112 Mr. W. W. Jacques on the Effect of the Motion of the 



balls, and their velocity measured with the anemometer. The 

 estimates of density have been made from the velocity of mo- 

 tion. Now the experiments of Professor Tyndall have shown 

 that currents of air of varying density form one of the chief 

 obstacles to the propagation of sound-waves. The author, in 

 repeating these experiments in a somewhat modified form, 

 found that such currents of air not only decrease the intensity 

 of a sound-wave, as Professor Tyndall has shown, but that 

 they actually modify its form, and so give rise to great indi- 

 stinctness. The experiments were made as follows : — At A 

 was placed a source of sound, . 



A+- + - + - + -+E, 



being in some cases an organ-pipe, in others a man who spoke 

 in a clear and distinct voice, and in others various musical 

 instruments on which simple combinations of notes were 

 played. Just below the points + + &c, were placed sub- 

 stances heated to such temperatures as to give rise to currents 

 of air corresponding in density to those found in an auditorium. 

 At E was placed the ear, which, though it be not so reliable 

 an instrument as the singing-flame of Professor Tyndall for 

 estimating intensities of sounds, is, of course, the best instru- 

 ment imaginable for determining their qualities. The results 

 of the experiments were as follows : — The ear being placed at 

 E, and a small lead organ-pipe, blown with a constant pressure 

 of air, at A, the heated bodies were placed under + +, &c. 

 A very decided decrease in the intensity of sound was noticed; 

 but it was also noticed that the previously clear note lost its 

 distinctness. The pipe was removed, and a man was placed 

 at A, who spoke in clear and well-defined tones. The effect 

 was not only to decrease the intensity of his voice, but to make 

 it slightly confused and indistinct, as if each syllable were re- 

 peated several times in very close succession. When a flute 

 was substituted for the voice the effect was the same. The 

 effect on a violin seemed to be considerably less. With a drum 

 no effect whatever was observed. The effect seemed to be 

 most marked on the man's voice, or a musical instrument in 

 which the overtones were comparatively small. The explana- 

 tion of this is very simple. The original ray of sound, striking 

 upon the first current of air, is partially reflected and partially 

 transmitted. The loss of the reflected portion causes a de- 

 crease in the intensity of sound. The transmitted portion, 

 striking upon a second current, is likewise divided, and its 

 transmitted portion continues to be so divided as many times 

 as there are variations in the density of the air. Its reflected 

 portion, as well as that of all the succeeding reflections; instead 



