which are all needed to make the protoplasm from which all the plantcells—that is, all parts of the plant-are built up. They must, besides, have iron, lime, soda, potassa, magnesium, flint, phosphorus, &c. In the lower plants, the whole surface absorbs these elements; in the higher, the roots are the mouths through which the plant drinks them in. The delicate hairs you see all over roots are so many pipes to help them in this, and they are further provided with countless little glands* over every part of them, to their utmost fibre, which also suck in all kinds of nourishment. You have seen that a plant is made up of minute walled cells; so it is clear that whatever is to pass into its vessels must be received either as a gas or a liquid. The carbonic acid gas plants need is mostly absorbed from the atmosphere, by the leaves, under the influence of light; the oxygen being separated from it and given off again. Very little of this gas seems to be received through the roots. Hydrogen is obtained by the decomposition of water within the plant. Oxygen, in combination with hydrogen, as water, and in other substances, is only too abundantly supplied, so that the excess has to be rejected. Yet it is of such vital necessity to the plant, that while the roots alone receive more than they require, it is constantly being absorbed, through the leaves, from the air, to neutralise the excess of carbonic acid gas. The whole plant needs, in fact, to breathe-in oxygen, just as we ourselves do; for the protoplasm, which is the life-blood of the plant, would die, but for oxygen thus being breathed perpetually through it. It is this inward breathing, if I may call it so, which, besides, creates the heat needed for various wants of plant life, just as it creates the warmth in our own blood. Nitrogen is absorbed through countless little mouths on the under side of the leaves, but also in ammonia through the roots, for all water contains more or less of that substance. Sulphur is obtained from the soil around, and so are the various mineral salts. The nourishment drawn in through the fine hairs of the root rises to the leaves in an ascending stream of SAP, which is largely water, with the various substances dissolved in it. Flowing in minute threads through the root hairs, it gathers into larger vessels as it advances through the roots, and finally rises up the stem in a steady flood, which divides itself again when it reaches the branches, and still more when it comes to the twigs and leaves, the finest nerves of which are moistened by it. From the surface of the leaves it passes off by evaporation into the air, just as perspiration docs from the skin of animals. It has been found that the quantity of water thus given off * glands-called Spongioles, or little sponges. in a few days by vigorous plants, is often equal to even a hundred times the whole bulk of the plant. This stream of sap flows up in great measure through the part of the plant just below the bark; but it is not confined to it alone. The whole of the woody fibres of the plant aid in it. If a coloured liquid, such as the red juice of cherries, be let soak through the roots of a plant, it will be found before long to have penetrated the whole structure of the plant, and to show itself even in the sheaths of the flowers. VII.-WAVES OF THE SEA. 1. I SUPPOSE every one has stood, some time or other, beside a millpond when the wind was blowing lightly over it. Did you notice how, as it did so, it raised the water into ripples, which spread over the whole pond? Or, perhaps, you have sometimes been puffing a toy-boat across a basin of water, and have seen how your breath roughened the surface into little waves in the same way. The waves of the sea are caused by just such a blowing of strong winds over it; and as these winds often blow from different directions, at different parts, the waves they raise often cross each other, and make the sea so broken that it tosses the ships, and makes them roll, by turns. 2. If you ever took a trip to the open sea, you would notice, further, that the waves grew larger and larger as you got farther out. This is because the height of waves depends on the depth of the water on which they rise, and the extent of its surface. You would not expect such waves on a mill-pond as on a great harbour, nor such waves on the harbour as on the open sea. According to Dr. Scoresby, the waves in the Atlantic rise, in a storm, from twenty-four to forty-three feet above the level of the ocean. Captain Wilkes found the highest in the Southern Pacific to be about thirty-two feet; and Sir James Ross, and the French naturalists who sailed in the frigate Venus round the world, agree in fixing the height of the highest waves they met at twenty-two feet. On inland seas, like the Mediterranean, they range from thirteen to eighteen feet in a storm. Hence it is only fancy when we hear of waves running "mountains high." 3. The motion of the waves is something quite distinct from the motion of a single advancing mass of water. When a flood sweeps down a river-bed, from the bursting of a dam, the whole body of the water is in motion, and it sweeps everything before it. But if you drop even so light a body as a cork on a wave, you will notice that it does not move on with the wave, but rises to the top, and then sinks down into the valley of water behind. In the same way, a ship is not driven on by a wave as it would be by a flood broken loose; the wave sinks below it, and leaves it very nearly where it was Every one has seen how pieces of wood floating near the shore do not drive towards it as fast as the waves do, but let the waves pass under them and roll on alone. 4. A wave, then, is not the water in motion. If it were, the poor sailor whom it bears to the rock that saves him, would be dashed against it and killed at once. It is simply a mass of water raised by the wind, giving the same impulse to the water next it that it has itself received, and thus causing it to rise in turn. The water of the first wave, as it sinks, presses up the water next it into a second wave, and the second forces up a third, and so on. If you hold a long streamer in the wind, you will see exactly the same thing. It will flutter in waves from end to end, though it is still in your hand all the time; or, if you ever noticed men at one side of a large carpet shaking it, you would see it rise and fall in waves, all across, each time they shook it. This is how the waves pass over the sea; the motion passes, but the water, like the streamer or the carpet, remains where it was. 5. The rate at which this motion of the waves advances is, however, sometimes, very great. Commodore Wilkes estimated its rapidity at twenty-six miles an hour in some storms he saw; and Dr. Scoresby sets it down as nearly thirty-three miles an hour in a storm in the Atlantic. A wave is, on an average, fifteen times broader at its base than it is high. A wave of forty feet in height would thus have a breadth at its base of six hundred feet (40 × 15). 