III.]

COLOUR OF THE SEA. SEA WAVES.

39

taken from the water; there are a kind of animals called corals, the greater part of whose body consists of a hard skeleton of lime. They do not move about, but live, almost like plants, fixed in the same spot; and when they die, their skeletons remain, or being broken up by the waves and re-cemented, form masses of limestone which serve as supports for others to grow upon. In this way vast masses

of stony substance accumulate, and new rocks and islands are formed. The Maldives and Laccadives to the southwest of India are entirely islands of this class.

In the neighbourhood of the coast the colour of the seawater is generally green; this is not the true colour of pure sea-water, but is owing to the mud brought into it by rivers and that washed from the coast. In mid-ocean, the colour is a pure deep indigo blue, and the water so transparent, that if a white plate be thrown into it, it can be seen after it has sunk many fathoms below the surface. Certain inland seas, so called because although they communicate with the ocean they are in great part surrounded by land, have waters of peculiar tints which are in general due to the presence of some foreign substance, such as mud or minute floating plants. Thus the Red Sea has received its name from the frequent appearance of a reddish or rather brownish scum in patches on the surface of the water; and the Yellow Sea is so named from the quantity of yellowish mud poured into it by the Hoang Ho, a large river in Northern China. These, however, but slightly affect the general blue or green colour of the water.

The surface of the sea is seldom quite smooth; this is the case only when there has been no wind in the neighbourhood for one or two days. Most frequently it is raised in waves, which roll along in the direction of the wind, or are simply propagated by the water from some distant place where a strong wind is blowing. These latter are spoken of as the swell of the sea; and the rolling of ships as they sail over it, is to many persons, who are unused to it, peculiarly unpleasant, producing sickness. When these long waves reach the shore, such a shore for instance as extends

40

SEA-CLIFFS.

[CHAP.

along the face of the Súndarbans or the coast of Orissa and Madras, they become higher, and the crest, curling over, breaks in a mass of foam, which makes it dangerous for boats, except such as are built with planks sewn together, like the Masúlah boats of Madras. Such waves are termed breakers, and in storms they have great destructive power, and soon dash to pieces and break up any ill-fated ship that may have been driven on the coast, and so become exposed to their impact. On a coast such as that which extends along the Indian side of the Bay, where the land is almost everywhere low, the coast is not much destroyed by the action of the waves; but on rocky coasts, where the edge of

the land is high above the sea-level, they pound and break down the exposed face of the rocks and sweep away the abraded material, leaving a cliff, such as is shown in the above woodcut.

This woodcut represents a part of the chalk cliffs near

III.]

MARINE SEDIMENT.

41

England, as they appeared
The great shaded mass in

Brighton, on the south coast of about forty years ago (in 1836). the mid-distance and the smaller cliffs at its base have long ago been swept away by the sea. The flatter strip of land at the base of the cliffs on which the boat rests is termed the beach, and consists chiefly of a mass of well-rounded flint pebbles originally derived from the bands of flints in the chalk. These being very hard, resist for a long time the grinding action of the sea-waves; but at length they are worn down to sand, which is swept further out and spread abroad on the sea-bottom.

The waves of the sea subserve one very important purpose, viz. they facilitate the aëration of the water; that is, they help the air becoming dissolved in the water; without which fish and most other animals that live in water would be unable to breathe.'

In the introductory chapter we learned that the greater part of the mud that renders our rivers so turbid is carried down into the sea with the water; and I have now shown that, in addition to this, a great deal is carried away from certain coasts which are being eaten away by the waves. All this sinks gradually to the bottom; but before it finally settles, it is often carried many hundreds of miles by those movements of the water of which we have yet to speak. However, sooner or later, it settles down, and forms new layers of mud and sand on the floor of the ocean. In deep seas, these accumulations of necessity form very slowly; but they accrete more rapidly near the mouths of large rivers and certain coasts that are wasting rapidly. The dead bodies of fishes and other marine animals, sinking to the bottom, are sometimes buried in this mud and sand; and the hard parts, such as shells and bones, are thus frequently preserved. Ages afterwards, when, by some of those changes that rocks are always undergoing, these layers of mud and sand have become converted into stone, and being

Most, but not all. Whales, porpoises, seals, dugongs, and all such animals as suckle their young, also turtles and sea-snakes, breathe air, for which purpose they come occasionally to the surface.

