It teaches us, as far as such things can be explained, why the world is as we see it. Not indeed why it exists, any more than it teaches us why we ourselves exist. Such questions cannot be answered by any science. But it will teach us that every change that we see in progress goes on regularly, however irregular it may seem at first sight, and that everything is subject to change. Moreover that nothing takes place without a cause. When we have found out an explanation, and found that it is always true, such as that which we arrived at a short time ago, viz., that rivers are fed by rain, and always flow down a slope towards the sea, or that land is wasting away and is somehow renewed, we call it a natural law, and the object of Physical Geography is to find out what are the laws according to which changes take place around us.

CHAPTER I.

THE EARTH AS A PLANET.

THOSE of my readers who live in Máldah, Púrnia, or the northern part of Bhágalpúr, on looking to the north some fine October morning, may have noticed along the horizon (that is, where the land seems to end and the sky to begin) a line of bright white mountain tops glistening in the morning sun. They are the snow-covered crests of the Himálaya. Viewed from so great a distance, it is hard to believe that they are of the great height that we know them to be; for the snow does not lie much below 16,000 feet, or at a height of nearly three miles above the level of the rice-fields from which I suppose them to be seen. But a great part of the lower slopes of the mountains are hidden by the seemingly level plain on which the observer stands. This may seem strange; but it is only an example of a very familiar kind of appearance, from which we learn that the earth we live upon is really not flat, but a globe or ball; but which is so large, that so much of it as we can see at any one time seems to be flat. The surface of the sea, when calm, is much smoother and apparently flatter than the rice-fields of Bengal; and here it may be seen almost daily, that distant objects, ships for instance, are partly hidden by its really curved surface. When the man on the look-out, at the mast head of a vessel, first descries a distant steamer, he probably sees nothing but the smoke. Presently, as it draws nearer, the tops of the masts seem to emerge from the water on the horizon. Then they rise higher and the funnel appears, and

CHAP. 1.] FORM OF THE EARTH AND MOON.

II

lastly the hull of the steamer comes into sight. It does not matter from what direction the steamer comes,-north, south, east, or west,—the several parts of the steamer come into sight successively in the order I have mentioned; the highest parts first, the lower last. Now if the earth were really flat, this would not be the case. The steamer would at first seem a mere speck in the far distance, and as soon as its parts could be made out distinctly, the hull would be seen as well as the masts and funnel. The annexed diagram will help to make clear the above description of what is actually seen, and how from it we learn that the world is round. o is the observer at the mast head, s the sea, h the horizon that bounds the visible part of the sea-surface, and st the distant steamer. I must however observe that, in comparison with the curvature of the earth, the size of these vessels is greatly exaggerated in the figure; in reality, if only the tops of the steamer's masts were seen by the observer at o, the two vessels must be many miles apart.

The moon is a globe like the earth, but not so large. With a telescope we can see that she has the appearance of a ball. We are all familiar with the changes in her appearance, which are called her phases; how from a narrow crescent of light she grows to a semicircle and thence to a full round. If we take a ball of white clay and hold it before a single lamp, so that only one side is illuminated, then, looking at it successively from every side, it will present a series of appearances very like those of the moon. If we could stand on the moon and look at the earth, our earth would look very much like the moon as we see it (except that it would appear much larger), and would go through

12

FORM OF THE EARTH.

[CHAP. a similar series of changes, because both the moon and the earth are illuminated in the same way by the sun. But sometimes when the moon is full, the earth passes between her and the sun, throwing a shadow on her illuminated surface, just as a second ball will do if interposed between the lamp and the white ball that it illuminates. Now this shadow is always round, such a shadow as a ball casts on a white wall; it is round, whichever face of the earth is turned towards the moon, and since a ball is the only form of body that always throws a round shadow and no other, this again assures us that the earth is a ball.

I might easily multiply proofs of the roundness of the earth were it necessary, but since one complete proof, such as the shape of its shadow, is really all that we need, perhaps it will be more useful if we now consider the matter and see if there are no difficulties to be met. One difficulty will perhaps occur to some of my readers. If the earth be a ball, it will be readily understood that those who live on the top should remain standing, but it will be asked, 'do people live on the other side of it, and if so, why do they not fall off?' Now it is unquestionable that people do live on the other side of the earth, viz., those who live in Peru and Chili in South America, for when it is about midnight with us and everything is dark and quiet, it is a little after noonday with them, and the sun is shining brightly over their heads. When a telegraphic message is sent to America, which, with a little preparation beforehand, may be done so quickly that it reaches America almost at the same instant of time that it leaves Calcutta; if it is sunset when the signalling clerk in Calcutta is sending the message, it is sunrise in America to the clerk there who is receiving it. Again, ships often sail right round the world, going always to the east, and yet returning to the port from which they set out. Yet those who live in America, and the sailors in a ship sailing round the world, see the earth or water at their feet, and the sky above them, just as we do, and they stand as firmly; and if one of them throws a ball into the air, it fall back again to the ground just as it does here.

1.]

GRAVITATION.

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This is perhaps, at first, not very easy to understand; but we may understand it, if we only think what we really judge by, when we speak of throwing a ball up and of its falling down. We really refer up and down to the position of our own bodies as we stand; and we call the direction in which our heads point, upwards, and that in which our feet point, downwards. And since Americans stand just as we do, that which they call up is almost1 identically the same direction in space that we call down. Consequently when a ball is thrown up into the air in Peru, it moves away from the earth, in almost the same direction as another which is falling towards the earth in Bengal. With a large ball before him to represent the earth, such as a Kadú, or a common round earthen pot with the neck knocked off and filled with clay, rounded off at the top, if a terrestrial globe be not at hand, to represent the earth, my readers may, with a little thought, realize these facts to their mind. And when they have clearly realized them, they will have gained an important step in knowledge, viz., that all we know about upwards and downwards is the direction judged of by our own position in standing.

This much being clearly understood, we have yet to explain why a ball always falls to the earth, and why we stand firmly on it, when our heads point in one direction and those of the Peruvians in a direction almost exactly opposite. This is because the earth, or globe, on which we live, attracts or draws everything towards it. And not only does the earth do this, but everything that we can see and handle, everything that has weight, does exactly the same. The ball that falls to the earth attracts the earth towards it in the same way that it itself is attracted by the earth, but the strength of the pull exerted by each is in proportion to its size and its intrinsic heaviness, since a ball of lead exercises a stronger pull than a ball of cotton of the same size. Now the earth is enormously large, the ball very small indeed; and therefore to our senses, the ball seems to be the

1 I say almost, for the part of the earth directly opposite to Bengal, and called its antipodes, is in the Pacific Ocean and not in America.