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portion of it is stopped in its progress by a submarine bank near Cape Agulhas, and deflected back in an easterly direction. Of this current our mariners take advantage in their outward voyage to Australia.

The great POLAR CURRENTS flow from each of the poles in the direction of the equator. It is owing to these currents that immense masses of floating ice are carried annually from the polar to the temperate regions, which render navigation so dangerous in the northern parts of the Atlantic Ocean, particularly in foggy weather. And it was by the prevalence of these currents that Captain Parry's celebrated attempt to reach the north pole by means of boat-sledges and reindeer was frustrated. He found that as they advanced over the fields of ice to the northward, they were actually carried at a quicker rate to the southward.

If from the middle of a tub filled with water you take up the full of a quart, for instance, you will observe the water rushing from the sides of the tub towards the centre, till an equilibrium or level is produced. This is a familiar illustration of the causes which produce the currents of water from the poles towards the equator. The great quantity of water raised by evaporation in the equatorial regions produces a deficiency, and destroys the level of the waters in the middle part of the globe; and hence, from the colder regions, north and south, currents flow towards the equator, to supply the deficiency and to restore the equilibrium.

This appears at variance with the description which we have given of the western tendency of the waters within the tropics; but the following explanation will reconcile the discrepancy. The waters, as they advance from the polar seas towards the equator, come from regions where the rotatory motion of the earth is less, to those where it is greater; and before they have time to acquire the rapid velocity with which the earth moves in the equatorial regions, they are left rather behind, that is, to the westward;

a Icebergs from the polar seas have been met with near the Azores in the northern hemisphere, and near the Cape of Good Hope in the southern.

Pine trees in great numbers are frequently cast upon the northern coast of Iceland, which supply the inhabitants with much of their fuel. These pines are brought by polar currents from Siberia and North America.

b See note, page 54,

the earth turning round in the opposite direction. Hence, as the water does not move eastward along with the earth, it will apparently move westward; and this apparent motion will be kept up by the continual arrival of water, to which the increased velocity of the equatorial regions has not been communicated.

CONTRARY CURRENTS flowing side by side are often met; and of the existence of under currents there can be no doubt that is, of currents flowing in one direction on the surface, while at a greater depth below them, currents flow in different and even in opposite directions. Thus, in the Cattegat, one current enters the Baltic along the coasts of Jutland, while another glides out of it by the shores of Sweden; and it is concluded that the Mediterranean, into which a powerful current constantly flows through the Strait of Gibraltar, sends back, by under currents, a portion of its water to the Atlantic.a A similar interchange of waters takes place between the Red Sea and the Indian Ocean. A constant current flows into it through the Strait of Bab-elMandeb, for which a compensating quantity of water is sent back by an under current.

When two opposing currents of equal force meet, particularly in narrow channels, they sometimes, by turning upon a centre, assume the form of eddies or whirlpools. It is thus the celebrated Charybdis, the terror of ancient mariners, is formed; and the modern, and far more dangerous Charybdis, the Maelstrom on the coast of Norway. The power of the latter is such, that ships and even whales have been drawn in and ingulfed from a distance of two or three miles. It roars, especially in storms, with a tremendous noise, and its influence is often felt at the distance of seven or eight miles.b

The Antarctic Current, which seems to be the primum

a In some parts of the Caribbean Sea, where the equatorial currents run strongly, it has been affirmed that a boat might be kept in a fixed position by suspending from it a heavy body at a depth sufficient to place it beyond the influence of a current on the surface. In this case it is assumed, that the immersed body would be impelled by an under and contrary current, in a direction opposite to that of the boat, and with equal force.

The dangers of the Maelstrom have been greatly exaggerated. It is only when its current is opposed by a strong wind from the N.W. that it deserves this description.

mobile of all the great currents we have been describing, originates in the wide expanse of the Antarctic Ocean. The general flow of its waters is in a north-east direction, towards the western coasts of South America. On reaching the shores of Patagonia, it bends to the north, and flows along the coast till it comes opposite to about the middle of Peru, where it turns to the west, and gives the first impulse to the great equatorial current of the Pacific Ocean. This current, in its northward course, is called the Peruvian Current. The Arctic waters, of which it consists, impart a cooling and refreshing effect to the temperature along the whole coast of Chili, and a portion of that of Peru.

There are many other important currents which cannot be noticed in a work of this kind. In charts and works for the use of mariners they are indicated and described, in order that they may avoid those that are adverse, and take advantage of those that are favourable to them.

The currents of the ocean," by circulating from cold to warm regions, not only tend to equalize the temperature of its waters, but have a similar effect upon the temperature of the earth. Extreme cold would render many parts of the earth uninhabitable, but for the milder temperature of the adjoining seas.

QUESTIONS FOR EXAMINATION ON CHAP. X.

