Generally we look up at the clouds, but I remember once looking down and seeing them below me. I had climbed a high mountain, and just when I got to the top it happened that the peak was quite clear, but around it, a little lower down, a wreath of white cloud was floating. Every now and then, through a rift in the cloud, I could see the beautiful valley below, with its smiling fields and winding river, and far away there was the sea, with hundreds of green islands; all this I saw for a moment, as if through a soft thin veil, and then the cloud closed again, and shut out the view. I can quite understand travellers saying how lovely it is when they sail through the air in balloons, to get up into a clear still height, and see the "plains of clouds" below them. But there is one thing which makes voyages in balloons dangerous. The higher people go, the more thin and difficult to breathe the air becomes. One celebrated traveller, when he had got as high as seven miles in his balloon, lost his senses, and his companion was nearly frozen to death by the piercing cold. This traveller tells us that about six or seven miles above the earth no sound can reach the ear to break the perfect stillness and silence. This is because the air at this height is so thin. On the top of Mont Blanc a pistol-shot can scarcely be heard even though it is fired quite close; but if the same pistol were to be fired off in the next field you would hear it, and put your hand to your ears because the report was so loud. But what makes the report? The pistol was fired into the air, and hit nothing. It was the air which was struck, and which sent back the sound. You remember learning how light is turned back or reflected. Just as the light-waves come back again, so do the sound-waves; very quickly if the reflecting surface is near; after some time if it is far off. You know what an echo is. There is a lovely place where some children I know used often to go for a picnic. What they cared for most in Coombe Dingle was a wood which they called the "Echo wood." They would stand beside a gate, and call across the fields, and then listen. Very soon their own words, and even their own tones, were sent back to them. The waves of air carried the sounds along until they reached a pine wood which shut in the field. They struck the tall trees, and were reflected, or sent back again, almost as clearly as when first spoken. Just in this way echoes of sound are, like birds, ever on the wing: the whole air is alive with them. The walls of our rooms give back the tones of our voices, but we hear no echo, because they are so near that the repeating of the sound comes almost at the same moment as the sound itself. There are echoes on all sides of us, and no sound is ever lost. How can this be?