But it is in the caves and hollows of the earth that this dissolved matter is built up into the most beautiful forms. If you have ever been to Buxton in Derbyshire, you will probably have visited a cavern called Poole"s Cavern, not far from there, which when you enter it looks as if it were built up entirely of rods of beautiful transparent white gla.s.s, hanging from the ceiling, from the walls, or rising up from the floor. In this cavern, and many others like it,*(See the picture at the head of the lecture.) water comes dripping through the roof, and as it falls carbonate of lime forms itself into a thin, white film on the roof, often making a complete circle, and then, as the water drips from it day by day, it goes on growing and growing till it forms a long needle-shaped or tube-shaped rod, hanging like an icicle. These rods are called stalact.i.tes, and they are so beautiful, as their minute crystals glisten when a light is taken into the cavern, that one of them near Tenby is called the "Fairy Chamber." Meanwhile, the water which drips on to the floor also leaves some carbonate of lime where it falls, and this forms a pillar, growing up towards the roof, and often the hanging stalact.i.tes and the rising pillars (called stalagmites) meet in the middle and form one column. And thus we see that underground, as well as aboveground, water moulds beautiful forms in the crust of the earth. At Adelsberg, near Trieste, there is a magnificent stalact.i.te grotto made of a number of chambers one following another, with a river flowing through them; and the famous Mammoth Cave of Kentucky, more than ten miles long, is another example of these wonderful limestone caverns.
But we have not yet spoken of the sea, and this surely is not idle in altering the shape of the land. Even the waves themselves in a storm wash against the cliffs and bring down stones and pieces of rock on to the sh.o.r.e below. And they help to make cracks and holes in the cliffs, for as they dash with force against them they compress the air which lies in the joints of the stone and cause it to force the rock apart, and so larger cracks are made and the cliff is ready to crumble.
It is, however, the stones and sand and pieces of rock lying at the foot of the cliff which are most active in wearing it away.
Have you never watched the waves breaking upon a beach in a heavy storm? How they catch up the stones and hurl them down again, grinding them against each other! At high tide in such a storm these stones are thrown against the foot of the cliff, and each blow does something towards knocking away part of the rock, till at last, after many storms, the cliff is undermined and large pieces fall down. These pieces are in their turn ground down to pebbles which serve to batter against the remaining rock.
Professor Geikie tells us that the waves beat in a storm against the Bell Rock Lighthouse with as much force as if you dashed a weight of 3 tons against every square inch of the rock, and Stevenson found stones of 2 tons" weight which had been thrown during storms right over the ledge of the lighthouse. Think what force there must be in waves which can lift up such a rock and throw it, and such force as this beats upon our sea-coasts and eats away the land.
Fig. 28 is a sketch on the sh.o.r.es of Arbroath which I made some years ago. You will not find it difficult to picture to yourselves how the sea has eaten away these cliffs till some of the strongest pieces which have resisted the waves stand out by themselves in the sea. That cave in the left-hand corner ends in a narrow dark pa.s.sage from which you come out on the other side of the rocks into another bay. Such caves as these are made chiefly by the force of the waves and the air, bringing down pieces of rock from under the cliff and so making a cavity, and then as the waves roll these pieces over and over and grind them against the sides, the hole is made larger. There are many places on the English coast where large pieces of the road are destroyed by the crumbling down of cliffs when they have been undermined by caverns such as these.
Thus, you see, the whole of the beautiful scenery of the sea - the sh.o.r.es, the steep cliffs, the quiet bays, the creeks and caverns - are all the work of the "sculptor" water; and he works best where the rocks are hardest, for there they offer him a good stout wall to batter, whereas in places where the ground is soft it washes down into a gradual gentle slope, and so the waves come flowing smoothly in and have no power to eat away the sh.o.r.e.
And now, what has Ice got to do with the sculpturing of the land?
First, we must remember how much the frost does in breaking up the ground. The farmers know this, and always plough after a frost, because the moisture, freezing in the ground, has broken up the clods, and done half their work for them.