Pages 151-155.-The tides are caused by? 2. Can you describe them? 3. Why not two tides every twenty-four hours? 4. How much the tides later every day? 5. Why? 6. How are the waters of the sea preserved from putrefaction? 7. Can you show how the tides are produced? 8. Why is the tide not at its highest when opposite to the moon? 9. How illustrate this by the facts referred to in the note? 10. How is it that the waters on the side of the earth, nearest and farthest from the moon, are equally raised in tides at the same time? 11. The illustration? 12. Can you explain the tides by a diagram P 13. Can you show how the spring and neap tides are caused? 14. When have we spring and when neap tides? 15. Why are the tides higher towards the equator? 16. Where are the tides most regular? 17. What produces irregularities in the time and height of tides? 18. Why tides almost imperceptible in the Mediterranean and the Baltic? 19. Why regular, and often very high tides in Baffin and Hudson Bays, and

• Within the temperate zones, as far at least as the parallel of 45o or 50° on both sides of the equator, the general direction of the currents is from west to east. In higher latitudes, particularly in the polar seas, the general set of the currents is to the southward within the northern hemisphere, and to the northward within the southern hemisphere.

L

in the Red Sea? 20. Where do tides often rise dangerously high? 21. Why is it high water sooner on the western than on the eastern coasts of the British Islands? 22. What is meant by the tidal wave?

Pages 155-161.-The use of currents in the ocean? 2. Can you describe the great EQUATORIAL CURRENT? 3. Can you give a description of the GULF STREAM? 4. The velocity of the Gulf Stream? 5. How distinguished from the water round it? 6. How the Gulf Stream instrumental in leading to the discovery of America? 7. The South Connecting current? 8. Can you describe the great equatorial current in the Pacific Ocean? 9. The Mozambique current? 10. The Agulhas current? 11. How the existence of the POLAR CURRENTS proved by the failure of Captain Parry's attempt to reach the north pole? 12. Why navigation dangerous in the northern parts of the Atlantic? 13. Can you give a familiar illustration of the causes which produce the polar currents? 14. How the inhabitants of Iceland supplied with much of their fuel? 15. The Antarctic current ? 16. The Peruvian current? 17. Why do the polar currents, as they approach the equatorial parts of the earth, take a westerly direction? 18. What is said of cONTRARY and UNDER currents? 19. How are EDDIES and WHIRLPOOLS formed? 20. Can you describe the Maelstrom ? 21. Where Charybdis? 22. The utility of currents?

CHAPTER XI.

THE ATMOSPHERE-ITS PROPERTIES, And uses.

THE ATMOSPHERE" is that transparent, elastic, and invisible fluid which encompasses the earth on all sides to the height of about forty-five miles." It revolves with the earth round its axis, and is carried with it in its orbit or course round the sun. The existence of this fluid is essential not only to animal, but also to vegetable life. Where it is very rare, as on the tops of lofty mountains, respiration or breathing is found to be very difficult; and it is known that if an

Atmosphere. That is, the vapour of the sphere. The air was formerly considered to be an elementary or simple substance; but it is now ascertained to be compounded of two distinct and very different ingredients, namely, oxygen and nitrogen gas. In every 100 parts of atmospheric air the constituent parts are, according to Dr. Murray:—

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The exact height of the atmosphere cannot be determined, but above this distance from the earth, it ceases from its great rarity to reflect the rays of light from the sun.

animal be placed within the exhausted receiver of an airpump, it will immediately die.

Without an atmosphere there would be neither rain nor dews to refresh and fertilize the earth. It decomposes and dissipates the mephitic or infectious vapours which are continually exhaling from the earth; and it is the grand agent which, by tempering the extremes of heat and cold, renders every clime habitable to man. It is to its REFLECTING and REFRACTING powers that we owe the morning and the evening TWILIGHT, and the general diffusion of the sun's light over the whole heavens. If it were divested of these powers, instead of the gradual approach of night which prepares the world for the change, the transition from the brightness of day to the darkness of midnight would be instantaneous. And it is entirely owing to the REFLECTIVE powers of the atmosphere that the sun is enabled to light up the heavens. For if his rays were not reflected and diffused through the heavens, only that part of the sky in which he appears would be enlightened, while in every other direction the sky would be as dark as midnight, and the stars would be visible at noonday.

REFLECTION-TWILIGHT.

In the morning, when the sun is eighteen degrees below the horizon, his rays pass over our heads into the higher regions of the atmosphere, from which they are reflected towards the earth.

The day is

then said to dawn; and the light continues to increase till the sun appears above the horizon. In the evening, in like manner, we have light from the sun till after he has sunk eighteen degrees below the horizon. This light, which grows fainter and feinter till it is lost in the darkness of night, is called TWILIGHT. The duration of twilight varies with the latitude of the place, the season of the year, and the height and state of the atmosphere. In the equatorial parts of the earth the twilight is at all seasons very short, while in the polar regions it lasts often for several months. For as the sun during the winter, in those regions, is scarcely ever more than eighteen degrees below the horizon, the twilight, during that season, scarcely ever terminates. The inhabitants of those dreary regions, therefore, though deprived for a great part of the year of the pre

• In the equatorial parts of the earth, the sun descends beneath the horizon in the evening, and approaches it in the morning, in a direct, and therefore, speedier course; while in the other parts of the earth, particularly in the direction of the poles, he moves slantingly from and towards the horizon (in the evening and morning).

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