Page:Popular Science Monthly Volume 14.djvu/79

Rh 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 sixty 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 three or four 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 one-eighth of an 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 at every toot of the pipe.



We make a long selection from Chapter VIII., which treats of the "Interferences of Sonorous Vibrations, and of the Beats of Sound," in order to set forth the manner in which the author has knit together his simple experiments.