Imágenes de páginas
PDF
EPUB

Constance, Baikal, &c. Several lakes of this kind may be considered as expansions of the rivers which flow through them. The great lakes of North America, for instance, may be considered as expansions of the St. Lawrence; Geneva, of the Rhone; Constance, of the Rhine; Lake Dembea, of the Abyssinian Nile; Loughs Allen, Ree, and Derg, of the Shannon.

The FOURTH class consists of those which receive streams, and often large rivers, but which have no visible outlet; as the Caspian Sea, the Sea of Aral, the Dead Sea, the Great Salt Lake in North America, and Lake Tchad in Africa. In all such lakes the rainfall received by the lake exactly balances the evaporation from its surface.

Lakes of the fourth class are usually salt, as the Caspian Sea, the Sea of Aral, Lake Van in Armenia, and Urumiyah, one of the largest lakes in Persia. In hot countries, and in the dry and desert regions of Asia, Africa, and America, salt lakes are often found, which, evaporating during the hot season, leave saline incrustations upon their beds, by which the natives are abundantly supplied with salt.

Some lakes contain nutron or soda, which collects on the bottom. The most celebrated are the six Natron lakes of Egypt, which furnish large quantities for commerce. In Maracaybo (South America) there is a lake which deposits more than 1,000 lbs. in two years, which is taken up from the bottom by Indian divers. There are some lakes of this nature in Hungary.

Some lakes appear and disappear periodically; as Lake Zirknitz in Illyria, and Lake Xarayes to the eastward of the Paraguay river. The latter, from this circumstance, has been often drawn and effaced on the maps of South America. The appearance and disappearance of such lakes are produced by the increase or diminution of the sources from which they derive their waters.

We shall conclude this chapter by giving the estimated areas of the principal lakes in the world, in English miles

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed]

QUESTIONS FOR EXAMINATION ON CHAP. IX.

Pages 145-148.-1. The origin of rivers? 2. The size and character of a river depend upon? 3. The extent of the basin of the Amazon? 4. The velocity of rivers depends upon? 5. How are cascades and cataracts formed? 6. How are rivers often carried over plains? 7. Can you give instances of this? 8. How do rivers differ with regard to their mouths? 9. The breadth of the estuary of the La Plata? 10. What produces floods periodically in some rivers? 11. Can you repeat the instances mentioned in the text? 12. The danger from floods? 13. The utility of them? 14. What does Gibbon say of the Nile? 15. Can you describe the several classes into which rivers have been divided? 16. Has the length of rivers been accurately ascertained? 17. How should these rivers be studied? 18. In what class is the largest river in Europe? 19. In which the largest in Ireland and England?

a In most cases, the estimated areas of these lakes are to be considered as mere approximations to the truth. Even with regard to European lakes, the authorities in some cases differ.

b Lake Superior is the largest body of fresh water in the world. It is equal in extent to the whole of Ireland.

e Several extensive lakes have been recently discovered in Southern Africa; as Ngami, Nyassa or Maravi, Tanganyika, and Nyanza.

d Nicaragua lies principally between the 11th and 12th degrees of north latitude, and the 84th and S6th of west longitude; about 12 miles in a direct line from the Pacific, and 90 miles from the Caribbean Sea; with which it is connected by the River St. Juan. The junction of the Atlantic and Pacific Oceans, by means of this lake and river, has often been proposed.

e Titicaca, the largest of the South American lakes. See p. 120.

Pages 148-150.-How are lakes classed? 2. Give examples of each kind? 3. The most usual class? 4. Such lakes may be considered expansions of ? 5. Can you give instances? 6. How do lakes of the fourth class generally differ from the others? 7. Where the Natron lakes? 8. What is said of Lake Zirknitz? 9. And of Xarayes? 10. How is this accounted for? 11. The largest lake in the world? 12. Its area in square miles? 13. The area of Lake Superior? 14. Of Geneva? 15. Lough Neagh? 16. Windermere? 17. What have you to observe regarding Nicaragua ?

CHAPTER X.

TIDES AND CURRENTS.

TIDES, or the alternate flowing and ebbing of the sea, are produced by the attraction of the moon and sun, but principally by the attraction of the moon. For the moon being so much nearer to the earth than the sun, has a much greater attractive influence on its waters than the sun.

The ancients looked upon the flowing and ebbing of the tides as one of the greatest mysteries in nature; and but for the more than human intellect of Newton, it is probable that it would have remained a mystery to this day."

