70 



TEE POPULAR SCIENCE MONTHLY. 



Experiment 60. — Cut out two small triangles of copper-foil or 

 tinsel, of the same size, and with wax fasten one on the end of each of 

 the prongs of a tuning-fork. Put the fork in the wooden block, and 

 set up the guide (as in experiment, Fig. 21). Prepare a strip of smoked 

 glass, and then make the fork vibrate and slide the glass under it, and 

 get two traces, one from each prong. 



Holding the glass up to the light, you will see the double trace, as 

 shown in Fig. 37= You observe that the wavy lines move apart, and 



Fig 37. 



then draw together. This shows us that the two prongs, in vibrating, 

 do not move in the same direction at the same time, but always in op- 

 posite directions. They swing toward each other, then away from each 

 other. 



Experiment 61. — What is the effect of this movement of the prongs 

 of the fork on the air ? A simple experiment will answer this question. 

 Place three lighted candles on the table at A^ B, and G (Fig. 38). 

 Hold the hands upright, with the space between the palms opposite A, 

 while the backs of the hands face the candles £ and C. Now move 

 the hands near each other, then separate them, and make these motions 

 steadily and not too quickly. You thus repeat the motions of the 

 prongs of the fork. While vibrating the hands, observe attentively 



the flames of the candles. When 

 the hands are coming nearer each 

 other, the air is forced out from 

 between them, and a puff of air is 

 driven against the flame A^ as is 

 shown by its bending away from 

 the hands. But, during the above 

 movement, the backs of the hands 

 have drawn the flames toward them, 

 as shown in Fig. 38. When the 

 hands are separating, the air rushes 

 in between them, and the flame A 

 is drawn toward the hands by this 

 motion of the air, while at the same 

 time the flames at H and C are 

 driven away from the backs of the hands. From this experiment it is 

 seen that the space between the prongs and the faces of the prongs of 

 a fork are, at the same instant, always acting oppositely on the air. 

 This will be made clearer by the study of the diagram, Fig. 39. 

 This figure supposes the student looking down on the tops of the 

 prongs of the fork. Imagine the prongs swinging away from each 



Fig. 38. 



