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rains regularly descend at a certain season of the year, and never at any other. In fact, the only divisions of the year in those regions are the dry and rainy seasons. In the northern half of the torrid zone it is the rainy season, when the sun is north of the equator; and the dry season, when the sun is south of the equator. And in the southern half of the torrid zone, the seasons, in like manner, depend upon the place of the sun. Wherever the sun is vertical or overhead, the rains and clouds are almost constant, because the atmosphere is not able to contain all the vapours which are raised by the increased heat; and thus we see that a provision has been made for protecting the earth from the perpendicular rays of the sun.

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The effects of these periodical rains and droughts in the tropical regions of South America are thus described by Humboldt:

"The immense plains (called LLANOS), which in the rainy season display a beautiful verdure, gradually assume the aspect of a desert; the grass is reduced to powder, the earth cracks, and the alligators and the large serpents remain buried in the dried mud till the first showers of the year awaken them from their lethargy.

"The rainy season begins about the end of April. The sky becomes obscured, the azure disappears, and a gray tint is spread uniformly over it; at the same time the heat progressively increases, and soon dense vapours cover the heavens from one end to the other. The plaintive cry of the howling monkeys begins to be heard before the rising of the The atmosphere is at length convulsed by frequent thunderstorms, the rains descend in torrents, and the rivers rising rapidly above their banks overspread the plains with extensive inundations.'

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The suffering which the wild horses of the llanos endure on these occasions are thus described by the same author:

"In the rainy season the horses that wander in the savannah, and have not time to reach the rising grounds of the llanos, perish by hundreds amidst the overflowings of the rivers. The mares are seen followed by their foals swimming during a part of the day to feed upon the grass, the tops of which alone wave above the waters. this state they are pursued by the crocodiles; and it is by no means

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It is only in the temperate zones that the four seasons are known. From the sixtieth degree of latitude to the poles only two seasons take place, a long and severe winter, and a short, warm, but ineffectual summer; and within the tropics, it may be said to be perpetual summer. The rain does not fall during the night, nor till about the middle of the day, and it ceases in four or five hours.

• In the northern tropical regions of Africa showers commence in April, and increase till June, when torrents of rain begin to descend, and continue almost three months without intermission. The face of nature is soon changed; rivulets, before dried up, swell into large rivers; rivers overflow their banks; and the plains become vast lakes. In the course of September the rains cease, and not a drop falls till the following April.

Llanos is another name for pampas or selvas. See page 144.

uncommon to find the prints of the teeth of these carnivorous reptiles on their thighs."

In the West Indies, and in some other parts of the torrid zone there are two rainy seasons; but one of them is of much shorter duration, and has much lighter rains than the other. And in some parts of the same zone, from the effect of mountain-ranges and peculiar winds, places under the same parallel (as the countries divided by the Ghauts in the south of Hindostan) have their dry and rainy seasons at opposite periods of the year. In some parts of the torrid zone, too, as the Sahara of Africa, the desert of Gobi, and the plains of Peru, rain seldom or never falls. In Egypt too, as we have observed before, rain is almost unknown.

Generally speaking, in all parts of the world mountains and elevated regions are more subject to rain than plains and level countries in the same latitude; and the same may be said of places near the sea, as islands and coasts, when compared with places remote from the sea (as the interior of a country) though under the same parallel. The reason is obvious; the mountains, as we observed before, intercept the clouds and bring them down in rain; and thus they are often almost all brought down before they are carried over countries very remote from the sea; so that the nearer a place is to the sea, from which the vapours arise, the more frequently is its atmosphere saturated with them; and of course, the more likely it is to receive a large portion of them in rain.

The horse is not originally a native of these regions. first introduced by the Spaniards.

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The second rainy season occurs when the sun passes the zenith of the place on his return from the tropic, but there are many irregularities in this respect occasioned by local circumstances.