But this is not the chief work of ice. You will remember how we learnt in our last lecture that snow, when it falls on the mountains, gradually slides down into the valleys, and is pressed together by the gathering snow behind until it becomes moulded into a solid river of ice (see Fig. 29, Frontispiece). In Greenland and in Norway there are enormous ice-rivers or glaciers, and even in Switzerland some of them are very large. The Aletsch glacier, in the Alps, is fifteen miles long, and some are even longer than this. They move very slowly - on an average about 20 to 27 inches in the centre, and 13 to 19 inches at the sides every twenty-four hours, in the summer and autumn. How they move, we cannot stop to discuss now; but if you will take a slab of thin ice and rest it upon its two ends only, you can prove to yourself that ice does bend, for in a few hours you will find that its own weight has drawn it down in the centre, so as to form a curve.
This will help you to picture to yourselves how glaciers can adapt themselves to the windings of the valley, creeping slowly onwards until they come down to a point where the air is warm enough to melt them, and then the ice flows away in a stream of water. It is very curious to see the number of little rills running down the great ma.s.ses of ice at the glacier"s mouth, bringing down with them gravel, and every now and then a large stone, which falls splashing into the stream below. If you look at the glacier in the Frontispiece, you will see that these stones come from those long lines of stones and boulders stretching along the sides and centre of the glacier. It is easy to understand where the stones at the side come from; for we have seen that damp and frost cause pieces to break off the surface of the rocks, and it is natural that these pieces should roll down the steep sides of the mountains on to the glacier. But the middle row requires some explanation. Look to the back of the picture, and you will see that this line of stones is made of two side rows, which come from the valleys above. Two glaciers, you see, have there joined into one, and so made a heap of stones all along their line of junction.
These stones are being continually, though slowly, conveyed by the glacier, from all the mountains along its sides, down to the place where it melts. Here it lets them fall, and they are gradually piled up till they form great walls of stone, which are called moraines. Some of the moraines left by the larger glaciers of olden time, in the country near Turin, form high hills, rising up even to 1500 feet.
Therefore, if ice did no more than carry these stone blocks, it would alter the face of the country; but it does much more than this. As the glacier moves along, it often cracks for a considerable way across its surface, and this crack widens and widens, until at last it becomes a great gaping chasm, or creva.s.se as it is called, so that you can look down it right to the bottom of the glacier. Into these creva.s.ses large blocks of rock fall, and when the chasm is closed again as the ice presses on, these ma.s.ses are frozen firmly into the bottom of the glacier, much in the same way as a steel cutter is fixed in the bottom of a plane. And they do just the same kind of work; for as the glacier slides down the valley, they scratch and grind the rocks underneath them, rubbing themselves away, it is true, but also sc.r.a.ping away the ground over which they move. In this way the glacier becomes a cutting instrument, and carves out the valleys deeper and deeper as it pa.s.ses through them.
You may always know where a glacier has been, even if no trace of ice remains; for you will see rocks with scratches along them which have been cut by these stones; and even where the rocks have not been ground away, you will find them rounded like those in the left-hand of the Frontispiece, showing that the glacier- plane has been over them. These rounded rocks are called "roches moutonnees," because at the distance they look like sheep lying down.
You have only to look at the stream flowing from the mouth of a glacier to see what a quant.i.ty of soil it has ground off from the bottom of the valley; for the water is thick, and coloured a deep yellow by the mud it carries. This mud soon reaches the rivers into which the streams run; and such rivers as the Rhone and the Rhine are thick with matter brought down from the Alps.
The Rhone leaves this mud in the Lake of Geneva, flowing out at the other end quite clear and pure. A mile and a half of land has been formed at the head of the lake since the time of the Romans by the mud thus brought down from the mountains.
Thus we see that ice, like water, is always busy carving out the surface of the earth, and sending down material to make new land elsewhere. We know that in past ages the glaciers were much larger than they are in our time; for we find traces of them over large parts of Switzerland where glaciers do not now exist, and huge blocks which could only have been carried by ice, and which are called "erratic blocks," some of them as big as cottages, have been left scattered over all the northern part of Europe. These blocks were a great puzzle to scientific men till, in 1840, Professor Aga.s.siz showed that they must have been brought by ice all the way from Norway and Russia.
In those ancient days, there were even glaciers in England; for in c.u.mberland and in Wales you may see their work, in scratched and rounded rocks, and the moraines they have left. Llanberis Pa.s.s, so famous for its beauty, is covered with ice-scratches, and blocks are scattered all over the sides of the valley. There is one block high up on the right-hand slope of the valley, as you enter from the Beddgelert side, which is exactly poised upon another block, so that it rocks to and fro. It must have been left thus balanced when the ice melted round it. You may easily see that these blocks were carried by ice, and not by water, because their edges are sharp, whereas if they had been rolled in water, they would have been smoothed down.
