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duced by the rotation of the earth, differences of temperature | rial Current. It originates in the general inflowing of waters in the waters of the ocean, and other causes not yet fully from the temperate and polar regions, but especially from the understood. Periodical currents are principally due to the Antarctic Ocean and neighboring seas. These waters, over. action of tides, though they are doubtless affected somewhat balancing the heated and therefore expanded and specifically by the land and sea breezes and monsoons. Variable currents | lighter waters at the equator, cause them to rise above the are occasioned by tides, winds, and the melting of ice in the general level and overflow in a constantly spreading stream. polar regions. Counter currents are the streams that flow But as the inflowing waters come from parts of the earth’s alongside or beneath, and in opposite directions to, other surface which lie nearer the poles or the earth's axis, and currents. Drift currents are the effect of permanent and pre- have a less rapid diurnal movement (from west to east) than vailing winds upon the surface of the sea, and a variety of the parts near the equator, they accordingly partake of this other causes.
movement; and hence, as they approach the equator, fall be358. EASTERN POLAR OR GREENLAND CURRENT.—The East- hind the more swiftly advancing points of its surface, and thus ern Polar or Greenland Current originates in the Arctic Ocean acquire a western tendency. Accordingly the Equatorial Curnorth of Asia. It sweeps around the northern shores of Spitz rent, fed by these westerly moving waters, has a resulting bergen and Iceland, and flows southwesterly between the latter westward course; and, crowding against the eastern shores island and Greenland. Near Cape Farewell ir sends off a of the continents and of the great islands which separate the branch which runs into Baffin Bay, but its principal flow ap- Pacific from the Indian Ocean, divides into various streams, pears to be southwesterly between the Gulf Stream and the
most of which flow off to the north and south. neighboring coast of America.
363. In the Indian Archipelago and neighboring seas, how359 The breadth of this current is in some places from ever, it separates into numerous small branches that take 250 to 300 miles. Its velocity varies, in different parts of
different courses according to the channels through which they its course, from eight or nine to fifteen or sixteen miles per flow: hence the variable currents prevailing in the Indian day. The icy masses it bears along, and which are frequently Ocean, which render navigation so dangerous. A large volswept around Cape Farewell, are supposed to be about two
ume of water forces its way through the islands, and joins the months in making the circuit to Baffin Bay and thence to the
great equaorial current which moves on toward the eastern coast of Labrador,
coast of Afrca. The greater portion of the stream flows round 360. This current is distinguished for the grea 'amount of north of the island of Madagascar, and sweeps through the drift-wood which it floats along and casts upon ne shores of channel of Mozambique, after which, being joined by other Spitzbergen, Iceland, and other lands lying in itway. The
currents from the east, it moves toward the southern exmasses of floating wood thrown upon the island ofJan Mayen tremity of Africa, where it is said to unite with a current often equal, it is said, the whole of the island in eent. It is descending along the western coast of the grand division, and supposed that this timber comes from the forests of beria, and
to flow thence to the Antarctic Ocean.
364. The Equatorial Current, on reaching Cape St. Roque, 361. Recent observations in high northern latifles show
the most eastern point of South America, is separated into two that the Eastern Polar or Greenland Current preses one of | branches. One proceeds southward along the coast of South the most formidable difficulties in exploring the pol regions. America, under the name of the Brazilian Current, and termiParry, who attempted to reach the north pole byneans of nates in a region of variable currents, the chief of whichboat-sledges and reindeer, traveled over the surface che deep observed as moving to the eastward with an increasing velocity to nearly lat. 83°, which seemed to be the utmost lit of an.
