PIERCE. — A METHOD OF MEASURING THE INTENSITY OF SOUND. 391 



evidence, because of the possible actual disturbance of the stationary 

 system by the reversal of the telephone, yet the result seems highly 

 probable on account of its agreement with the familiar fact that the 

 maximum of motion of the air column of a tubular resonator is outside 

 the end of the resonator. The sound-collecting cone of the present 

 apparatus is a resonator for the pitch employed — in fact, the particu- 

 lar pitch was selected by a preliminary experiment which showed that 

 the air column of this cone was in resonance with the pitch — and this 

 resonant air column, according to deductions from the above experi- 

 ment, is thrown into most active vibration when a region just outside 

 (5.7 cm.) the opening of the cone is coincident with a region of large 

 displacement. 



This result enables us to locate the actual position of the nodes and 

 loops of Curve 4, Figure 8. Each point of the curve belongs to a 

 region of space 5.7 cm. nearer to the wall than the corresponding 

 abscissa ; therefore, the first maximum of motion, which was obtained 

 with the opening of the cone 5 cm. from the wall, is really .7 cm. 

 behind the wall, — that is to say, pi-actically coincident with the wall. 



In order to examine the distribution of sound intensity in the 

 neighborhood of another portion of the w?,ll of the room, the telephone 

 receiver and its track were placed at 2mii Figure 7, and the galvan- 

 ometer readings were taken with the opening of the cone turned 

 toward the wall and placed at various distances from the wall. Curve 

 6 of Figure 8 was obtained as representative of the distribution at this 

 position. Here again the corrected position of the first maximum is 

 practically coincident with the wall. The interference system in this 

 locality is much more irregular than in position 1, and the maxima 

 with the exception of the maximum at 90 cm. are less intense than 

 those at position 1. This is interesting when we note the fact that 

 the distance of the position 2 from the source of sound is only one 

 half as great as the distance of position 1. For hearing this particu- 

 lar note the position at the back of the room is more favorable than 

 the much nearer position at the side of the room, notwithstanding the 

 fact that the side position was directly in front of the lip of the pipe 

 and was unobscured by intervening objects, while a line running from 

 the source of sound to the position in the rear of the room passed 

 immediately over the backs of numerous benches with which the room 

 was furnished. 



At a third position in the room, position 3, Figure 7, an interval 

 of 100 cm. was investigated. The results obtained are shown in 

 Curve 7, Figure 8. These distances (abscissae) are measured from 

 an arbitrary origin. The opening of the cone of the telephone was 



