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SUMMARY.

[CHAP. to the 22nd September,' and night during the remaining six months, while at the South Pole the case is exactly the reverse. At both poles therefore the summer is all day, the winter all night.

On the 20th March the sun is rising on the North Pole and setting on the South Pole, and on the 22nd September it is setting on the former and rising on the latter. On these days, on all other parts of the earth, the day and equal, viz. twelve hours each.

night are

Such then is the cause of the seasons, of the difference in the relative length of the day and night at opposite periods of the year, and also of the great differences of the summer heat and winter cold, at places far from the Equator. But while the former depend solely on the position of a place on the earth, the latter, as we have seen, are much modified by other circumstances, such as the prevalence of land or water and certain others of which more will be said in another part of this work.

In this chapter we have learned that the atmosphere that everywhere surrounds our earth is not much more than 50 miles thick, and that its lower layers are more dense than those above them, because those which are lowest are pressed upon by those above them. It consists chiefly of two gases, which, when heated, expand and become lighter, and when cooled contract and become heavier or denser, but which always remain gaseous, and mixed in the same proportions. It contains also water vapour, the quantity of which is always varying. This is furnished chiefly by evaporation from seas and oceans. When much cooled, it condenses and forms clouds, rain, snow, and hail; or when condensed on the surface of grass, leaves and the like in the night-time, dew and hoar-frost. Winds are produced by the

By the terms day and night in this description I mean the intervals between the rising and the setting of the sun, and vice versa. It is familiar to us all that darkness does not at once set in when the sun sets, nor does it last until the sun rises again. At the poles twilight begins some days (periods of twenty-four hours each) before the sun rises, and lasts for some days after it has set.

II.]

SUMMARY.

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Heated air rises and

unequal heating of the air by the sun. colder air flows in below to take its place, while that which has ascended is cooled by its ascension, and so forms winds in the upper atmosphere which blow in the opposite direction to the former. At last it descends to earth, to take the place of that which has been drawn towards the heated region. In India the winds blow pretty steadily from the south, south-west, or south-east (according to the locality) during a part of the year, and from the north-west, north or north-east during another part of the year; and these winds are called the monsoons. The former, coming from the ocean, bring us our rains. The latter, blowing from the land towards the sea, have little vapour and bring fine clear weather. On the coast of China and that of North Australia and Africa there are also monsoons, but their directions differ somewhat from ours. On the two great oceans, the Pacific and the Atlantic, as well as on the tropical part of the South Indian Ocean, where the winds blow the same way all the year through, they are called trade-winds; and in Europe where they are much less regular, this irregularity is owing to there being two principal winds, one of which is chiefly from north-east, the other from south-west, which continually displace each other, instead of one being always above and the other below, as is the case in the region of the trade-winds. All these movements and changes of the atmosphere are due to the heat of the sun; to the facts that land is both heated and cooled much more quickly than water; and that the heat is most intense where the sun shines directly overhead, and when the days are longest. The alternation of summer and winter, and indeed of the seasons generally, arises from the inclination of the earth's axis; from which it follows that the Northern and Southern Hemispheres alternately, enjoy a greater and less share of the sun's heat and light, as the earth revolves in its orbit around the sun.

CHAPTER III.

THE SEA.

In this chapter I shall have to describe things probably unfamiliar to most of my readers. But it would be impossible for them to understand much of what I shall have to say afterwards about the land, if they have not first gained some acquaintance with the nature and movements of the ocean; and fortunately it requires no great exertion of the imagination, even for those who have never seen the sea, to conceive a vast body of water, filling all the greater depressions of the earth's surface, and as may be seen on any map of the world or a terrestrial globe, occupying very nearly threefourths of that surface.

