duced by the rotation of the earth, differences of temperature 358. EASTERN POLAR OR GREENLAND CURRENT.-The Eastern Polar or Greenland Current originates in the Arctic Ocean north of Asia. It sweeps around the northern shores of Spitzbergen and Iceland, and flows southwesterly between the latter island and Greenland. Near Cape Farewell it sends off a branch which runs into Baffin Bay, but its principal flow appears to be southwesterly between the Gulf Stream and the neighboring coast of America. 359 The breadth of this current is in some places from Its velocity varies, in different parts of 250 to 300 miles. its course, from eight or nine to fifteen or sixteen miles per day. The icy masses it bears along, and which are frequently swept around Cape Farewell, are supposed to be about two months in making the circuit to Baffin Bay and thence to the coast of Labrador. 360. This current is distinguished for the grea amount of drift-wood which it floats along and casts upon he shores of Spitzbergen, Iceland, and other lands lying in itsway. The masses of floating wood thrown upon the island ofJan Mayen often equal, it is said, the whole of the island in eent. It is supposed that this timber comes from the forests of beria, and is carried into the Arctic Ocean by the streams of nobern Asia. 361. Recent observations in high northern latifles show that the Eastern Polar or Greenland Current preses one of the most formidable difficulties in exploring the pol regions. Parry, who attempted to reach the north pole byneans of boat-sledges and reindeer, traveled over the surface che deep to nearly lat. 83°, which seemed to be the utmost lit of animal life. Here he found that when, according to hreckoning, he had traveled ten or eleven miles toward the rth, he had actually gone four miles to the south, owing the curThe success of the expedition was thus rende hopeless. The two vessels of the Grinnell Expedition wh were sent out, under the command of Lieut. De Haven, search for Sir John Franklin, after having penetrated far i: Wel lington Channel, were inclosed firmly in the ice, anrifted backward through Baffin Bay, a distance of not less thæfteen hundred miles, thus defeating the plan of one of the blest and most humane enterprises ever undertaken. rent. 362. EQUATORIAL CURRENT.-The most extensive ment of the ocean is that which proceeds from east to von each side of the equator, and is therefore called the ato Questions. -358. Origin of Eastern Polar or Greenland Current? Its course? ch? Principal flow? 359. What is the breadth of this current? Its velocity? 860. hat is it distinguished? 361. What do recent observations in high northern latituc? Describe the attempt of Parry to reach the north pole. The drift of the two vesthe Grinnell Expedition. 862. Which is the most extensive movement of the ocean,hat is it called? How does it originate? Cause of this movement of the waters ? do the inflowing acquire a peculiar tendency? rial Current. It originates in the general inflowing of waters from the temperate and polar regions, but especially from the Antarctic Ocean and neighboring seas. These waters, overbalancing the heated and therefore expanded and specifically lighter waters at the equator, cause them to rise above the general level and overflow in a constantly spreading stream. But as the inflowing waters come from parts of the earth's surface which lie nearer the poles or the earth's axis, and have a less rapid diurnal movement (from west to east) than the parts near the equator, they accordingly partake of this movement; and hence, as they approach the equator, fall behind the more swiftly advancing points of its surface, and thus acquire a western tendency. Accordingly the Equatorial Current, fed by these westerly moving waters, has a resulting westward course; and, crowding against the eastern shores of the continents and of the great islands which separate the Pacific from the Indian Ocean, divides into various streams, most of which flow off to the north and south. 363. In the Indian Archipelago and neighboring seas, however, it separates into numerous small branches that take different courses according to the channels through which they flow: hence the variable currents prevailing in the Indian Ocean, which render navigation so dangerous. A large volume of water forces its way through the islands, and joins the great equatorial current which moves on toward the eastern coast of Africa. The greater portion of the stream flows round north of the island of Madagascar, and sweeps through the channel of Mozambique, after which, being joined by other currents from the east, it moves toward the southern ex tremity of Africa, where it is said to unite with a current descending along the western coast of the grand division, and to flow thence to the Antarctic Ocean. 364. The Equatorial Current, on reaching Cape St. Roque, the most eastern point of South America, is separated into two branches. One proceeds southward along the coast of South America, under the name of the Brazilian Current, and terminates in a region of variable currents, the chief of whichobserved as moving to the eastward with an increasing velocity -has been designated as the South Connecting Current. 365. The other and principal branch of the Equatorial Current of the Atlantic is known as the Guiana Current. It runs from off Cape St. Roque, across the mouth of the Amazon, and after skirting the low coast of Guiana, and passing through the Caribbean Sea, it enters the Gulf of Mexico between the island of Cuba and the peninsula of Yucatan. 