-has been designated as the South Connecting Current. imal life. Here he found that when, according to hreckon
365. The other and principal branch of the Equatorial Curing, he had traveled ten or eleven miles toward the rth, he rent of the Atlantic is known as the Guiana Current. It runs had actually gone four miles to the south, owing the cur
from off Cape St. Roque, across the mouth of the Amazon, and rent. The success of the expedition was thus rende: hope. after skirting the low coast of Guiana, and passing through the less. The two vessels of the Grinnell Expedition wh
Caribbean Sea, it enters the Gulf of Mexico between the island sent out, under the command of Lieut. De Haven, search
of Cuba and the peninsula of Yucatan. for Sir John Franklin, after having penetrated far i: Wel- 366. MEXICAN GULF STREAM.—This is the most powerful lington Channel, were inclosed firmly in the ice, anxifted current known, and the most important in consequence of the backward through Baffin Bay, a distance of not less théfteen
extent to which it affects the navigation of the Atlantic. It hundred miles, thus defeating the plan of one of thelest
originates in the Gulf of Mexico, the waters of which are and most humane enterprises ever undertaken.
characterized by a remarkably high temperature. It pours 362. EQUATORIAL CURRENT.—The most extensive mnent
forth at a rate of from three to five miles an hour through the of the ocean is that which proceeds from east to v straits of Florida, and flows in a northeasterly direction along each side of the equator, and is therefore called the ato
the whole coast of the United States, expanding in volume
is carried into the Arctic Ocean by the streams of nonern Asia.
Questions. - 858. Origin of Eastern Polar or Greenland Current ? Ils course ?ch ? Principal flow? 359. What is the breadth of this current? Its velocity? 860.'bat is it distinguished ! 361. What do recent observations in high northern latitucw ? Describe the attempt of Parry to reach the norih pole. The drift of the two vestbe Grinnell Expedition. 862. Which is the most extensive movement of the occan, hat 1s it called ? How does it originate? Cause of this movement of the waters i do the infowing acquire a peculiar tendency?
Questions. - Result concerning the Equatorial Current ? 868. What happens on its reaching the Indian Archipelago? Describe its subsequent course. 864. Its branches in the Atlantic ! What of its southern branch ? 865. What is the name of the other branch, and where does it run ? 866. What is said of the Mexican Gulf Stream ? Where does it originate ? Its velocity through the struits of Fiorida ? Its subsequent course ? Ils course after striking the banks of Newfoundland ? Great whirlpool of the Atlantic ?
and diminishing in rapidity.* On striking the banks of New- . 368. Coming from the heated caldron of the Gulf of Mexico, foundland, it sets to the east: the northern portion, however, the waters of the Gulf Stream have a high temperature, which sweeps toward Iceland, Norway, and the British Isles; the is gradually lost as they reach higher latitudes. “The maxisouthern portion flows to the Azores, where it turns south and mum temperature of the Gulf Stream is 86°, or about go above enters the Equatorial Current on the coast of Africa, and is the ocean temperature due the latitude. Increasing its laticonducted again to the west, to re-enter into itself in the Gulf tude 10°, it loses 2° of temperature. And, after having run of Mexico. Thus the waters of the Atlantic Ocean between 3,000 miles toward the north, it still preserves, even in winter, the parallels of 11° and 43° constitute a whirlpool of prodigious the heat of summer. With this temperature it crosses the extent, by which a single particle of water describes a circuit 40th degree of north latitude, and there, overflowing its liquid of over 11,000 miles in the space of two years and ten months. banks, it spreads itself out for thousands of square leagues
367. The Gulf Stream, as it issues from the straits of Florida, over the cold waters around, and covers the ocean with a is a dark indigo-blue; the line of junction between it and the mantle of warmth that serves so much to mitigate in Europe green waters of the Atlantic is plainly seen for hundreds of the rigors of winter. Moving now more slowly, but dispensmiles. This line is finally lost to the eye as the stream goes ing its genial influences more freely, it finally meets the British north, though it is preserved to the thermometer for several Islands. By these it is divided, one part going into the polar thousand miles. From observations made with the deep-sea basin of Spitzbergen, the other catering the Bay of Biscay, but thermometer, it has been ascertained that “the stream, as far
each with a warmth considerably above ocean temperature. as the banks of Newfoundland, flows through a bed of cold Such an inmense volume of heated water can not fail to carry water, which cold water performs to the warm the office of
with is beyond the seas a mild and moist atmosphere. And banks to a river.”
this it is which so much softens the climate there.”I
Questions - What is its maximum or greatest temperature, and how many degrees is it above that due the latitude ? Describe how the Gulf Stream serves to moderate the climate of Europe.