It is only of late years that we have gained any accurate knowledge of its depth, and even now there are immense tracts, especially in the Pacific and Indian Oceans, of the depth of which we know very little. But the third great section, the Atlantic, or rather the northern part of it which extends between Europe and North Africa on one side and America on the other, has been much more closely examined; and not only do we know pretty well how deep it is in different parts, but specimens of the bottom and of the sea animals that live on it have been brought up from a depth of nearly four thousand fathoms. The bottom of the

Depths of the sea are usually given in fathoms, each of which is equal to two yards, or four haths, or six feet. The naturalists on board the Challenger have obtained a specimen of the sea-bottom and the animals living on it from a depth of 3,875 fathoms, or nearly four and a half miles.

CHAP. III.] DEPTH AND SALTNESS OF THE OCEAN.

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Atlantic shows irregularities of the same kind as the land surface of continents. It has submarine mountain ranges, table-lands, and valleys. And this we might have expected, for geology tells us that much of what is now covered by the ocean, was at some former time dry land; and on the other hand, we know that in some of the loftiest mountains, the Himalaya for instance, the shells and other hard parts of animals that once lived on the sea-bottom are embedded in the solid rock of the mountain sides; proving to us, that although they are now high above the sea, they once formed part of the bed of the ocean. The greatest depths of the Atlantic are depressed below the sea-surface, to a depth much greater than the height of the highest mountain masses that now tower above it. One large part of it is rather more than 5,000 fathoms, a depth in which, if the three loftiest peaks of the Himalaya were buried, there would still be from 150 to 300 fathoms of water over their summits. It is probable that in the Southern Ocean there are depths exceeding the above, but we cannot be sure of this without further measurements. We do, however, know that the average depth of the ocean much exceeds the average height of the land. The Bay of Bengal is very deep in some parts. On the Indian side the depth increases very gradually, and the deepest part1 lies about 50 miles eastward from the middle of the bay. It shelves down very gradually too from the coast of Bengal; except at one place, about midway between the Hooghly and the Megna, where there is a very deep and abrupt channel in the bed of the sea called "the Swatch;" the depth of which is said to exceed 2,000 fathoms, or more than two miles. This, however, requires verification.

The water of the ocean, unlike that of most lakes and rivers, is very salt; so much so, that a great part of the salt consumed in Madras (and formerly in Bengal also,) is obtained from it, by collecting the water in shallow reservoirs, and allowing it to evaporate. As only the water goes

I

1,785 fathoms was the greatest depth indicated by the dynamometer in laying the telegraph cable between Penang and Madras.

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SALTS IN SEA-WATER.

[CHAP. off as vapour, all the salt is left behind. The salt so btained is however far from pure. It has a more bitter taste than pure salt, owing to its impurities, which consist of salts of lime and magnesia, with other substances, for a knowledge of which my readers must refer to books on chemistry.

The saltness of the water is nearly the same in all parts of the ocean. In the neighbourhood of any coast where a large river enters the sea, the admixture of fresh water makes it somewhat less salt; and inland seas, connected with the ocean, but almost entirely surrounded by land, are either fresher or salter, according as the rivers that enter them supply more fresh water than evaporates from their surface or the reverse. Thus the Red Sea, into which no river flows, and from the surface of which great evaporation is always going on, is salter than the ocean; while the Baltic, which receives many of the large rivers of Northern Europe, and which, being in a cool climate, gives off comparatively little evaporation, is always very much fresher.

If it be asked why the sea is salt, we can only reply that it has probably been so from the time when it began to exist as an ocean. From time to time small tracts of it may have been cut off from the main ocean in some dry region and so dried up, in which case the salt has been left behind as a solid mass called rock-salt. But any loss of salt that it may suffer in this way is compensated by that brought into it by rivers, all of which contain some traces of salt; and by that which the sea itself washes out of the land.

I have already spoken of the lime in sea-water. There is very little, but there would be much more, were it not that lime is continually abstracted from the water by the animals that live in it. Many of these form hard shells, which consist chiefly of a compound of lime, and this lime is

In parts of Bengal the lime eaten with pân is generally made by burning the large shells collected in the salt lakes of the Sundarbans. In Madras large quantities of sea-shells are collected for the lime used to stucco the interiors of houses.