Lord Rayleigh's Acoustical Observations. 459 



The principal obstacle throwing the shadow was the corner 

 of a large house ; and among the sources of sound tried were 

 the human voice, tuning-forks, whistles steadily blown, and a 

 small electric bell, of which the last (which was employed in 

 Professor Eeynolds's acoustical experiments) proved to be as 

 convenient as any. The source was placed close to the south 

 side of the house, at a distance of eight or ten yards from the 

 south-west corner, while the observer took up a corresponding 

 position on the west side. With these arrangements the sound- 

 shadow was pretty good, though far from perfect. "When, 

 however, a flat reflector, such as a drawing-board of moderate 

 dimensions, was held at the proper angle by an assistant placed 

 at some distance outwards from the corner, the augmentation 

 of sound was immense, and the hearer realized for the first 

 time how very good the shadow really was. 



A screen made by stretching a ' Times ' over a hoop about 

 2 \ feet in diameter gave apparently as good a reflection as the 

 drawing-board ; but when calico was substituted for the paper 

 the reflecting-power was very feeble. By wetting the calico, 

 however, it could be made to reflect very well. These results 

 are in agreement with the striking experiments described by 

 Professor Tyndall. 



Audibility of Consonants. 



I suppose it must have been noticed before now that the s 

 sound is badly returned by an echo. Standing at a distance 

 of about 150 yards from a large wall, I found that there was 

 scarcely any response to even the most powerful hiss. Sh was 

 heard a little better ; m, k, p, g pretty well ; r very well ; h 

 badly ; t badly; b seemed half converted into p by the echo. 

 The failure of the hiss seems to be the fault of the air rather 

 than of the wall, for a powerful hiss heard directly at a dis- 

 tance of 200 yards had very little s left in it. 



Interference of Sounds from two unisonant Tuning-forks. 



In ordinary experiments on interference the sounds are only 

 approximately in unison, and consequently the silences result- 

 ing from antagonism of the vibrations are of only momentary 

 duration. I thought it of interest, therefore, to arrange an 

 experiment in which the sounds should be pure tones, abso- 

 lutely in unison, and should proceed from sources at a consi- 

 derable distance apart. With the aid of electromagnetism the 

 solution of the problem was comparatively easy. An intermit- 

 tent electric current, obtained from a fork interrupter making 

 128 vibrations per second, excited by means of electromag- 

 nets two other forks, whose frequency was 256. These latter 