• Different opinions have been formed respecting the cause of the Gulf Steam. It is supposed by some, that the waters of the Mexican Gulf have a higher level than those of the Alantic in consequence of the trade-winds and the influx of the Eqiatorial Current; and that the current is merely the running off of the water in orderlo restore an equilibrium. Accordingly, the stream has been likened to “an immense ri/er descending from a higher level into a plain.” But Lieut. Maury has very satisfacorily disproved this theory, and shown that, “ instead of descending, its bed (the bel of the stream) represents the surface of an inclined plane from the north, up which the lower depths of the stream must Asrend." It is safe to assume, respecting the cause of this remarkable current, that it is influenced much by the excessive temperature inparted to the waters of the Gulf of Mexico. The course of the Gulf Stream has been essigned to the differ
ence in density btween the waters of the Caribbean Sea and the Gulf of Mexico, and
greater during the night than the day; and rather more
abundant on the summit of high mountains than on plains. COMPOSITION OF AIR.
373. Oxygen and nitrogen are extremely different in their TMOSPHERE is the name of properties. Oxygen gas is a supporter of combustion, and is
that thin, transparent, and high- required for the support of animal life, while nitrogen, in its ly elastic fluid which surrounds unmixed state, is destructive to both. Without oxygen, fires the earth on every side, and
would cease to burn, and all animals would immediately expire. accompanies it in its diurnal | By the process of breathing it is taken into the lungs and goes revolution upon
its axis and its to purify the blood. When the blood is brought into the lungs annual motion round the sun. it is of a dark purple color, but it then throws off the hydrogen It is lighter than either land or and carbon, and receives oxygen, which gives it a bright red water, and rises above them, color. A portion of the nitrogen that is received by the lungs but is kept by the force of appears to be absorbed, while the other and larger part is
gravity close to the surface of rejected and thrown back again into the atmosphere in which the earth, where its use is indispensable to all living creatures. it immediately rises, being lighter than air. It is the medium through which sound, light, and odor are transmitted; it is the vehicle in which moisture is raised and diffused; and the agent by which that diversity of color so pleasing to the eye is produced in natural objects 370. Atmosphere is unlike the great divisions of land and
PROPERTIES OF THE ATMOSPHERE.
374. The general properties of the atmosphere are translighter substances with which it comes in contat, but, when parency, fluidity, weight, and elasticity. Transparency is that greatly agitated, uproots trees, crumbles rocks, ad overturns
state or property it possesbuildings. Its motion is applied as a mechanical orce, and as
ses by which it suffers rays
of light to pass through such is of vast use to man in wafting his vessels ove the ocean. 371. The atmosphere is composed principally of to different
it, so that objects can be gases, termed oxygen and nitrogen, the relative roportions
distinctly seen through it.
The various degrees of being 21 parts of the former to 79 of the latter. contains a
clearness in the atmosphere small but variable proportion of aqueous vapor,nd a still smaller proportion of carbonic acid gas. The prortions of
are owing to particles of
vapor and other substances oxygen and nitrogen are definite, but the amountf aqueous
which float in it. Distant vapor fluctuates. Under ordinary circumstances, tl composition of 1000 parts of the atmosphere may be stated follows:
objects sometimes appear
twice as near as at others, a pheOxygen.
nomenon occasioned by the differ
ence in the purity of the atmosphere, Aqueous vapor.. Carbonic acid ..
or its freedom from aqueous and
other particles. 094.0
375. By the fluidity of the atmo 872. The same proportions of oxygen and nitrogere found sphere is meant that quality it possesses which renders it imin the atmosphere of all countries, and at all elevais, over
pressible to the slightest force, and by which the particles land and over sea, on the summit of the highest mouins and easily move or change their relative positions. Fluidity is a at their base, at the equator, and in high northern anguthern
property common to liquid and aeriform substances. The atlatitudes. The quantity of carbonic acid gas is, wever, mosphere, like other fluids, presses in all directions, upward as greater near the level of the sea in summer than vinter; well as downward, and is capable of supporting light bodies.
Questions.—869. Of what is atmosphere the name ? What is said of ghtness ? What else is remarked of it? 870. How is atmosphere unlike the great dive of land and water? Its effects when in motion? Its motion how applied ? 371. hat is the atmosphere principally composed, and in what proportions? What other suces does It contain ? State the composition of 1000 parts of the atmosphere. 572. is said of the invariable proportions of oxygen and nitrogen in common air ? In wcalities, and at what times, is the quantity of carbonic acid greater ?