366. MEXICAN GULF STREAM.-This is the most powerful current known, and the most important in consequence of the extent to which it affects the navigation of the Atlantic. It originates in the Gulf of Mexico, the waters of which are characterized by a remarkably high temperature. It pours forth at a rate of from three to five miles an hour through the straits of Florida, and flows in a northeasterly direction along the whole coast of the United States, expanding in volume 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 straits of Florida? Its subsequent course? Its course after striking the banks of Newfoundland? Great whirlpool of the Atlantic? and diminishing in rapidity.* On striking the banks of Newfoundland, it sets to the east: the northern portion, however, sweeps toward Iceland, Norway, and the British Isles; the southern portion flows to the Azores, where it turns south and enters the Equatorial Current on the coast of Africa, and is conducted again to the west, to re-enter into itself in the Gulf of Mexico. Thus the waters of the Atlantic Ocean between the parallels of 11° and 48° constitute a whirlpool of prodigious extent, by which a single particle of water describes a circuit of over 11,000 miles in the space of two years and ten months. 367. The Gulf Stream, as it issues from the straits of Florida, is a dark indigo-blue; the line of junction between it and the green waters of the Atlantic is plainly seen for hundreds of miles. This line is finally lost to the eye as the stream goes north, though it is preserved to the thermometer for several thousand miles. From observations made with the deep-sea thermometer, it has been ascertained that "the, stream, as far as the banks of Newfoundland, flows through a bed of cold water, which cold water performs to the warm the office of banks to a river."t Question 8.-367. What is the color of the Gulf Stream? What fact has beer ascertained respecting it? 868. What is said of its temperature? * 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 Atlantic in consequence of the trade-winds and the influx of the Equatorial Current; and that the current is merely the running off of the water in orderto 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 bed of the stream) represents the surface of an inclined plane from the north, up which the lower depths of the stream must ascend." It is safe to assume, respecting the cause of this remarkable current, that it is influenced much by the excessive temperature imparted to the waters of the Gulf of Mexico. The course of the Gulf Stream has been assigned to the differ 368. Coming from the heated caldron of the Gulf of Mexico, the waters of the Gulf Stream have a high temperature, which is gradually lost as they reach higher latitudes. "The maximum temperature of the Gulf Stream is 86°, or about 9° above the ocean temperature due the latitude. Increasing its latitude 10°, it loses 2° of temperature. And, after having run 3,000 miles toward the north, it still preserves, even in winter, the heat of summer. With this temperature it crosses the 40th degree of north latitude, and there, overflowing its liquid banks, it spreads itself out for thousands of square leagues over the cold waters around, and covers the ocean with a mantle of warmth that serves so much to mitigate in Europe the rigors of winter. Moving now more slowly, but dispensing its genial influences more freely, it finally meets the British Islands. By these it is divided, one part going into the polar basin of Spitzbergen, the other entering the Bay of Biscay, but each with a warmth considerably above ocean temperature. Such an immense voltime of heated water can not fail to carry with is beyond the seas a mild and moist atmosphere. And this it is which so much softens the climate there."‡ 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. " for ence in density btween the waters of the Caribbean Sea and the Gulf of Mexico, and those of the Balic aid the North seas. The waters of the former contain a larger proportion of salt and re consequently heavier than sea water; while those of the latter, being only sightly impregnated with saline matter, are much lighter than common sea water. This diferene in density destroys the equilibrium and produces a current ; wherever equilibriunbe destroyed, it is restored by motion, and motion among fluid particles gives rise to crents, which, in turn, constitute circulation." It is more probable, however, that its cuilinear direction is due to the less rapid eastward movement of the earth's surface as it proaches the higher latitudes. Ibid. + Lieut. M. F. Mry. LESSON I. COMPOSITION OF AIR. TMOSPHERE is the name of that thin, transparent, and highly elastic fluid which surrounds the earth on every side, and accompanies it in its diurnal revolution upon its axis and its annual motion round the sun. It is lighter than either land or water, and rises above them, but is kept by the force of gravity close to the surface of the earth, where its use is indispensable to all living creatures. 