292 COMPOSITION OF SOUND 



positions of full sound and four positions of perfect silence will be 

 found. When a side of the fork is parallel to the ear, the sound 

 is plainly audible ; when a corner of a prong is turned toward the 

 ear, the waves from one prong completely destroy the waves started 

 by the other. The interference is complete. 



(c.) Over a resonant jar, as shown in Fig. 226, slowly turn a 

 vibrating tuning-fork. In four positions of the fork we have loud, 

 resonant tones ; in four other positions we have complete inter- 

 ference. If, while the fork is in one of these positions of inter- 

 ference, a pasteboard tube be placed around one of the vibrating 

 prongs, a resonant tone is instantly heard ; the cause of the inter- 

 ference has been removed. 



4:52. Beats. If two tuning-forks, A and B, vibrating 

 respectively 255 and 256 times a second, be set in vibration 

 at the same time, their first waves will meet in like phases 

 and the result will be an intensity of sound greater than 

 that of either. After half a second, B having gained half 

 a vibration upon J, the waves will meet in opposite phases 

 and the sound will be weakened or destroyed. At the end 

 of the second we shall have another reinforcement ; at the 

 middle of the next second another interference. This 

 peculiar palpitating effect is due to a succession 

 of reinforcements and interferences, and is called 

 a beat. The number of beats per second equals the dif- 

 ference of the two numbers of vibrations. 



(a.) In a quiet room, strike simultaneously one of the lower white 

 keys of a piano and the adjoining black key. The beats will be 

 heard. 



(6.) If the two tuning-forks described in 443, one being loaded 

 as there mentioned, be simultaneously sounded, the beats will be 

 very perceptible. Replacing the 3-cent piece success] vely by a silver 

 half-dime and a dime, the number of beats will be successively 

 increased. 



(c.) If two large organ pipes, having exactly the same tone, be 

 simultaneously sounded, a low, loud, uniform sound will be pro- 

 duced. If an aperture be made in the upper part of one of the 

 walls of one of the pipes and closed by a movable plate, the tone 



