396 WRITINGS OF JOSEPH HENRY. fl874 



tion of different pitches with the siren alone, the trumpet 

 being removed. The result was as follows: 



The siren was sounded at five different pitches, the arti- 

 ficial ear being at such a distance as to be near the limit of 

 disturbance by the sound. In this condition the lowest 

 pitch gave no motion of sand. A little higher, slight motion 

 of sand. Still higher, considerable motion of sand ; and 

 with a higher pitch again, no motion of sand. The best 

 result obtained was with a revolution which gave 360 im- 

 pulses in a second. 



3. An attempt was made to determine the most effective 

 pitch or tone of the steam whistle. It was started with 

 what appeared to be the fundamental note of the bell, which 

 gave slight motion of sand ; a higher tone a better motion; 

 still higher, sand briskly agitated ; next, several tones lower, 

 no motion ; higher, no motion ; still higher, no motion. The 

 variation in the tone was made by altering the distance be- 

 tween the bell and the orifice through which the steam was 

 ejected. 



The result of this experiment indicated nothing of a 

 definite character other than that with a given pressure 

 there is a maximum effect produced when the vibrations of 

 the sheet of air issuing from the circular orifice are in unison 

 with the natural vibrations from the cavity of the bell, a 

 condition which can be determined in any case only by 

 actual experiment. In practice, Mr. Brown was enabled to 

 produce the best effect by regulating the velocity until the 

 trumpet gave the greatest penetrating power, as indicated by 

 an artificial ear of little sensibility ; which latter was adopted 

 in order that it might be employed for determining the rela- 

 tive power while the observer was but a few yards from the 

 machine. These experiments have been made in an apart- 

 ment of less than 80 feet in length, in which the sounding 

 apparatus was placed at one end, and the artificial ear at the 

 other, substituting fine shot in the place of sand. 



The experiments with the siren however, indicate the fact 

 that neither the highest nor the lowest pitch of an instru- 

 ment gives the greatest penetrating power, but one of a 

 medium character. 