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 division: of land and water in not being perceptible to the touch unles in agitation. Its existence as a material substance is evident the moment it is set in motion. It not only carries away in its orogress the lighter substances with which it comes in contat, but, when greatly agitated, uproots trees, crumbles rocks, ad overturns buildings. Its motion is applied as a mechanical ›rce, and as such is of vast use to man in wafting his vessels ove the ocean. 371. The atmosphere is composed principally of to different gases, termed oxygen and nitrogen, the relative roportions being 21 parts of the former to 79 of the latter. contains a small but variable proportion of aqueous vapor,nd a still smaller proportion of carbonic acid gas. The prortions of oxygen and nitrogen are definite, but the amountf aqueous vapor fluctuates. Under ordinary circumstances, tl composition of 1000 parts of the atmosphere may be stated follows: 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 div of land and water? Its effects when in motion? Its motion how applied? 371. at is the atmosphere principally composed, and in what proportions? What other suces does It contain? State the composition of 1000 parts of the atmosphere. $72. is said of the invariable proportions of oxygen and nitrogen in common air? In wcalities, and at whet times, is the quantity of carbonic acid greater? greater during the night than the day; and rather more abundant on the summit of high mountains than on plains. 373. Oxygen and nitrogen are extremely different in their properties. Oxygen gas is a supporter of combustion, and is required for the support of animal life, while nitrogen, in its unmixed state, is destructive to both. Without oxygen, fires would cease to burn, and all animals would immediately expire. By the process of breathing it is taken into the lungs and goes to purify the blood. When the blood is brought into the lungs it is of a dark purple color, but it then throws off the hydrogen and carbon, and receives oxygen, which gives it a bright red color. A portion of the nitrogen that is received by the lungs appears to be absorbed, while the other and larger part is rejected and thrown back again into the atmosphere in which it immediately rises, being lighter than air. LESSON II. PROPERTIES OF THE ATMOSPHERE. 374. THE general properties of the atmosphere are transparency, fluidity, weight, and elasticity. Transparency is that state or property it possesses by which it suffers rays of light to pass through it, so that objects can be distinctly seen through it. The various degrees of clearness in the atmosphere are owing to particles of vapor and other substances which float in it. Distant objects sometimes appear twice as near as at others, a phenomenon occasioned by the difference in the purity of the atmosphere, or its freedom from aqueous and other particles. sphere is meant that quality it possesses which renders it im375. By the fluidity of the atmopressible to the slightest force, and by which the particles easily move or change their relative positions. Fluidity is a mosphere, like other fluids, presses in all directions, upward as property common to liquid and aeriform substances. The atwell as downward, and is capable of supporting light bodies. Questions.-373. What is said of oxygen gas? Of nitrogen gas? What would happen 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 the lungs? 874. What are the general properties of the atmosphere? What is transparency? what are the various degrees of clearness owing? Why do distant objects appear sometimes twice as near as at others? 875. What is meant by the fluidity of the atmosphere? How does the atmosphere press? Το 376. The air is ponderable, or has weight. The pressure or weight exerted upon every square inch of the earth's surface is equal to about 15 pounds. In consequence of its fluidity it presses equally in every direction, and the human body, of the ordinary size (supposed to measure 15 square feet), sustains the enormous pressure of 31,536 pounds or 14 tons. We do not feel the pressure, owing to its acting uniformly on all sides, and because the air within our bodies perfectly counterpoises the external pressure. 377. The weight of a column of the entire atmosphere is equal to that of a column of water of the same base about 34 feet high, or a similar column of mercury 30 inches high. The pressure diminishes as we ascend, according to a scale, which is nearly certain. From numerous observations it has been ascertained that 3 2 378. The pressure of the atmosphere is indicated by the barometer (the measure of weight), an instrument consisting of a column of mercury poised or pressed up into a vacuum by the weight of the atmosphere. The mercury rises or falls according to the pressure of the atmosphere, its range, at the level of the sea, being from about 28 to 31 inches. 379. The barometer is used for determining the height of mountains. At the level of the sea the pressure is greatest in 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 pressure is diminished, and consequently the mercury falls. Thus, Humboldt, at the foot of Mount Chimborazo, found the barometer to stand exactly at 30 inches; but on ascending the mountain to the elevation of 19,000 feet, it was very little higher than 14 inches. In the pass of Antarangra (one of the highest passes of the Andes), Lieut. Herndon found the barometer to stand at 16.73 inches, indicating an elevation of 16,044 feet. Experiments have proved that the mercury will fall about 'th of an inch for every 100 feet of perpendicular height, or one inch for every 1,000 feet. 380. The density or pressure of the atmosphere is, by another method, made subservient to the measurement of heights; namely, by observing the boiling point of water, which decreases in a ratio nearly equivalent to the decrease of atmospheric pressure. At the level of the sea, water boils, or passes into the state of steam, at 212° Fahrenheit, but at the Hospice of the Great St. Bernard it boils at the lower temperature of 203°, and on the top of Mont Blanc at 1860. In the pass of Antarangra, Lieut. Herndon found water to boil at 1820.5. From these and other observations, it may be inferred that a difference of one degree in the boiling point of water, as indi Questions.