The south-west monsoon parts with most of the vapours with which it is charged as it blows over the lofty range of the Western Ghauts, and there is consequently only a small supply of moisture left for the high table-lands in the interior: and by the time it reaches the eastern or Coromandel coast it has become a dry or rainless wind. During the continuance of this monsoon it is the rainy season on the Malabar coast, and in the regions of the Western Ghauts. The reverse of this is the case on the Coromandel coast, and in the regions of the Eastern Ghauts, during the continuance of the north-east monsoon.

The south-east trade-winds retain almost all the vapours with which they are charged while passing over the comparatively low mountains of Brazil, but on approaching the Andes they part with the whole of them in those heavy rains which feed the streams of the Amazon and its numerous affluents. These same winds, on the other side of the Andes, as in Peru, have little or no moisture to communicate, and the region is consequently almost rainless.

• At Bergen the annual fall of rain is 89 inches, while on the other side of the mountains, at Christiana, the fall is but 20 inches.

DEW.

That moisture which in warm and dry weather, after sunset, is insensibly precipitated from the atmosphere upon the surface of the earth, is called DEW. In the warm regions of the earth, particularly in tracts of country destitute of rain, the dews are exceedingly heavy; and it need scarcely be observed that they are of the greatest use in refreshing the earth and promoting vegetation.

It was formerly thought that dews were produced by the cooling of the atmosphere after the disappearance of the sun, which, of course, would diminish its solvent power, and cause all the vapours which it could not hold in solution to descend in moisture to the earth. But it has recently been proved that they are caused by the previous cooling of the surface of the earth, which radiates or parts with its heat much more rapidly than the surrounding air. After sunset, therefore, the surface of the ground having become cooler than the atmosphere which is in contact with it, condenses and draws from it the vapours which it held in solution during the heat of the day. When a glass of cold water is brought into a warm room, particularly in summer, the outside of the glass will soon become covered with moisture, though it was perfectly dry when brought into the apartment. Now this familiar fact exemplifies the formation of dew. The outside or surface of the glass is colder, in consequence of the water which it contains, than the atmosphere of the room, and hence it condenses and forms into dew the vapoury particles with which it comes in contact. The outside of a bottle of wine brought fresh from a cold cellar into a warm dining-room will exemplify the same fact; and it is upon the same principle that moisture is formed upon the inner side of the glass in the windows of a close carriage, or of a heated room."

It has been observed that in cloudy nights there is little or no dew. This is because the earth's surface on such occasions has much the same temperature as the surrounding air; for the heat that it radiates or gives out is reflected back to it again by the clouds; whereas, in clear and cloudless nights, the radiation of heat from the earth passes without obstacle to the higher regions of the atmosphere, and its surface is in consequence rapidly cooled.

It has also been observed, and the fact is worthy of our admiration, that in the same locality the deposition of dew is, generally speaking, in proportion to the necessity there is for it. It has been found, for instance, that a thermometer laid upon grass marked 16° lower than one laid upon a gravel walk beside it; and hence we may infer that the grass in a dewy night would receive a large portion of it, while little or none would be deposited on the gravel walk. And

The glass, in consequence of its exposure to the external air, is colder than the atmosphere within, and hence it condenses and forms into dew the vapoury particles which come in contact with it.

generally, we may infer that little or no dew falls upon rocks, roads, sands, or water, even while grass and vegetables in the same locality are covered with it.

When the dew which falls upon the earth is frozen by the cold, it is called HOAR-FROST. But MILDEW is not of atmospheric origin.

SNOW AND HAIL.

The other forms which the vapours of the atmosphere assume are SNOW and Hail. When the aqueous particles or rain-drops at the moment of formation are crystallized, or frozen by the cold, they assume the form of snow; and if, while falling through the atmosphere, they are still further united and congealed, they assume the form of hail; for hail may be regarded as a species of snow or snowy-rain. Judging from appearances, we might conclude that the cold humidity of snow must be very detrimental to vegetation, but the contrary of this is the case. Snow, particularly in those regions where the ground is covered with it during the winter months, is of great service to vegetation, by protecting corn and other vegetables from the intenser cold of the air, and especially from the cold piercing winds. It also serves to moisten gradually those lands from which, owing to their situation, the rain is soon carried off. Except in mountainous and elevated districts, snow is unknown in the warm regions of the earth; but hail has been known to fall in all parts of the earth, and at all seasons of the year, even in summer. The formation of hail

seems to be owing to electrical agency; one proof of which is, that thunder and lightning are often attended with hail showers; and in fact, artificial hail can be produced by means of an electrical apparatus.

