EESEARCIIES IN SOUND. 475 



siren was sounded with diiferent velocities of rotation of the perforated 

 disk, the pressure of steam remaining at 90 pounds per square inch. 

 The effect upon the artificial ear in causing greater or less agitation of 

 sand was taken as the indication of the penetrating x^ower of the difter- 

 eut tones. The number of revolutions of the disk in a gi^en time was 

 determined by a counting apparatus, consisting of a train of wheels and 

 a series of dials showing tens, hundreds, and thousands of revolutions ; 

 this was temporarily attached to the projecting end of the spindle of 

 the revolving disk by pushing the projecting axis of the instrument into 

 a hole in the end of the spindle. 



From the whole of this series of experiments it appeared that a revolu- 

 tion which gave 400 impulses in a second was the best with the siren 

 when furnished with a trumpet. On reflection however it was con- 

 cluded that this result might not be entirely due to the i^itch, but in 

 part to the perfect unison of that number of impulses of the siren with 

 the natural tone of the trumpet. To obviate this complication, a series 

 of experiments was next day made on the penetration 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 artificial ear be- 

 ing 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 3G0 impulses 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 funda- 

 mental 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 between the bell 

 and the orifice through which the steam was ejected. 



The result of this experiment indicated nothing of a definite charac- 

 ter, other than that with a given pressure there is a maximum effect 

 in^oduced when the vibrations of the sheet of air issuing from the circu- 

 lar orifice are in unison with the natural vibrations from the cavity of 

 the bell, a condition which can only be determined in any case by actual 

 experiment. In practice, JMr. 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, 

 in order that it might be employed for determining the relative power 

 while the observer was but a few yards from the machine. 



These experiments have been made in an apartment of less than 80 

 feet in length, in which the sounding apparatus was placed at one end 