-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 not feel the pressure? 877. To what is the weight of a column of atmosphere equal? 878. By what 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 proved? 380. By what other method may the density of the atmosphere be made subservient to the measurement of heights? cated by the common thermometer, answers very nearly to 550 feet of elevation. 381. The elasticity of the atmosphere is the property it possesses of occupying less space under the influence of certain forces, and returning to its original volume when the influence is withdrawn. Hence its density is not uniform, but, as before explained, diminishes from below upward. The height of the atmosphere is not known, but it is supposed to extend to about fifty miles. By far the greater portion of it is within fifteen or twenty miles of the earth's surface; and at a much less distance it becomes so rarefied as to be incapable of supporting life. 382. Travelers on high mountains have experienced sensible, and sometimes painful, proofs of the rarefied state of the air. In very elevated regions the thinness of the air diminishes the intensity of sound, renders breathing difficult, and produces a loss of physical strength. The blood burst from the ears and lips of Humboldt in attempting to reach a high elevation in the He experienced the same difficulty in kindling and Andes. maintaining a fire at great heights, which Marco Polo felt on the mountains of central Asia. In the high regions of the Andes a malady prevals, called veta, which is thus spoken of Lieut. Herndon: "Yeta is the sickness caused by the rarity of the atmosphere at these great elevations. The Indians call it veta or vein, because they believe it is caused by veins of metal diffusing around a poisonous infection. The affection displays itself in a violent head:che, with the veins of the head swollen and turgid, a difficuty of respiration, and cold extremities." LESSON III. WINDS. 383. THE mosphere remains at rest so long as its density is unchanged but as soon as the equilibrium is broken by any cause whatev, a motion occurs, which is called wind. If, in one part of e atmosphere, the air becomes dense, it passes away to thosparts where the density is less, in the same manner as air mpressed in a pair of bellows escapes by the orifice. Wmay compare this displacement of air to that of water in rivs; it is a flowing of the aerial ocean from one region towa another. 384. The arrents of the atmosphere perform many invaluable servicto man... They renew the air of cities; and they mitigate thaimates of the north by bringing to them the heat of the sout They transport the clouds from the sea to the interior of ntinents, thus aiding to fertilize regions which would otheise become arid and uninhabitable. They waft the sails one navigator around the globe, bring distant nations into piliarity, and are thus greatly instrumental in the diffusion ofvilization and Christianity throughout the world. Questions. 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 Antarangra ? Welevation is found to answer the difference of one degree in the boiling point? 381. is the elasticity of the atmosphere? How does its density diminish? What is the sued height of the atmosphere? 382. What have travelers on high mountains experien What effects are produced by the thinness of the air? 383. How long does the atmore remain at rest? What occurs when this equilibrium is broken? If in one part of thosphere the air becomes dense, what follows? To what may this displacement of ir be compared? 884. Describe the uses of the atmospheric currents. 385. To indicate the direction of the wind the horizon is divided into eight equal parts, and the wind is designated by giving it the name of the points of the horizon whence it blows. The eight kinds of winds are north, northeast, east, southeast, south, southwest, west, and northwest. It is customary to write merely the initial of these words, that is: N., N. E., E., S. E., S., S. W., W., N. W. 386. The general direction of the wind near the surface of the earth is indicated by vanes. They are commonly placed on elevated buildings, such as steeples, towers, etc., so that small variations, resulting from accidents of the ground, may not have any action on them. Clouds indicate the direction of the upper aerial currents, and show that it differs often very from the direction of the wind on the surface of the earth. 