Questions.-373. What is said of oxygen gas? Of nitrogen gas? What would bappen without oxygen? What is the color of the blood when brought into the lungs? What change then takes place ? What becomes of the nitrogen received into thu lungs ? 874. What are the general properties of the atmosphere? What is transparency? To what are the various degrees of clearness owing? Why do distant objects appear sometimes twice as near as at others ? 375. What is meant by the auidity of the atmosphere? How does the atmosphere press ?
2 4 8
376. The air is ponderable, or has weight. The pressure or cated by the common thermometer, answers very nearly to 550 weight exerted upon every square inch of the earth's surface is feet of elevation. it
it presses equally in every direction, and the human body, of the sesses of occupying less space under the influence of certain ordinary size (supposed to measure 15 square feet), sustains the forces, and returning to its original volume when the influence enormous pressure of 31,536 pounds or 14 tons. We do not is withdrawn. Hence its density is not uniform, but, as before feel the pressure, owing to its acting uniformly on all sides, and explained, diminishes from below upward. The height of the because the air within our bodies perfectly counterpoises the atmosphere is not known, but it is supposed to extend to about external pressure.
fifty miles. By far the greater portion of it is within fifteen 377. The weight of a column of the entire atmosphere is equal or twenty miles of the earth's surface; and at a much less to that of a column of water of the same base about 34 feet high, distance it becomes so rarefied as to be incapable of supportor a similar column of mercury 30 inches high. The pressure di- ing life. minishes as we ascend, according to a scale, which is nearly cer- 382. Travelers on high mountains have experienced sensible, tain. From numerous observations it has been ascertained that and sometimes painful, proofs of the rarefied state of the air. 34
In very elevated regions the thinness of the air diminishes the 7
intensity of sound, renders breathing difficult, and produces a 101 at the 14 miles above the level 16 times lighter than at
loss of physical strength. The blood burst from the ears and height of 171 of the sea, the air is 32 the earth's surface.
lips of Humboldt in attempting to reach a high elevation in the 21 241
He experienced the same difficulty in kindling and 128
maintaining a fire at great heights, which Marco Polo felt on the 378. The pressure of the atmosphere is indicated by the
mountains of central Asia. In the high regions of the Andes barometer (the measure of weight), an instrument consisting of a malady prevjáls, called veta, which is thus spoken of Lieut. a column of mercury poised or pressed up into a vacuum by the
Herndon : “Jeta is the sickness caused by the rarity of the atweight of the atmosphere. The mercury rises or falls according mosphere at these great elevations. The Indians call it veta or to the pressure of the atmosphere, its range, at the level of the
vein, because they believe it is caused by veins of metal diffusing sea, being from about 28 to 31 inches.
around a poisonous infection. The affection displays itself in 379. The barometer is used for determining the height of
a violent headsche, with the veins of the head swollen and mountains. At the level of the sea the pressure is greatest in
turgid, a difficuty of respiration, and cold extremities." consequence of the weight of all the superincumbent atmo. sphere, and hence at that point the highest column of mercury will be sustained; but as we ascend, this superincumbent press
1 ure is diminished, and consequently the mercury falls. Thus,
LESSON III. Humboldt, at the foot of Mount Chimborazo, found the barom
WINDS. eter to stand exactly at 30 inches; but on ascending the mountain to the elevation of 19,000 feet, it was very little higher than 383. The mosphere remains at rest so long as its density 14 inches. In the pass of Antarangra (one of the highest passes is unchanged but as soon as the equilibrium is broken by any of the Andes), Lieut. Herndon found the barometer to stand cause whatev, a motion occurs, which is called wind. If, in at 16.73 inches, indicating an elevation of 16,044 feet. Experi- one part of le atmosphere, the air becomes dense, it passes ments have proved that the mercury will fall about 'th of an away to thosparts where the density is less, in the same maninch for every 100 feet of perpendicular height, or one inch for ner as air mpressed in a pair of bellows escapes by the every 1,000 feet.
