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this is so you may prove for yourself by the following experiment:

EXPERIMENT 44.-Being careful not to move the glass ,plate from its present position (Experiment 43), stick it with wax to the tumbler. Pour a little silica into the tumbler, and then hold it horizontally, and vibrate the fork near its opening, observing attentively how the silica powder is acted on by the inclosed vibrating air.

EXPERIMENT 45.-Take a piece of thin linen paper about 4 inches square, and having wetted it paste it over the mouth of the tumbler. When the paper has dried it will be stretched tightly. Take a sharp penknife and carefully cut away the paper so as to make an opening as shown at B in Fig. 27. Make this opening small at first, and very gradually make it larger and larger. Hold the fork over the opening after each increase in its size, and you will soon discover the size of the opening which causes the air inclosed in the tumbler to vibrate with the fork, and thus greatly to strengthen its sound. You have now a mass of air in tune with the fork, and inclosed in a vessel which has one of its walls formed of a piece of elastic paper. With this instrument, which I have invented for you, you must make some charming experiments.

EXPERIMENT 46.-If the air in the tumbler vibrates to the A-fork, it will, of course, vibrate to the A-pipe, which gives the same note as the fork. Scatter some sand on the paper, and then sound the A-pipe a foot or two from it. The sand dances vigorously about, and ends by arranging itself in a nodal line parallel to the edges of the paper, in the form of a U with its two horns united by a straight line. The vibrations of the pipe can only reach the tumbler by going through the air, and, as the sand vibrates when the tumbler is placed in any position

about the pipe, it follows that the air all around the pipe vibrates while the pipe is sounding.

EXPERIMENT 47.-Sprinkle a small quantity of sand on the paper, and then, placing a thin book under the tumbler, so incline it that the sand just does not run down the paper, as shown in B, Fig. 27. Now go to the farthest end of the room and blow the pipe in gentle toots, each about one second long. At each toot, your friend, standing near the tumbler, will see the sand make a short march down the paper; and soon by a series of marches it makes its way to the edge of the paper and falls into the tumbler. I have, in a large room, gone to the distance of 60 feet (18.28 metres), and the experiment worked as I have just described it.

EXPERIMENT 48.-Again arrange the experiment as in Experiment 47, and standing 3 or 4 feet from the tumbler try how feeble a sound will vibrate the paper. If every part of the experiment is in good adjustment, you will find that the feeblest toot you can make will set the sand marching. To keep it at rest you must keep silent.

EXPERIMENT 49.-To show these experiments on a greatly magnified scale, place the tumbler in front of the heliostat (see "Light," page 79) so that the sun's rays just graze along the inclined surface of the paper. Cut off a piece of a match inch long, and split this little bit into four parts. Place one of these on the inclined paper. Of course, the image of the tumbler is inverted, so the bit of wood appears to adhere to the lower side of the paper. If a little paper mouse cut out of smooth paper is used in place of the bit of wood, it is really amusing to see the mouse make a start to every toot of the pipe. I trust my reader will not think me unscientific for making a little fun. Singing the note A, instead of sounding it

on the pipe, produces the same effects in the above experiments.

EXPERIMENT 50.-If you sing or sound some other note than the A, you will find it powerless to move the sand over the tumbler.

EXPERIMENT 51.-The experiments just made with the tumbler, partly covered with the glass plate or stretched paper, may be modified in a way that makes one of the most beautiful and instructive experiments.

Take a pint bottle half filled with distilled or rain water, and put into it one ounce of shavings of white castile soap; then shake the bottle. If the soap does not all dissolve, add more water till you have a clear solution. Then add a gill of glycerine, shake, and allow to settle. This solution is the best for making soap-bubbles.

Pour out the soap-solution into a basin; then dip the mouth of a deep tumbler (one 5 or 6 inches deep is the best) into it. The glass plate is now slid through the soapwater under the mouth of the tumbler. Take the tumbler, with the glass on it, out of the basin and stand it erect on the table. Vibrate the A-fork, and hold it over the edge of the tumbler while you slide the glass plate across its mouth, as we did in our other experiments. The opening which is thus made, between the rim of the tumbler and the edge of the glass plate, will have a soapfilm over it. Adjust the size of this opening till it tunes the air in the tumbler to vibrate to the fork. When this takes place, a loud sound issues from the tumbler, and the delicate soap-bubble is violently agitated; its surface is chased and crinkled in so complicated a manner that its appearance cannot be described.

This experiment succeeds best with a very deep tumbler, like the one we have used, and with a C-fork and

pipe. The soap-film covers nearly half of the mouth of the tumbler when the latter is in tune to the C-fork.

To see well the vibrating surface of soap-film, you must reflect from it the light of the sky.

EXPERIMENT 52.-By the aid of the heliostat and a lens the experiment may be made one of great beauty. With some wax stick the glass plate to the tumbler, so that the soap-film may be placed upright and inclined to the beam of light coming from the heliostat. With a planoconvex lens placed between the film and the screen obtain a magnified image of the soap-film (see "Light," page 79).

As the soap-film is upright it drains thinner and thinner, while the image of the film grows more and more brilliant. Magnificent bands of reddish and bluish light appear, and stretch across the screen. Now sound the

fork or pipe near the film. The vibrations bend and undulate the colored bands, and the colors chase each other over the screen like waves on a troubled sea. On the sound ceasing, the bands straighten, and a comparative calm spreads over the screen.

EXPERIMENTS WITH THE SENSITIVE-FLAMES OF GOVI AND BARRY, AND OF GEYER.

EXPERIMENT 53.-In Fig. 28, A is an upright wooden rod nailed to a block D. At B is a piece of stout wire bent in the form of a ring, 5 inches (12.7 centimetres) in diameter, and then bent at a right angle and stuck in the upright rod. On the ring is laid a piece of wire gauze that has about 30 meshes to the inch. E is a glass tube joined to a rubber tube that leads to the nearest gasburner. To make this glass tube or jet, take a piece of glass tube, about inch outside diameter and 6 inches

(15.2 centimetres) long, and, holding its ends in the hands, heat the tube, at about 1 inch from its end, in a spiritflame or the flame of a Bunsen burner till it softens ; then pull it out till it is reduced about one-quarter in diameter. When it is cold, draw the edge of a file across this narrow part, and snap the tube asunder. Now heat in like manner the middle of this tube, and bend it into a

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right angle, as shown in Fig. 28, and, with wax, stick it upright on a block of wood, with the tip of the jet about 2 inches (5.1 centimetres) below the wire gauze.

Turn on the gas and light it above the gauze, where it will burn in a slender, conical flame, about 4 inches high, with its top yellow and its base blue. This forms the "sensitive-flame" invented by Prof. Govi of Turin, and afterward by Mr. Barry of Ireland.

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