387. That the direction of the wind in the upper regions is often the reverse of what it is in the lower, has been conclusively proved. During an eruption of the volcano of St. Vincent in 1812, the ashes were conveyed in great quantities to the island of Barbadoes, situated to the east. These islands lie in the range of the trade-winds, which blow from the east toward the west; but the ashes, having been launched into the air as high as the region of the upper current, were transported by it in the direction from west to east. At the summit of the Peak of Teneriffe almost all travelers have found west winds, while east winds prevail at the level of the sea. On the 25th of February, 1835, the ashes emitted from the volcano of Coseguina, in the state of Guatemala, obscured the light of the sun for five days; they rose into a high region of the atmosphere, and fell a short time afterward in the streets of Kingston, in Jamaica, which is situated to the N. E. of Guatemala, the winds near the surface all the while blowing toward the S. W. 388. Daily experience teaches us the unequal force of the wind, exhibiting every conceivable variety, from the almost insensible breeze to the hurricane which prostrates the monarch of the forest. The following facts respecting the velocity and force of winds have been ascertained: cates, are very irregular as to time, direction, and force, and seldom continue to blow for many days. They prevail in the temperate and frigid zones, those of the torrid zone being, for the most part, either permanent or periodical. 391. We are not fully acquainted with the causes which produce these partial and ever-fluctuating aerial currents, but there is no doubt that they are mainly due to the unequal states of the temperature of land and sea. Although these winds alternately come from every point of the compass, changing frequently from one point to the opposite in a very short space of time, it has been observed that different seasons are characterized by winds from different directions. Franklin long ago observed in North America that in summer the winds come from the south and in winter from the north. 392. From numerous observations made in different parts of Europe, the following laws have been established: In winter, the direction of the wind is principally from the south, its force being greatest in January. In spring, east winds are common at certain places in March, and at other places in April. In summer, especially in July, the winds blow chiefly from the west, and in autumn the south winds more frequently blow, particularly in October. 393. Designating the total number of winds that blow in a given time by 1000, the following table shows their relative frequency in the countries named: E. S. E. S. ..225....171....120 Countries. N. N.E. S. W. W. N. W. North America... 96....116.... 49....108....123....197....101....210 England..... 82. .111.... 99.... 81.. 111. France. .126....140.... 84.... 76....117....192....155....110 Germany. 84.... 98....119.... 87.... 97....185. .198....131 Denmark. 65.... 98....100....129.... 92....198.. .161....156 Sweden ...102... 104.... 80....110....128. ..210. .159....106 Prussia...... 99....191.... 81....130.... 98.. .143....166....192 394. There is a certain class of variable winds known to possess peculiar properties, such as the hot winds felt on the northern coast of Africa, in Persia, India, and China, the cold winds of Siberia, the pestilential simoon or samiel of Africa, Arabia, and Mesopotamia, etc. Winds partake of the property of the regions from which they come; thus, in Europe, the west winds which blow from the Atlantic are more moist than the east, which sweep over the continent. On the Atlantic coast of the United States, the winds which come from the northeast are remarkable for their chilliness, and for the disagreeable storms which accompany them. Variable winds may be subdivided into cold and hot winds. 395. The cold winds of the north temperate zone are those which blow from the north and northeast. In Europe the 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 Questions.-885. Into how many parts is the horizon divided to indicate the direction of the wind? How is the wind designated? What are the eight kinds of winds? Their initials? 886. How is the general direction of the winds near the surface of the earth indicated? Where are they commonly placed, and why? What do clouds indicate? 887. What is said of the reverse direction of upper and under currents? Of the eruption of the volcano of St. Vincent? What have travelers found at the summit of the Peak of Teneriffe? What further illustration of this fact was afforded by the transportation of ashes emitted from the volcano of Coseguina? 888. What is said of the unequal force of the wind? Describe the force, and mention the characteristic, of wine having a velocity of 1 mile per hour. 2 miles per hour 5 miles per hour. Questions.-10 miles per hour. 20 miles per hour. 80 miles per hour. 40 miles per hour. 50 miles per hour. 60 miles per hour. 80 miles per hour. 100 miles per hour. 889. Into what three classes may winds be divided? 890. What are variable winds? Where do they prevail? 891. To what are they no doubt mainly due? What has been observed respecting them? What did Franklin observe? 892. What is the general direction of the winds of Europe in winter? In spring? In summer? In autumn? 394 What is said of the properties of certain winds? Of the west winds in Europe? Of the northeast winds on the Atlantic coast of the United States? How may variable winds be subdivided? 895. What is said of the cold winds of the north temperate zone? Of the northeast winds in Europe? Of the north winds in the south of Europe? |