EXPERIMENTS IN SOUND. 



69 



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 vi- 

 brates while the pipe is sounding. 



Experiment ^7. — 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. 



Mcperiment Jf.8. — 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 Jf9. — To show these experiments on a greatly magni- 

 fied 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. 



Fi&. 21. 



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. 



