite hemisphere at d. this quarter, a wave advances into the Indian Ocean, reaching Different causes have been assigned for this phenomenon, but it the island of Sumatra, the southern point of Hindoostan, the
Question8.-336. What is the crest of a wave ? The trough? 837. 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 lides? From what do they arise ? Describe the successive movements of the sea. 340. How may the theory of the tides be explained ?
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 ? 848. 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. 847. What is supposed to be the cradle of the tides ? Describe the movements of a wave proceeding from this quarter.
. Rev. Thomas Milner,
island of Madagascar, and the Cape of Good Hope about the 352. Currents are due to a variety of causes; as the influence same time. [See map No. 1.]
of tides and winds,-the evaporating power of the sun,--the 348. Entering the Atlantic, the wave proceeds northerly and expansion and contraction of water by heat and cold,—and the westerly, bringing high water successively to all parts on the revolution of the earth upon its axis. western shores of Africa and eastern shores of America. It 353. The effect of the rise and fall of tides in producing an moves with much greater rapidity through the central parts of alternate flowing of currents in opposite directions, is perthe ocean than along the coast–in consequence of which it ceived in channels between islands, or between islands and the reaches the islands of Cuba and Newfoundland, and Cape mainland. Thus, in the channel which connects Long Island Blanco, in Africa, simultaneously. The Atlantic coast of the Sound with the Harbor of New York, known as the East River, United States receives the wave from the east, while the west- strong currents alternately prevail in opposite directions, as the crn coast of Europe receives it from the west, the direction, in tide ebbs or flows. the latter case, being nearly the reverso of what it had pre- 354. Evaporation by solar heat is another cause of oceanic viously to doubling the Cape of Good IIope.
currents. Large quantities of water raised from one tract of 349. Along the shores of South America, from Rio Janeiro the ocean are transported to some other, where the vapor
is 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.
TIDE TABLE FOR THE COAST OF THE UNITED STATES.
Interval betw, time
time of th water.
Rise and Fall.
Mean. Spring. Neap.
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 11. M.
to dispose of the quantity of water flowing inward; hence the
inference that the inward current goes to supply the waste New Bedford Harbor...
caused by an enormous evaporation. New Haven..
355. The expansion and contraction of water by heat and cold
are perhaps the principal causes to which currents are due. Smithville..
Warm water is specifically lighter than cold, and when certain Key West....
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. LESSON XVI.
356. The revolution of the earth upon its axis is still another
powerful cause in producing currents, particularly those of the THE OCEAN-(CONTINUED).
equatorial regions, which have commonly a westerly direction. 351. CURRENTS constitute the third oceanic movement. They The winds of tropical climates, which blow continuously, or consist of vast oceanic streams which keep up a perpetual circula- during long periods in one direction, also lend their influence in lion of the waters, transferring them from one hemisphere to an- affecting this class of oceanic movements.
ther—from the Pacific to the Atlantic, and to the Pacific again 357. Currents may be classed into constant, periodical, vari—and from the polar seas to the warm regions of the torrid zone. able, counter, and drift currents. Constant currents are pro
6 14 6 16 6 50 6 21 6 2+ 6 0 6 1 5 54 6 1 .8 4 6 59
6 12 6 18 5 33 6 8 6 5 6 25 6 25 6 83 6 26 7 22 5 25
1.5 55 7.6 2.8
0.9 8.8 5.5 0.7
Questions.—348. Its progress after entering the Atlantic Orean. What three points are re: ched simultaneously? 349. What is the direction of the wave south of Rio Janeiro ? Describe its movements on the western coast of South America. On the western coast of North America. 350. Mention the height to which tides rise at different places.
Questions.- 351. Of what do currents consist? 352. To what are they due ? 353. Where is the effect of the rise and fall of tides in producing currents perceived ? Example. 854. How does evaporation by solar beat 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 current ? Periodical currents ? Variable currents ? Counter currents? Drift currents ?
The author is indebted for the above table to the kindness of A. D. Bache, Superin. tendent of the United States Coast Survey.