"I F there were no air around us," Uncle Paul went on, "the bell, the wine-glass, and the violin-string would vibrate to no purpose; we should hear nothing, there being no sound for us to hear. Silence would reign even where now is deafening uproar; the lightning's flash would be followed by no thunderclap. Do you ask why? Because in the absence of air sound-waves would no longer be possible and the ear would receive nothing to convey the sense of sound. If you wish for a proof that silence follows when air is removed, I will give you one.
"In the middle of a hollow glass globe of considerable size is hung a small bell. By means of a stop-cock communication with the interior of the glass receptacle may be suspended or reëstablished at will. At first it is full of air, as is naturally the case with any vessel that we call empty. If we shake it so as to disturb the bell, we hear the latter ring, the enclosed air transmitting the sound-waves beyond the confines of the glass chamber; and this is true even though the stop-cock be closed, because the undulatory movement of the inner air is transmitted through the glass and into the outer air.
"Now let us exhaust the air within by means of an air-pump. To describe this pump would take us too far out of our way and would involve details too difficult for your comprehension. Let us pass on, then, and suppose the glass globe emptied of all the air it at first contained, and the stop-cock closed to prevent the entrance of any air from without.
"In that condition the globe may be shaken as much as we choose and, though we see the bell swinging and its clapper beating the metal, there comes to our ears no sound whatever. The bell does actually move and is struck by the tongue—that is indisputable—but its vibrations are mute because they start no sound-waves for lack of air. Now we open the stop-cock, and the outer air rushes in and fills the globe, whereupon the bell is heard as in the beginning. The fact is established that with air there is sound; without air, silence.
"Let us return for a moment to the concentric circles formed on the surface of a sheet of water by the falling of a stone. These circles, these waves, are seen to make their way far out from the center, and their rate of progress is slow enough to admit of being followed by the eye. If we wished, we could, with a little care and by the help of a watch having a second-hand, ascertain the time consumed by one of these waves in going from any given point to any other, and thus the distance covered in one second. Sound-waves are transmitted in a similar manner, and so we say that sound travels. How far does it travel in a second? That would be something worth finding out. Let us look into the matter a little.
"You are standing, we will say, in full view of a belfry some distance off. You see the bell swing and even make out the clapper as it strikes the metal. When it strikes you know that it makes a sound, and yet you hear nothing immediately. The sound reaches you a little later, when the bell is swinging back for another blow from the clapper. How is it that the eye sees before the ear hears?
"The message to the eye is transmitted by light, that to the ear by sound-waves. Now, light travels with inconceivable speed, and for it the greatest terrestrial distances are nothing. Its rapidity of movement is comparable with that of electricity along a wire. We see, then, the clapper striking the bell at the very instant of the concussion, for light no sooner starts on its journey than it arrives at its destination.
"But with sound-waves it is a very different matter. They travel faster than the circular ripples on water, but still they take an appreciable length of time in reaching us. Hence they are behindhand from the moment the clapper strikes the bell, and they fall more and more behind the farther they have to go.
"You watch from a distance a hunter about to shoot a bird. All attention, you wait for the discharge with the lively interest felt by those of your age in the fall of the tiniest chaffinch from its perch. The shot is fired, you see both the flash and the smoke, but the report does not reach you until a little later. The explanation is the same as in the case of the bell. Light, moving with unparalleled rapidity, shows us the flash and the smoke as soon as the discharge takes place, whereas the sound-waves, which are relatively slow of movement, bring us only later the report of the huntsman's fowling-piece.
"Let us suppose exactly one second to intervene between our seeing the flash caused by the explosion of the gunpowder and our hearing the report. Then, if our distance from the hunter be carefully measured, it will be found to be three hundred and forty meters. Accordingly, it has taken sound one second to travel that distance. By similar experiments it has been ascertained that all sound, whether loud or subdued, shrill or the reverse, caused in one way or in another, travels through the air at the same invariable rate of speed, three hundred and forty meters per second.
"At this point it may be interesting to make a practical application of what we have just learned. Suppose we wish to know how far away are the lightning and thunder we have just seen and heard. If the flash of lightning and the peal of thunder reach us at the same time, the electric discharge was very near, since the sound, despite its relatively low rate of speed, shows no such tardiness in reaching us as it would have done had it come a long distance. But usually the thunder follows the lightning after an appreciable interval, indicating that the electric discharge is not very near.
"To ascertain just how near or how far away the lightning is, we simply have to know how many seconds elapse between the flash and the first thunderclap. If we have no watch by which to count the seconds—as, if we are school-children, it is more than likely we shall not have—let us simply count, 'One, two, three, four,' and so on, without haste, but also without lagging. In that way we shall be able to measure the time accurately enough.
"Now, then, attention! Watch the storm-cloud yonder. There comes a flash of lightning, and we count: one, two, three, and so on up to twelve. Ha! the thunder at last. Twelve seconds passed between flash and report, and so it must have taken that length of time for the sound to reach us. The point where the discharge took place is therefore distant from us twelve times three hundred and forty meters, or about four kilometers. What do you say? Isn't that simple enough? How easy it is with a little intelligence, to solve problems that at first seemed extremely perplexing! To find out how far away the thunder is we only have to count one, two, three, and so on.
"The circular waves on the water have something else very interesting to tell us. Let us go back to them once more. The sheet of water, an enclosed basin let us say, is bounded by a wall that serves to hold the water in. Watch closely what takes place where water and wall come in contact. The ripples produced by the fall of a stone make their way toward the wall in ever-widening circles, and reach the wall one after another. Then, as soon as they touch the wall, they start back again; always preserving their circular form and their fixed distance from one another, they move now in the opposite direction.
"Thus the surface of the water becomes ruffled with two series of waves, one moving toward the wall, the other away from it; and these waves, traveling in contrary directions, meet and pass one another with no confusion or hindrance whatever. To designate this change of direction we say that the waves are reflected by the wall. Those moving toward it are direct waves; those returning from it are reflected waves.
"Confronted by a wall, a rock, or any other obstacle barring their passage, sound-waves behave exactly like water-waves: they are reflected by the obstacle and go back in the opposite direction. Hence it is that we may hear the same sound twice in the same place, with an interval between the first and second hearing. The first is caused by direct waves, the second by reflected waves. When the sound reaches us the second time, we call it an echo.
"In future when you hear your own voice from a distance as if some mischievous sprite were mocking you, you will know that the repetition is produced by some opposing object, some wall or rock or other obstruction, that sends back the sound-waves to you by reflecting them. You first received the sound-waves by your voice directly, and then you received the sound-waves reflected to you by the obstructing object."