orifice. Wmay compare this displacement of air to that of 380. The density or pressure of the atmosphere is, by an- water in rivs; it is a flowing of the aerial ocean from one other method, made subservient to the measurement of heights; region towa another. namely, by observing the boiling point of water, which de- 384. The arrents of the atmosphere perform many invalcreases in a ratio nearly equivalent to the decrease of atmo- uable servic to man. They renew. the air of cities; and they spheric pressure. At the level of the sea, water boils, or passes mitigate thdimates of the north by bringing to them the heat into the state of steam, at 212° Fahrenheit, but at the Hospice of the sout They transport the clouds from the sea to the of the Great St. Bernard it boils at the lower temperature of interior of ntinents, thus aiding to fertilize regions which 203°, and on the top of Mont Blanc at 186o. In the pass of would othțise become arid and uninhabitable. They waft
. Antarangra, Lieut. Herndon found water to boil at 1820.5. the sails ope navigator around the globe, bring distant naFrom these and other observations, it may be inferred that a tions into piliarity, and are thus greatly instrumental in the difference of one degree in the boiling point of water, as indi
diffusion olvilization and Christianity throughout the world.
fulestions.-376. What is the pressure or weight on every square inch of the earth's surface? What pressure does the human body of the ordinary size sustain? Why do we pot feel the pressure ? 877. To what is the weight of a column of atmosphere equal? 878. By wbat instrument is the pressure of the atmosphere indicated ? What is the range of the mercurial column at the level of the sea ? 879. For what is the barometer used ? Where will the highest column of mercury be sustained, and why? Why does it fall as we ascend? What did Humboldt discover ? Lieut. Herndon ? What have experiments pruved? 380. By what other method may the density of the atmosphere be made subservient to the measurement of heights ?
Qrles'ions.—hat degree of Fahrenheit does water boil at the level of the sea? At the Hospice of Great St. Bernard ? On the top of Mount Blanc? In the pass of ADturangra ? Wilevation is found to answer the difference of one degree in the boiling point? 381. Vis the elasticity of the atmosphere? How does its density diminish ? What is the syed height of the atmosphere? 392. What have travelers on high mountains experien What effects are produced by the thinness of the air? 383. How long does the atmcte remain at rest? What occurs when this equilibrium is broken? If in one part of tubosphere the a'r becomes dense, what follows? To what may this dis placement orir be compared ? 884. Describe the uses of the atmospheric currente.
385. To indicate the direction of the wind the horizon is cates, are very irregular as to time, direction, and force, and divided into eight equal parts, and the wind is designated by seldom continue to blow for many days. They prevail in the giving it the name of the points of the horizon whence it blows. temperate and frigid zones, those of the torrid zone being, for The eight kinds of winds are north, northeast, east, southeast, the most part, either permanent or periodical. south, southwest, west, and northwest. It is customary to write 391. We are not fully acquainted with the causes which merely the initial of these words, that is: N., N. E., E., S. E., | produce these partial and ever-fluctuating aerial currents, but S., S. W., W., N. W.
there is no doubt that they are mainly due to the unequal states 386. The general direction of the wind near the surface of the temperature of land and sea. Although these winds alof the earth is indicated by vanes. They are commonly placed | ternately come from every point of the compass, changing on elevated buildings, such as steeples, towers, etc., so that frequently from one point to the opposite in a very short space small variations, resulting from accidents of the ground, may of time, it has been observed that different seasons are characnot have any action on them. Clouds indicate the direction terized by winds from different directions. Franklin long ago of the upper aerial currents, and show that it differs very often observed in North America that in summer the winds come from the direction of the wind on the surface of the earth. from the south and in winter from the north.
