﻿of just audible Aerial Waves. 3G7 



difficulty in converting energy into sound upon a small seal*', 

 and thus in reducing the distance of audibility to such a 

 figure as 30 metres. Under these circumstances the obser- 

 vations are much more manageable than when the operators 

 are separated by half a mile, and there is no reason to f ear 

 disturbance from atmospheric refraction. 



The fork is mounted upon a stand to which is also firmly 

 attached the observing-microscope. Suitable points of light 

 are obtained from starch grains, and the line of light into 

 which each point is extended by the vibration is determined 

 with the aid of an eyepiece-micrometer. Each division of 

 the micrometer-scale represents '001 centim. The resonator, 

 when in use, is situated in the position of maximum effect, 

 with its mouth Uuder the free ends of the vibrating prongs. 



The course of an experiment was as follows : — In the first 

 place the ratei of dying down were observed, with and with- 

 out the resonator, the stand being situated upon the grouid 

 in the middle of a lawn. The fork was set in vibration with 

 a bow, and the time required for the double amplitude to fall 

 to half its original value was determined. Thus in the case 

 of a fork of frequency 256, the time during which the vibra- 

 tion fell from 20 micrometer-divisions to ."'O micrometer- 

 divisions was 16 s without the resonator, and 9 s when the 

 resonator was in position. These times of halving were, as 

 far as could be observed, independent of the initial amplitude. 

 To determine the minimum audible, one observer (myself) 

 took up a position 30 yards (27 "4 metres) from the fork, and 

 a second (Mr. Gordon) communicated a large vibration to the 

 fork. At the moment when the double amplitude measured 

 20 micrometer-divis ; ons the second observer gave a signal, 

 and immediately afterwards withdrew to a distance. The 

 business of the first observer was to estimate for how many 

 seconds after the signal the sound still remained audible. In 

 the case referred to the time was 12 s . When the distance 

 was reduced to 15 yards (13*7 metres), an initial double 

 amplitude of 10 micrometer-divisions was audible for almost 

 exactly the same time. 



These estimates of audibility are not made without some 

 difficulty. There are usually 2 or 3 seconds during which 

 the observer is in doubt whether he hears or only imagines, 

 and different individuals decide the quest : on in opposite ways. 

 There is also of course room for a real difference of hearing, 

 but this has not obtruded itself much. A given observer on 

 a given day will often agree with himself surprisingly well, 

 but the accuracy thus suggested is, 1 think, illusory. Much 

 depends upon freedom from disturbing noises. The wind 