The phenomenon of the tides has been observed in every part of the earth which is washed by the sea. For about six hours the sea gradually swells, so that it enters the mouths of harbours and rivers, and comes nearer to the coasts. This is called FLOOD TIDE. For about twelve minutes it rests or remains in equilibrio; during which it is said to be HIGH WATER. It then begins to ebb, and continues to do so for about six hours, when it pauses again for about twelve minutes; during which it is said to be LOW WATER. It then begins to flow again for six hours; and so on, alternately. Hence in every twenty-four hours and fifty minutes there are two tides."

⚫ Galileo, Descartes, and particularly Kepler, made some successful approaches towards ascertaining the cause. That the tides had some connexion with the moon was natural to suppose, for they were observed always to follow her motion. The moon comes every day later to the meridian than on the preceding day, by about 50 minutes; and the tides in every part of the world happen exactly so much later every day as the moon comes later to the meridian.

If the moon were stationary, the same part of our globe would return under it every 24 hours, and there would in consequence, be two tides every 24 hours; but while the earth is turning once upon its axis, the moon moves forward in her orbit 13°, and hence it takes the earth about 50 minutes more to bring the same meridian under or opposite to the moon.

The continual flowing and ebbing of the sea, combined with the WAVES which agitate it, and the CURRENTS which run through it, keep its waters in constant motion, and thus prevent them from becoming stagnant and corrupt. But the SALTNESS of the sea, which is found to increase in warm climates where it is most required," contributes chiefly to preserve its waters from putrefaction.

The tides, as we have stated, are produced principally by the attraction of the moon. As the earth turns round on its axis it presents every part of its surface, in succession, to the moon, which, from the nature of attraction, exerts a greater attractive influence upon those parts of the earth's surface that are turned towards her, or nearest, than upon those that are turned from her, or most remote. Hence, as seas pass under the moon-or, as is said, when the moon comes to the meridian of the place the fluid particles of which they are composed, being more easily separated and attracted than particles of earth, are drawn more strongly towards her, which causes them to swell and flow, till the impulse is overcome by the attraction of the other watery particles, as they are brought, by the rotation of the earth, under the more direct influence of the moon."

d

The highest waves known are those which occur off Cape Horn, and those produced by a north-west gale off the Cape of Good Hope, some of which are nearly 40 feet high from the trough to the crest.

b The greater the heat, the greater the evaporation, and hence, in warm regions, the waters of the sea must be salter than they are in cold regions where the evaporation is less; for while the water ascends in vapour, the salt which it contained is left behind.

• If the waters of the sea became stagnant and corrupt, the earth would be rendered uninhabitable; for the atmosphere, instead of being purified, as it now is, by the perpetual agitations of the ocean, would be polluted by the exhalations arising from it, and thus unfitted for animal and vegetable life.

See Coleridge's "Ancient Mariner" for a touching and most poetical picture of the effects produced by a long calm near the equator― particularly the verse beginning with,

"The very deep did rot."

a The tide is not at its highest when directly under the moon, but in two or three hours after; and often, according to the obstructions it meets with from projecting coasts, and in narrow channels, it is not high water till after the moon has passed the meridian several hours. But even where there are no interruptions, as in great oceans, it is not high water till the moon has passed the meridian two hours or so; for the waters continue to accumulate till the moon, by an overcoming mpulse, draws them away. Compare the facts stated in note", p. 44.

While the water is thus attracted and heaped up on the side of the earth which is nearest to the moon, it is at the same time equally elevated on the other side of the earth or the side which is farthest from the moon; and hence there are always two tides at the same time, one on the side of the earth next to the moon, and the other on the opposite side.

That the moon should attract and raise up the waters of the earth that are under her, is easy to conceive; but that the same cause should, at the same time, raise them up on the opposite side of the earth, seems strange and incredible. It is, however, perfectly true, and is easily demonstrated. The general principle is, that as those parts of the earth which are nearest to the moon are more strongly attracted towards her than the parts which are most remote, the sea which covers the surface of the earth on the side farthest from the moon is less strongly attracted than the land which is under it, and which is consequently nearer to the moon. Hence, the body of the earth being more strongly attracted than the waters which cover its side farthest from the moon, is drawn away from these waters, and the same result is produced as if they had risen in tides.

The following diagram will give a clearer conception of the causes of the tides than any thing we can say on the subject. To simplify the matter, let us suppose the earth to be a regular and uniform sphere covered with water; and if there is no attraction or influence from any external body, the water will, in obedience to the laws of gravitation, arrange itself regularly and uniformly around the earth, forming a coating like the rind of an orange, and everywhere of the same depth. Now, let us suppose that the earth is brought under the attractive influence of an external body, like the moon, and the effects which we have already described will be produced.

[blocks in formation]

In the above diagram, A B D C is the earth, M the moon,

« AnteriorContinuar »