387. That the direction of the wind in the upper regions is 392. From numerous observations made in different parts often the reverse of what it is in the lower, has been conclu- of Europe, the following laws have been established: în winsively proved. During an eruption of the volcano of St. Vincent ter, the direction of the wind is principally from the south, its in 1812, the ashes were conveyed in great quantities to the force being greatest in January. In spring, east winds are island of Barbadoes, situated to the east. These islands lie in common at certain places in March, and at other places in
of the trade-winds, which blow from the east toward April. In summer, especially in July, the winds blow chiefly the west ; but the ashes, having been launched into the air as from the west, and in autumn the south winds more frequently high as the region of the upper current, were transported by it blow, particularly in October. in the direction from west to east. At the summit of the Peak 393. Designating the total number of winds that blow in of Teneriffe almost all travelers have found west winds, while a given time by 1000, the following table shows their relative east winds prevail at the level of the sea. On the 25th of Feb- frequency in the countries named : ruary, 1835, the ashes emitted from the volcano of Coseguina,
W. N. W. in the state of Guatemala, obscured the light of the sun for five North America... 96. 116.... 49....108....123....197....101....210 days; they rose into a high region of the atmosphere, and fell England. ... 82. .111.
iil....120 a short time afterward in the streets of Kingston, in Jamaica,
France ... .126. 140.... 84.... 76. ...117....192.... 155....110
84.... 98....119.... 87.... 97....185. which is situated to the N. E. of Guatemala, the winds near the
65.... 08....100....129. 92. ...198....161....156 surface all the while blowing toward the S. W.
...106 388. Daily experience teaches us the unequal force of the Prussia
99...,191.... 81. .130.... 98....143....166....192 wind, exhibiting every conceivable variety, from the almost insensible breeze to the hurricane which prostrates the monarch
394. There is a certain class of variable winds known to of the forest. The following facts respecting the velocity possess peculiar properties, such as the hot winds felt on the and force of winds have been ascertained :
northern coast of Africa, in Persia, India, and China, the cold
winds of Siberia, the pestilential simoon or samiel of Africa, Velocity of the Perpendicu'ar force
Characteristics. Arabia, and Mesopotamia, etc. Winds partake of the property per hour.
pounds avoirdupois. 1 .005 Hardly perceptible.
of the regions from which they come; thus, in Europe, the .020 Just perceptible.
west winds which blow from the Atlantic are more moist than 6 .123
Gentle, pleasant wind. the east, which sweep over the continent. On the Atlantic 10 .492 Brisk gale.
coast of the United States, the winds which come from the 20 1.968 Very brisk.
northeast are remarkable for their chilliness, and for the disa4.429
High wind. 40 7.873
greeable storms which accompany them. Variable winds may
Very high wind. 60 12.300 A storm.
be subdivided into cold and hot winds. 60 17.715 A violent storm.
395. The cold winds of the north temperate zone are those 80 31.490 A hurricane.
which blow from the north and northeast. In Europe the 100 49.200 A violent hurricane.
northeast winds are cold, deriving their character from the 389. Winds may be divided into three classes,-Variable, very low temperature which prevails in northern Europe and Permanent, and Periodical.
Asia. In the south of Europe the north winds are of great 390. VARIABLE WINDS.--Variable winds, as their name in- violence and severity, owing to the contrast between the
wind in miles
he square foot in
Questions.-885. Into how many parts is the horizon divided to ingicate the direction Questions.---10 miles per hour. 20 miles per hour. 80 miles per hour. 40 miles per of the wiod? How is the wind designated? What are the eight kinds of winds? Their hour. 50 miles per hour. 60 miles per hour. 80 miles per hour. 100 miles per hour. initials? 836. How is the general direction of the wiods near the surface of the earth 889. Into what three classes may winds be divided ! 890. What are variable wiods? indicated ? Where are they commonly placed, and why? What do clouds indicate ? Where do they prevail ! 891. To what are they no doubt mainly due ? What has been 887. What is suid of the reverse direction of upper and under currents? Of the eruption observed respecting them ? What did Franklin observe ? 892. What is the general of the volrano of St. Vincent ? What have travelers found at the summit of the Peak direction of the winds of Eorope in winter? In spring? In summer? In autumn ? of Teneriffe? What further illustration of this fact was afforded by the transportation 394 What is said of the properties of certain winas? of the west winds in Europe ? of asbes emitted from the volcano of Cosegulna ? 888. Wbat is said of the unequal of the northeast winds on the Atlantic coast of the United States ? How may variable force of the wind : Describe the force, and mention the characterisac, of wino having a i winds be subdivided ? 895. What is said of the cold winds of the north temperate zone ? velocity of 1 mile per bour. 2 miles per hour 6 miles per hour.
of the northeast winds in Europe ! or the north winds to the south of Europe ?