If you hiss, whistle, shake a bunch of keys, or clap the hands, the flame at once roars, and, shrinking down to the gauze, becomes entirely blue and almost invisible. It is called a "sensitive-flame," because it is sensitive to sonorous vibrations, and shows us their existence in the air.

EXPERIMENT 54.—Mr. Geyer, of the Stevens Institute of Technology, has made an addition to the Govi-Barry flame, which heightens its sensitiveness, and makes it utter a musical note while disturbed by vibrations; while, in another modification of the experiment, the flame sings continuously, except when agitated by external sounds. I give his experiments in his own words:

It

"To produce them it is only necessary to cover Barry's flame with a moderately large tube [see Fig. 28, in which, however, the tube is represented of somewhat too great a diameter], resting it loosely on the gauze. A luminous flame, 6 or 8 inches long, is thus obtained, which is very sensitive to high and sharp sounds. If, now, the gauze and tube be raised, the flame gradually shortens, and appears less luminous, until at last it becomes violently agitated, and sings with a loud, uniform tone, which may be maintained for any length of time. Under these conditions, external sounds have no effect upon it. The sensitive musical flame is produced by lowering the gauze until the singing just ceases. is in this position that the flame is most remarkable. At the slightest sharp sound, it instantly sings, continuing to do so as long as the disturbing cause exists, but stopping at once with it. So quick are the responses that, by rapping the time of a tune, or whistling or playing it, provided the tones are high enough, the flame faithfully sounds at every note. By slightly raising or lowering the jet, the flame can be made more or less sensitive, so that a hiss in any part of the room, the rattling of keys even in the pocket, turning on the water at the hydrant, folding up a piece of paper, or even moving the hand over the table, will excite

the sound. On pronouncing the word 'sensitive,' it sings twice; and, in general, it will interrupt the speaker at almost every 's,' or other hissing sound.

"The tube chiefly determines the pitch of the note, shorter or longer ones producing, of course, higher or lower tones respectively. I have most frequently used either a glass tube, 12 inches long and 11 inch in diameter, or a brass one of the same dimensions. Out of several rough pieces of gas-pipe, no one failed to give a more or less agreeable sound. Among these gas-pipes was one as short as 7 inches, with a diameter of 1 inch; while another was 2 feet long and 14 inch in diameter. A third gas-pipe, 15 inches long and 4 inch in diameter, gave, when set for a continuous sound, quite a low and mellow tone.

"If the jet be moved slightly aside, so that the flame just grazes the side of the tube, a note somewhat lower than the fundamental one of the tube is produced. This sound is stopped by external noises, but goes on again when left undisturbed. All these experiments can be made under the ordinary pressure of street-gas,inch of water being sufficient."

CHAPTER VII.

ON THE VELOCITY OF TRANSMISSION OF SONOROUS VIBRATIONS, AND ON THE MANNER IN WHICH THEY ARE PROPAGATED THROUGH ELASTIC BODIES.

ON THE SPEED WITH WHICH SONOROUS VIBRATIONS TRAVEL.

WHEN in the country, you have seen a man chopping wood. If you stood near him, you observed that the blow and the sound of his ax came together. If you moved away from him, you may have noticed that, while you could see his ax fall, and hear the sound of the blow, the sound seemed to follow the blow. When you moved away several hundred feet, the interval of time separating the sight of the blow and its sound was readily noted. You may also have observed that some time passed between the flash of a gun or the puff of a steam-whistle and the report of the gun and the sound of the whistle. These things convince us that sonorous vibrations take time to move through the air.

This matter has been carefully examined by scientific men, and they have found that sound-vibrations move through the air at the rate of 1,090 feet (332.23 metres) in one second. This is the velocity of sound when the temperature is just at freezing, or at 32° Fahrenheit. For each degree above this, sound gains in speed one foot more.

For instance, upon a summer's day, the thermometer may stand at 80°. This is 48° above 32°, and the sound gains 48 feet, so that it moves at the rate of 1,138 feet a second at this temperature.

The velocity of sonorous vibrations in oxygen gas at 32° is 1,040 feet per second; in hydrogen gas it is 4,160 feet, just 4 times as great. As a cubic foot of hydrogen weighs 16 times less than a cubic foot of oxygen, and as 4 is the square root of 16, it follows that the speed of sonorous vibrations in gases varies inversely as the square roots of the weights of equal volumes of the gases.

Sonorous vibrations travel through water at the speed of 5,000 feet per second, and through iron at about 16,000 feet in a second.

EXPERIMENTS WITH GLASS BALLS ON A CURVED RAILWAY, SHOWING HOW VIBRATIONS TRAVEL THROUGH ELASTIC BODIES.

EXPERIMENT 55.-Fig. 29 represents a wooden railway about 6 feet (183 centimetres) long. It may be made of pine strips, 1 inch (3.8 centimetres) wide and inch (6 millimetres) thick, laid side by side about 1 inch

(25 millimetres) apart, and joined together by short crossstrips nailed on them. bles at the toy-shop.

Get six or seven large glass mar-
These are intended to roll be-

tween the two strips, just as balls roll in the railway of a bowling-alley. Place the railway on a table or board, and fasten it down at the middle with a screw in the cross-strip, and then raise each end and put a book or wooden block under it, as in Fig. 29.

Place the balls in the middle of the curving railway, and then bring one to the end and let it roll down against the others. Immediately the last ball will fly out and roll part way up the incline toward the other end of the rail

way. The first ball will come to rest beside the others, and the ball which has been shot up the railway will roll back against those at rest, and the same performance will be repeated till the motion has gone from the rolling balls. Let us examine this matter, and see what happens to these balls on the railway. First, you must observe that the balls are elastic, for experiment will show that they will bound like rubber balls when let fall on the hearth-stone.

EXPERIMENT 56.—To show that the ball is elastic, and flattens when it strikes the stone, make the following experiment Mix some oil with a little red-lead, or other colored powder, and smear it over a flat stone, like a flag