264 Davis — Locating JVodes and Zoqps of Sound, etc. 



In order to avoid reflection from walls, and the consequent 

 formation of stationary waves, the pipe was then carried out 

 of doors, and compressed air led to it by a long rubber pipe. 

 Here, in the open air, the mill was found to rotate very rapidly 

 when near the pipe, and the rate decreased with the distance 

 from the pipe, ceasing to rotate at about 60 feet from the pipe. 

 This distance could probably be very much increased by the 

 use of more delicate apparatus, especial care being taken with 

 the pivot and needle point. This furnishes a means of study- 

 ing the decrease of intensity with distance, and, with the aid 

 of the formula developed by Lord Rayleigh,* of measuring 

 the actual amplitude of the vibration at various points in the 

 open air. 



A very sensitive sound detector of this character might be 

 made by suspending a system of very small cylinders in the 

 mouth of the resonator by means of a quartz liber, and then 

 observing the deflection by a mirror and telescope. Such an 

 instrument ought to be as sensitive as the one constructed by 

 Boys, who used a suspended disc for the same purpose.^ An 

 instrument of this kind, perhaps, might be useful in investigat- 

 ing the acoustic properties of buildings, also in the study of 

 the reflection, refraction and absorption of sound. 



Physical Laboratory, Columbia University, 

 June 15, 1901. 



* Theory of Sound, II, pp. 195-200. 

 f Boys, Nature, vol. xlii, p. 604. 