We cannot here go into the history of that great Glacial Period long ago, when large fields of ice covered all the north of England; but when you read it for yourselves and understand the changes on the earth"s surface which we can see being made by ice now, then such grand scenery as the rugged valleys of Wales, with large angular stone blocks scattered over them, will tell you a wonderful story of the ice of bygone times.
And now we have touched lightly on the chief ways in which water and ice carve out the surface of the earth. We have seen that rain, rivers, springs, the waves of the sea, frost, and glaciers all do their part in chiselling out ravines and valleys, and in producing rugged peaks or undulating plains - here cutting through rocks so as to form precipitous cliffs, there laying down new land to add to the flat country - in one place grinding stones to powder, in others piling them up in gigantic ridges. We cannot go a step into the country without seeing the work of water around us; every little gully and ravine tells us that the sculpture is going on; every stream, with its burden of visible or invisible matter, reminds us that some earth is being taken away and carried to a new spot. In our little lives we see indeed but the very small changes, but by these we learn how greater ones have been brought about, and how we owe the outline of all our beautiful scenery, with its hills and valleys, its mountains and plains, its cliffs and caverns, its quiet nooks and its grand rugged precipices, to the work of the "Two great sculptors, Water and Ice."
Week 16
Lecture VI
THE VOICES OF NATURE AND HOW WE HEAR THEM
We have reached to-day the middle point of our course, and here we will make a new start. All the wonderful histories which we have been studying in the last five lectures have had little or nothing to do with living creatures. The sunbeams would strike on our earth, the air would move restlessly to and fro, the water-drops would rise and fall, the valleys and ravines would still be cut out by rivers , if there were no such thing as life upon the earth. But without living things there could be none of the beauty which these changes bring about. Without plants, the sunbeams, the air and the water would be quite unable to clothe the bare rocks, and without animals and man they could not produce light, or sound, or feeling of any kind.
In the next five lectures, however, we are going to learn something of the use living creatures make of the earth; and to- day we will begin by studying one of the ways in which we are affected by the changes of nature, and hear her voice.
We are all so accustomed to trust to our sight to guide us in most of our actions, and to think of things as we see them, that we often forget how very much we owe to sound. And yet Nature speaks to us so much by her gentle, her touching, or her awful sounds, that the life of a deaf person is even more hard to bear than that of a blind one.
Have you ever amused yourself with trying how many different sounds you can distinguish if you listen at an open window in a busy street? You will probably be able to recognize easily the jolting of the heavy wagon or dray, the rumble of the omnibus, the smooth roll of the private carriage and the rattle of the light butcher"s cart; and even while you are listening for these, the crack of the carter"s whip, the cry of the costermonger at his stall, and the voices of the pa.s.sers-by will strike upon you ear. Then if you give still more close attention you will hear the doors open and shut along the street, the footsteps of the pa.s.sengers, the sc.r.a.ping of the shovel of the mud-carts; nay, if he happen to stand near, you may even hear the jingling of the s...o...b..ack"s pence as he plays pitch and toss upon the pavement.
If you think for a moment, does it not seem wonderful that you should hear all these sounds so that you can recognize each one distinctly while all the rest are going on around you?
But suppose you go into the quiet country. Surely there will be silence there. Try some day and prove it for yourself, lie down on the gra.s.s in a sheltered nook and listen attentively. If there be ever so little wind stirring you will hear it rustling gently through the trees; or even if there is not this, it will be strange if you do not hear some wandering gnat buzzing, or some busy bee humming as it moves from flower to flower. Then a gra.s.shopper will set up a chirp within a few yards of you, or, if all living creatures are silent, a brook not far off may be flowing along with a rippling musical sound. These and a hundred other noises you will hear in the most quiet country spot; the lowing of the cattle, the song of the birds, the squeak of the field-mouse, the croak of the frog, mingling with the sound of the woodman"s axe in the distance, or the dash of some river torrent. And beside these quiet sounds, there are still other occasional voices of nature which speak to us from time to time.
The howling of the tempestuous wind, the roar of the sea-waves in a storm, the crash of thunder, and the mighty noise of the falling avalanche; such sounds as these tell us how great and terrible nature can be.
Now, has it ever occurred to you to think what sounds is, and how it is that we hear all these things? Strange as it may seem, if there were no creature that could hear upon the earth, there would be no such thing as sound, though all these movements in nature were going on just as they are now.
