Page:Popular Science Monthly Volume 14.djvu/78

68 much louder than any you have ever before heard from the fork alone. This is because the air in the tumbler is set in vibration, and adds the vibrations of its mass to those of the fork. That 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 four and a half 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