QUESTIONS FOR EXAMINATION.

Pages 166-167.-Why do the vapours ascend? 2. When do they become stationary? 3. When visible? 4. In what form? 5. How are the clouds classified? 6. Mists or fogs? 7. How and where do they render navigation dangerous? 8. Evaporation, how produced? 9. Describe the circulation of waters that is constantly going on, for the benefit of mankind, between the sea, the sky, and the earth?

6. What mark

Pages 167-170.-In what parts of the earth is rain most abundant? 2. Why should we expect this to be the case? 3. The computed annual average quantity of rain in the torrid zone? 4. In the north temperate zone? 5. What is meant by inches of rain? of design is there in the different quantities of rain that fall in different parts of the earth? 7. In what parts of the earth is rain most frequent, or the number of rainy days most? 8. How is this illustrated in the text? 9. How do you account for the regularity of the rain and other phenomena of the atmosphere in the torrid zone? 10. What is a pluviometer or rain-gauge? 11. How many inches of rain have been collected, in 24 hours, in the torrid zone? 12. In what parts of the world are the four seasons distinctly marked? 13. Where are there only two seasons? 14. Where only one? 15. How is the year divided in torrid and tropical regions? 16. When is it the dry and when the wet season? 17. The mark of design in this? 18. Can you give Hum

boldt's description of the effects of these periodical rains and droughts in those regions? 19. The sufferings of the wild horses in the rainy season? 20. Is the horse originally a native of the llanos? 21. In the northern tropical regions of Africa, when do the rains commence, and when end? 22. Describe their effects. 23. In what parts within the tropics are there two rainy seasons? 24. In what two respects do they differ from each other? 25. Have all places under the same parallel their dry and rainy seasons at the same period of the year? 26. What produces this irregularity? 27. Give examples. 28. Are there any places within the torrid zone that have no rainy season? 29. Can you explain this? 30. What is said of Egypt? 31. Generally speaking, in all parts of the world, do equal quantities of rain fall in equal latitudes? 32. What localities are most subject to rain? 33. Can you state the reason?

Pages 171-172.-What is dew? 2. In what regions of the earth, and in what tracts of country in particular, are the dews heaviest ? 3. The utility of dew? 4. Can you state what was the opinion formerly regarding the formation of dew? 5. What is the present theory? 6. What are the illustrations given in the text? 7. Should we expect more dew in a cloudy, than in a clear night? 8. Why little or no dew in cloudy nights? 9. Is dew equally distributed over the same localities ? 10. How has this been proved? 11. The general inference from this? 12. How is hoar-frost produced? 13. Is mildew of atmospheric origin? 14. What other forms do the vapours of the atmosphere assume? 15. How is snow produced? 16. And how hail? 17. If we judge from appearances, what would we conclude regarding the utility of snow? 18. In what two respects is snow stated to be of great use? 19. What is the difference between snow and hail, as to the part of the earth, and seasons of the year in which they fall? 20. How is hail supposed to be produced?

CHAPTER XIII.

WINDS.

THE primary cause of wind is the heat of the sun, which rarefies and expands that portion of the atmosphere which, by the motion of the earth, is more immediately exposed to his rays. The air when rarefied becomes lighter, and consequently ascends to the higher regions, while the surrounding air, which is less rarefied and heavier, rushes in to supply its place. This current or motion of the air is called WIND, and according to its velocity it is said to be a BREEZE, GALE, STORM, &C. Hence it is that there are always draughts or currents of air towards a fire, or into a warm room, if the window be raised.

Winds are generally divided into three classes-permanent, periodical, and variable. The permanent winds extend nearly 30 degrees on each side of the equator, and blow almost always in the same direction. They are also called TRADE winds,

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