Try and grasp this thoroughly, for it is difficult at first to make people believe it. Suppose you were stone-deaf, there would be no such thing as sound to you. A heavy hammer falling on an anvil would indeed shake the air violently, but since this air when it reached your ear would find a useless instrument, it could not play upon it. and it is this play on the drum of your ear and the nerves within it speaking to your brain which make sound. Therefore, if all creatures on or around the earth were without ears or nerves of hearing, there would be no instrument on which to play, and consequently there would be no such thing as sound. This proves that two things are needed in order that we may hear. First, the outside movement which plays on our hearing instrument; and, secondly, the hearing instrument itself.
First, then, let us try to understand what happens outside our ears. Take a poker and tie a piece of string to it, and holding the ends of the string to your ears, strike the poker against the fender. You will hear a very loud sound, for the blow will set all the particles of the poker quivering, and this movement will pa.s.s right along the string to the drum of your ear and play upon it.
Now take the string away from you ears, and hold it with your teeth. Stop your ears tight, and strike the poker once more against the fender. You will hear the sound quite as loudly and clearly as you did before, but this time the drum of your ear has not been agitated. How, then, has the sound been produced? In this case, the quivering movement has pa.s.sed through your teeth into the bones of your hear, and from them into the nerves, and so produced sound in your brain. And now, as a final experiment, fasten the string to the mantelpiece, and hit it again against the fender. How much feebler the sound is this time, and how much sooner it stops! Yet still it reaches you, for the movement has come this time across the air to the drums of your ear.
Here we are back again in the land of invisible workers! We have all been listening and hearing ever since we were babies, but have we ever made any picture to ourselves of how sound comes to us right across a room or a field, when we stand at one end and the person who calls is at the other?
Since we have studied the "aerial ocean," we know that the air filling the s.p.a.ce between us, though invisible, is something very real, and now all we have to do is to understand exactly how the movement crosses this air.
This we shall do most readily by means of an experiment made by Dr. Tyndall in his lectures on Sound. I have here a number of boxwood b.a.l.l.s resting in a wooden tray which has a bell hung at the end of it. I am going to take the end ball and roll it sharply against the rest, and then I want you to notice carefully what happens. See! the ball at the other end has flow off and hit the bell, so that you hear it ring. Yet the other b.a.l.l.s remain where they were before. Why is this? It is because each of the b.a.l.l.s, as it was knocked forwards, had one in front of it to stop it and make it bound back again, but the last one was free to move on. When I threw this ball from my hand against the others, the one in front of it moved, and hitting the third ball, bounded back again; the third did the same to the fourth, the fourth to the fifth, and so on to the end of the line. Each ball thus came back to its place, but it pa.s.sed the shock on to the last ball, and the ball to the bell. If I now put the b.a.l.l.s close up to the bell, and repeat the experiment, you still hear the sound, for the last ball shakes the bell as if it were a ball in front of it.
Now imagine these b.a.l.l.s to be atoms of air, and the bell your ear. If I clap my hands and so hit the air in front of them, each air-atom hits the next just as the b.a.l.l.s did, and though it comes back to its place, it pa.s.ses the shock on along the whole line to the atom touching the drum of your ear, and so you receive a blow. But a curious thing happens in the air which you cannot notice in the b.a.l.l.s. You must remember that air is elastic, just as if there were springs between the atoms as in the diagram, Fig. 31, and so when any shock knocks the atoms forward, several of them can be crowded together before they push on those in front. Then, as soon as they have pa.s.sed the shock on, they rebound and begin to separate again, and so swing to and fro till they come to rest. meanwhile the second set will go through just the same movements, and will spring apart as soon as they have pa.s.sed the shock on to a third set, and so you will have one set of crowded atoms and one set of separated atoms alternately all along the line, and the same set will never be crowded two instants together.
You may see an excellent example of this in a luggage train in a railway station, when the trucks are left to b.u.mp each other till they stop. You will see three or four trucks knock together, then they will pa.s.s the shock on to the four in front, while they themselves bound back and separate as far as their chains will let them: the next four trucks will do the same, and so a kind of wave of crowded trucks pa.s.ses on to the end of the train, and they b.u.mp to and fro till the whole comes to a standstill. Try to imagine a movement like this going on in the line of air- atoms, the drum of your ear being at the end. Those which are crowded together at that end will hit on the drum of your ear and drive the membrane which covers it inwards; then instantly the wave will change, these atoms will bound back, and the membrane will recover itself again, but only to receive a second blow as the atoms are driven forwards again, and so the membrane will be driven in and out till the air has settled down.