6. But though waves are not so high in the open sea, even in the greatest storms, as we might have supposed, their force drives them very much higher when they break on any rock, or precipitous shore. Thus, the lighthouse on the Bell Rock, though one hundred and twelve feet high, is often buried in the foam and spray of the waves, and the thick glass of a lighthouse on the northern coast of Scotland, six hundred feet above the sea, is often broken in a storm, by the stones hurled up against it by the raging waters. The power of waves is indeed awful. At Plymouth Breakwater they once lifted huge masses of stone, from two to five tons in weight, from the bottom of the outer side, and hurled them over the inner side; and when the Bell Rock Lighthouse was being built in 1807, great blocks of granite were lifted and hurled hither and thither. The force of the waves that break over the lighthouse is equal to three tons a square foot. VIII.-EFFECT OF THE WAVES ON COASTS.-THE TIDES. 1. No wonder, then, that the waves eat away the coasts against which they beat. At Sherringham, in Norfolk, Sir Charles Lyell ascertained, in 1829, that there was then a depth of water sufficient to float a frigate -twenty feet-at one point of the harbour, where only forty-eight years before there stood a cliff fifty feet high, with houses on it! In the same way the village of Reculver, in Kent, was a mile distant from the sea in Henry VIII.'s time, but in 1804 the waters had advanced till the village had been washed away, leaving the church where it still stands, a landmark on the very edge of the sea-cliff. 2. During the most violent gales the agitation of the waters reaches to a depth of three hundred to four hundred feet, and stones have been thrown up even from a depth of six hundred feet during very great storms. It is easy from this to see how beaches are formed. The heaviest stones fall first; the lighter next, and then the shingle or sand. Sometimes, indeed, there is no sand or shingle, as on the southeastern coast of Nova Scotia. There, the long swell of the Atlantic, having nothing else to throw up, has heaped a great broad breakwater of stones high above its waters, for many miles. 3. Did you ever think about the tides when you happened to be living at the sea shore, or on a river which rose and fell with them? What an awful power must be required to lift up the waters of the sea from their bed, and pour them in such a way towards the land! All the steam-engines in the world would not, I am sure, be able to raise more than an unnoticeable fraction of the water needed to make one tide along the coast of England alone, and tides rise and fall all round the world. The power or force required to cause them must be something beyond all our thought. 4. What no human power could do is done by the moon, and, at certain times by the moon and the sun together. It is they which lift up* the ocean from its lowest depths till it covers the bounds which a few hours before were dry. On all sea-coasts, except the coasts of inland seas, such as the Mediterranean, nearly shut off from the ocean, the waters rise and fall twice in every twenty-five hours. They rise for about six hours; then fall during about the same time; then rise and fall again, as before. Each day the tides are later than the day before, according to the time the moon takes at different periods of each revolution to make her apparent daily motion round the earth. When it is new, and when it is full moon, the tide rises and falls twice in twenty-four hours and thirtyseven minutes, but when it is half moon twenty-five hours and twentyseven minutes are required. 5. The attraction of the moon draws the waters upwards at the side next it, and at the same time raises those at the side away from it, by drawing the earth, as it were, from below them, and this necessarily makes a high tide at both sides of the earth at once. This attraction of the moon causes the waters to rise five feet higher at the two sides affected than it is at the other two. The sun's power also influences the tides, but from the great distance from which it acts, it only raises the water about two feet. The progress of the tides round the earth follows that of the moon. 6. You have doubtless heard of high or spring-tides, and of low or neap-tides. Spring-tides are caused by the tide made by the sun rising at the same time as that caused by the moon, so as to add its two feet to the moon's five, thus lifting the waters seven feet in all. Neap-tides happen when the tide caused by the sun is cross to the tide caused by the moon, so as to draw the water away. In this case, instead of the five feet of the moon's tide, there is only a rise of three. At the new moon, when the sun and moon are in the same quarter of the heavens, they act in the same direction, and cause a spring-tide, and the same happens when they are on opposite sides of the earth, or at full moon. At all other times they act in different directions, and when this opposition is greatest—that is, at half moon-we have the neap-tides. The height to which a tide may rise depends, however, on the configuration of the coast at any special place, and also on the direction and strength of the wind, and the weight or lightness of the air. At some parts, even the usual tide rises to twenty or, it may be, forty feet, and you can easily fancy what would happen if the rising of the waters were aided by the strong winds of a storm and by a heavy sea. Such a tidal wave has again and again risen over all the usual defences of low coasts, and carried death and ruin far and wide. Large districts of Holland have thus been permanently submerged at different periods, as many as 100,000 people perishing in one awful inroad of the ocean. JOHN TROT.*-THOMAS HOOD, 1799-1845, As York did ever rear- A sergeant soon came down to York, With ribbons and a frill; "My lads," said he, "let broadcast, be, And come away to drill." *This piece is given as an exercise on the different meanings of the same words, or of words of similar sound. The pupils may be asked to explain the play on the words and phrases "granny, grenadier, broadcast, drill, raw recruit, overdone, shoulder-knot, dumpy, high, more dumpy still, low, look up to me, a cunning woman, studied in a cup, he wanted to be raised, mourning, cry in vain, Cries of London, followed you, high degree, matched beneath yourself, out of place, short, turn out better, black and blue, to serve the country all at large, with a shilling cut him off, paternal land, cut him down." |