1888.] on Diffraction of Sound. 189 



Everything then turns upon the question of the wave-length. 

 The wave-length of the sound that I am using now is about half an 

 inch. That is its comj^lete length, and it corresponds to a note that 

 would be very high indeed on the musical scale. The wave-length 

 of middle C being four feet, the C one octave above that is two feet ; 

 two octaves above, one foot ; three octaves above, six inches ; four 

 octaves, three inches ; five octaves, one and a half inch ; six 

 octaves, three-quarters of an inch ; between that and the next octave, 

 that is to say, between six and seven octaves above middle C, is the 

 pitch of the note that I was just now using. There is no difficulty 

 in determining what the wave-length is. The method depends upon 

 the properties of what are known as stationary sonorous waves as 

 opposed to progressive waves. If a train of progressive waves are 

 caused to impinge upon a reflecting wall, there will be sent back or 

 reflected in the reverse direction a second set of waves, and the 

 co-operation of these two sets of waves produces one set or system of 

 stationary waves, the distinction being that, whereas in the one set 

 the places of greatest condensation are continually changing and 

 passing through every point, in the stationary waves there are 

 definite points for the places of greatest condensation (nodes), and 

 other distinct and definite (loops) for the places of greatest motion. 

 The places of greatest variation of density are the places of no 

 motion : the places of greatest motion are places of no variation of 

 density. By the operation of a reflector, such as this board, we 

 obtain a system of stationary waves, rin which the nodes and loops 

 occupy given positions relatively to the board. 



You will observe that as I hold the board at difierent distances 

 behind, the flame rises and falls — I can hardly hold it still enough. 

 In one position the flame rises, further off it falls again ; and as I 

 move the board back the flame passes continually from the position 

 of the node — the place of no motion — to the loop or place of greatest 

 motion and no variation of pressure. As I move back the aspect of 

 the flame changes ; and all these changes are due to the reflection of 

 the sound-waves by the reflector which I am holding. The flame 

 alternately ducks and rises, its behaviour depending upon the 

 different action of the nodes and loops. The nodes occur at distances 

 from the reflecting wall, which are even multiples of the quarter 

 of a wave-length; the loops are, on the other hand, at distances 

 from the reflector which are odd multiples, bisecting therefore the 

 positions between the loops. I will now show you that a very slight 

 body is capable of acting as a reflector. This is a screen of tissue 

 paper, and the effect will be apparent when it is held behind the 

 flame and the distances are caused to vary. The flame goes up and 

 down, showing that a considerable proportion of the sonorous intensity 

 incident upon the paper screen is reflected back upon the flame ; 

 otherwise the exact position of the reflector would be of no moment. 

 I have here, however, a difierent sort of reflector. This is a glass 

 plate — I use glass so that those behind may see through it — and it 