This you see is quite different to the waves of light which moves in crests and hollows. Indeed, it is not what we usually understand by a wave at all, but a set of crowdings and partings of atoms of air which follow each other rapidly across the air.
A crowding of atoms is called a condensation, and a parting is called a rarefaction, and when we speak of the length of a wave of sound, we mean the distance between two condensations, or between two rarefactions.
Although each atom of air moves a very little way forwards and then back, yet, as a long row of atoms may be crowded together before they begin to part, a wave is often very long. When a man talks in an ordinary ba.s.s voice, he makes sound-waves from 8 to 12 feet long; a woman"s voice makes shorter waves, from 2 to 4 feet long, and consequently the tone is higher, as we shall presently explain.
And now I hope that some one is anxious to ask why, when I clap my hands, anyone behind me or at the side, can hear it as well or nearly as well as you who are in front. This is because I give a shock to the air all round my hands, and waves go out on all sides, making as it were gloves of crowdings and partings widening and widening away from the clap as circles widen on a pond. Thus the waves travel behind me, above me, and on all sides, until they hit the walls, the ceiling, and the floor of the room, and wherever you happen to be, they hit upon your ear.
Week 17
If you can picture to yourself these waves spreading out in all directions, you will easily see why sound grows fainter at the distance. Just close round my hands when I clap them, there is a small quant.i.ty of air, and so the shock I give it is very violent, but as the sound-waves spread on all sides they have more and more air to move, and so the air-atoms are shaken less violently and strike with less force on your ear.
If we can prevent the sound-wave from spreading, then the sound is not weakened. The Frenchman Biot found that a low whisper could be heard distinctly for a distance of half a mile through a tube, because the waves could not spread beyond the small column of air. But unless you speak into a small s.p.a.ce of some kind, you cannot prevent the waves going out from you in all directions.
Try and imagine that you see these waves spreading all round me now and hitting on your ears as they pa.s.s, then on the ears of those behind you, and on and on in widening globes till they reach the wall. What will happen when they get there? If the wall were thin, as a wooden part.i.tion is, they would shake it, and it again would shake the air on the other side, and so anyone in the next room would have the sound of my voice brought to their ear.
But something more will happen. In any case the sound-waves. .h.i.tting against the wall will bound back from it just as a ball bounds back when thrown against anything, and so another set of sound-waves reflected from the wall will come back across the room. If these waves come to your ear so quickly that they mix with direct waves, they help to make the sound louder in this room than you would in the open air, for the "Ha" from my mouth and a second "Ha" from the wall come to your ear so instantaneously that they make one sound. This is why you can often hear better at the far end of a church when you stand against a screen or a wall, then when you are half-way up the building nearer to the speaker, because near the wall the reflected waves strike strongly on your ear and make the sound louder.
Sometimes, when the sound comes from a great explosion, these reflected waves are so strong that they are able to break gla.s.s.
In the explosion of gunpowder in St. John"s Wood, many houses in the back streets had their windows broken; for the sound-waves bounded off at angles from the walls and struck back upon them.
Now suppose the wall were so far behind you that the reflected sound-waves only hit upon your ear after those coming straight from me had died away; then you would hear the sound twice, "Ha"
from me and "Ha" from the wall, and here you have an echo, "Ha, ha." In order for this to happen in ordinary air, you must be standing at least 56 feet away from the point from which the waves are reflected, for then the second blow will come one-tenth of a second after the first one, and that is long enough for you to feel them separately.* Miss C. A. Martineau tells a story of a dog which was terribly frightened by an echo. Thinking another dog was barking, he ran forward to meet him, and was very much astonished, when, as he came nearer the wall, the echo ceased. I myself once knew a case of this kind, and my dog, when he could find no enemy, ran back barking, till he was a certain distance off, and then the echo of course began again. He grew so furious at last that we had great difficulty in preventing him from flying at a strange man who happened to be pa.s.sing at the time.
(*Sound travels 1120 feet in a second, in air of ordinary temperature, and therefore 112 feet in the tenth of a second.
Therefore the journey of 56 feet beyond you to reach the wall and 56 feet to return, will occupy the sound-wave one-tenth of a second and separate the two sounds.)