end. So, too, when the wind is blowing, if you hold one ear firmly against the side of a wooden telegraph pole, you will hear sounds like those of a great Æolian harp, that are caused by the wind striking the telegraph wires.

The string of a kite can also transmit sounds. Major Abbot describes a form of musical kite that is common in certain parts of Central Asia. This kite is easily made, and I would advise you to surprise your friends with one. The kite consists of a square, formed by two diagonal sticks firmly connected at right angles, and the edges connected by means of a tight string, thus forming the four sides of a square. A piece of paper is then pasted over the surface in the usual manner. The string for raising the kite is attached to a loose string, the ends of which are tied firmly to the ends of the upright diagonal. A tail, of the usual form, is then attached to the lower extremity of the kite, then one of the cross-sticks, say the horizontal one, is bent back like the string of a bow by means of a piece of catgut. This piece of catgut must be made so tight as to produce a musical sound when struck by the wind. When such a kite is raised, the musical note produced by the piece of catgut is transmitted to the ground by the string employed for raising the kite. In order to strengthen the tone of the catgut, the end of the string employed for raising the kite must be firmly attached to the top of an empty starch- or soap-box that has been securely fixed to the ground.

Sound is readily transmitted through the ground. Indians, and woodsmen generally, can often hear the approach of an animal, or an enemy, by listening with an ear close to the ground.

A medium is necessary for the production and transmission of sounds. This medium is ordinarily the air, so that, in the absence of air, all ordinary sounds would instantly cease. The natural philosopher, Hawksbee, in an experiment made before the Royal Society of London, proved that all sounds disappear in a vacuum. He suspended a bell

inside a glass jar placed on the plate of an air-pump, so that, while the jar was being exhausted, the bell would be sounded. As the air was gradually removed from the glass, the sounds produced became more and more feeble until, when nearly all the air was drawn from the glass, the sounds almost entirely ceased.

Various forms of apparatus have been devised for the purpose of repeating Hawksbee's experiment. In one form the bell mechanism is hung on strings, so that some of the vibrations would be communicated to the outside.

A very simple observation will convince you that time is required for the transmission of sound waves. When you are standing near a person who is talking to you, so very short a time is required for the sound waves to pass from the lips of the speaker to your ears that the transmission may seem to be instantaneous; but if you are at a considerable distance from the place where sound waves start, as, for example, when you see a man at a distance strike a blow with a hammer, you will see the blow struck some time before you hear the sound of the blow. A still better example is the firing of a cannon, for you can see the flash long before you hear the report.

Sound waves travel through air when at a temperature of melting ice, with a speed of about 1,090 feet a second. As the temperature increases, the velocity of sound slightly increases. The velocity of sound in liquids is much greater than in air, the velocity in water being about four and a half times that in air. The velocity of sound in most solids is greater than in water.

There is a matter connected with the reflection of sound waves that I will endeavor to explain to you. It is much easier to speak to an audience in a room than it is to speak in the open air; for, in the room, the sound waves coming directly from your mouth reach the ears of the audience at almost the same time that the reflective waves do from the walls and the ceiling. When the distance between the

walls reflecting the sound and the position occupied by the speaker is not too great, these reflected sounds, reaching the ear of the audience only slightly after the direct sounds, tend to prolong and strengthen them. In other words, the speaker's voice is said to be strengthened by resonance.

You may, possibly, have been surprised when you held a hollow shell to your ear to hear murmuring sounds not unlike those produced by the waves beating on the shores of the ocean. This is not due, as some ignorant people assert, to imprisoned sound waves, but to the strengthening of feeble noises in the atmosphere by the resonance of the shell.

CHAPTER VII

ECHOES

You have undoubtedly heard echoes, and been both pleased and amused at the accurate way with which they repeat sounds. As you know, these sounds may be either spoken words, or the words of a song, or of some musical instrument.

An echo never originates sound. It only repeats what it hears. Moreover, it is only able to make one hear the last portions of these sounds.

The ancients believed that echoes were produced by a nymph named Echo, a daughter of Air and Earth. At one time they declared poor Echo could talk like other people. Indeed, she was so fond of talking that she got into trouble, for she was employed by Jupiter to keep talking to his wife, Juno, so as to prevent her from watching him too closely. Juno, discovering what Echo was trying to do, punished her by forcing her, whether she wished to or not, to repeat the last few syllables of anything that had been uttered in her presence. She could not, however, speak anything else. You can understand how great a punishment this must have been to a young lady who was especially fond of hearing herself talk.

I am sure you will be surprised at some of the wonders of the atmosphere that are caused by echoes. Let us, therefore, endeavor to understand just how echoes are produced.

An echo in sound is what an image is in light. If a lighted candle be placed in front of a plane mirror you will see an image of the candle as far back of the mirror as the candle is in front of it. When the rays of light, coming from the

candle, strike the surface of the mirror, they are flung back, or reflected, and entering the eyes of the observer, produce an image that appears to be back of the glass.

In the same manner, if you stand in front of a good reflecting surface of fairly considerable size, such as a high, perpendicular cliff, and speak, shout, or sing, the last few syllables will be repeated, and will appear to come from some point back of the cliff. This sound will be similar to the sound you made; that is, will be the same words of the song, the same notes of the musical instrument, or the same words that were spoken. This is because the sound waves that strike the surface of the cliff are reflected, or flung back, and entering your ears, produce an echo.

That sound is capable of being reflected like light, can be shown in a variety of ways. Professor Tyndall showed the

The position of this focus was seen in a bright cone of light received on a cloud of smoke in front of the mirror. In this way the position of the focus was readily discovered.

Then, remembering the position of the focus, let us suppose it at W, in Fig. 26, if a person stood in front of the mirror with one of his ears at this focus, he would have no difficulty in hearing the ticking of a watch placed as shown