Sound through Narrow Slits. 159 



length was often observed, but the ceiling was somewhat 

 curved and there were o£ course other obstacles in the room. 

 Not much better success was attained out of doors, the 

 apparatus being placed upon a lawn and the slit facing 

 upwards. Here again the horizontal position of the terminal 

 disks should have eliminated reflexions from obvious obstacles, 

 but alternations in the period of the full wave-length were 

 usually apparent. The question would sometimes suggest 

 itself whether visible obstacles were necessary at all to cause 

 reflexions. In fog-signalling, echos, sometimes up to 30 

 seconds' duration, have been observed when the sea was 

 smooth and there was no visible cause of reflexion. But 

 these are usually attributed to a streaky condition of the air, 

 a cause which could scarcely have operated in evening 

 experiments over a lawn. That reflected waves, arriving 

 more or less horizontally, were still concerned is suggested by 

 the observation that the anomalous alternations were intensified 

 by holding the terminal disks nearly vertical so as to attenuate 

 the direct sound. Possibly there was diffuse reflexion from 

 the grass. Whatever the cause of disturbance may have 

 been, it rendered hopeless any attempt to compensate width 

 of slit by alteration of distance, as had been the original 

 intention. Indeed it would hardly have been worth while to 

 describe at so much length the difficulties encountered, were 

 it not that they may probably embarrass other observers 

 unprepared for them, and that the terminal disk apparatus 

 has an independent interest. 



The best that I have been able to do is by altering the 

 length of the slit so as to compensate variations of width. 

 For this purpose a sliding plate was provided cutting off equal 

 lengths from the two ends. Observations have been made 

 both in the laboratory and outside upon the lawn. In both 

 cases the slit faced upwards, and the sound (/ v ) was observed 

 by ear through a hearing-tube of rubber, provided at the 

 further end with the disk-apparatus already described and 

 held in a clip with stalk vertical. In the first arrangement 

 of the slit the width might be "002 inch and the length one- 

 half an inch. The sound reaching the ear would be observed 

 and as far as possible retained in the memory. An assistant 

 would then alter the width, say to *01U inch, and the length 

 say to one-quarter inch, and the observer would endeavour to 

 decide which of the two sounds was the louder. It was 

 found as the mean result of two observers that compensation 

 ensued when the length was reduced from '5 inch to '28 inch. 

 A similar change of length compensated an alteration of 

 width from '004 to '020 inch. 