42

ORIGIN OF FOSSILS.

TIDES.

[CHAP.

upheaved, once more form part of the dry land, such remains, now termed fossils, are discovered by geologists, who learn from them what kind of animals lived in the sea, it may be thousands and hundreds of thousands of years before. Sometimes, but more rarely, the bodies of land animals and the leaves and branches of plants are embedded in the same way; but these are more common in similar accumulations that are formed in lakes or in the deltas of great rivers, such as that on which we live in Bengal. In another chapter we shall see that by diligently collecting and examining remains of this kind, we have learned that the animals that live on the earth now are not like those that used to live there in former times; and that all of them have changed, not once only, but many times in succession. The sea is never at rest. Not only is the surface stirred by the wind, but the whole mass of the ocean is moved by tidal and local currents. Tidal currents flow first one way and then in the opposite direction, changing each way twice in about twenty-five hours, and producing a rise and fall of the water-level twice in the same period. The second, in most parts of the ocean, run in the same direction with but little variation all the year through; and the result is, that the whole water of the ocean is constantly but slowly moving from one part of the earth to another. The causes of these movements may be understood with a little careful attention.

I have said that the tide rises and falls twice in about twenty-five hours; those who live on the banks of the Hooghly or the Megna, or any of the large rivers of the Súndarbans near the sea, must be familiar with this as a fact of lifelong experience. This period of twenty-five hours, or more accurately, twenty-four hours, fifty-four minutes, is exactly that between the time that the moon is seen at the same place in the sky on two successive days; a fact, which would at once suggest that the moon is in some way connected with the tides. Now if the reader bears in mind what we learned a short time since, about gravitation, he will find little difficulty in understanding what this connection is. I must only add one fact, which I have not mentioned before, in

III.

CAUSE OF THE TIDES.

43

order to avoid complexity at the outset. It is this; that the farther any two bodies are from each other, the less is the pull they exercise on each other; in such measure, that if the centre of the moon were twice as far off the centre of the earth as it is, she would pull and be pulled by the earth with only one-fourth the actual force; if at three times the distance, with only one-ninth of that force; if at four times the distance, with one-sixteenth, and so on.1

Now let us see what consequence follows from this, if we suppose the earth to be a rigid ball; that is, one, the shape of which remains unaltered, however it may be pulled; while it is covered with water, which flows easily in any direction. And for the sake of greater simplicity, we will assume, first of all, that the earth is completely covered by water, as represented in Fig. 1, Plate I. When the moon is in the position represented in Fig. 2, the water on the side a, nearest to the moon, will be pulled more strongly than the rigid earth c, because it is nearer; and will be heaped up at a, being drawn away from all that part which lies towards d and e. Thus high water will be produced at a. In the same way, and for the same reason, the rigid earth c will be pulled away from the water at b, and another pile of water, producing another high tide at b, will be the consequence. At d and e the water will be low, so that two high tides and two low tides are produced at the same time. Now as the earth revolves on its axis, which we may suppose to be represented by a line at right angles to the plane of the paper, passing through the centre of c, every point on the earth will be successively in the positions adbea; and if the moon remained in the same place at M, every place on the earth would have high water twice and low water twice in twenty-four hours; that is to say, while the earth completes one revolution. But in the meantime, the moon has moved in her own orbit towards M', so

Common arithmetic shows that the denominators of these fractions are the squares of the figures expressing the distance; so that the law of gravitation is expressed by saying, that the force is inversely as the square of the distance of the two bodies.