'29S Lord Raylcigh on the Production 



to the horizontal plane, but within that plane spread without 

 nnich loss through all directions less inclined than 80°. 



In order to carry the demonstration a little nearer to what 

 would 1)6 required in i)ractice, I have lately experimented 

 with the sound from a reed organ-pipe, giving waves of length 

 equal to 8 inches (20 cms.) and thus easily audible. The 

 trumpet is of wood, pyramidal in form, and the section at the 

 mouth is 36 x 4 inches (91 x 10 cms.). The length (OB) is 

 6 feet (183 cms.). These dimensions were chosen so that OA, 

 00 should exceed OB by ^ X. A larger difference might entail 



too great a discrepancy of phase in the waves at A and B ; 

 a less difference might lead to an unnecessary prolongation 

 of the cone along the axis. The trumpet was so mounted 

 that its mouth just projected from an open window^ and that 

 it could be readily turned round OB as horizontal axis so as 

 to allow the length of the section (AC) to be either horizontal 

 or vertical. The observers took up various positions on a 

 lawn at a moderate distance from the window. 



To observers in the line of the axis OB it should make 

 no difference how^ the trumpet is rotated. On the whole this 

 expectation was confirmed, but a little precaution is required. 

 As usual the phenomenon was complicated by reflected sounds 

 (doubtless from the ground). It w^as well in every case to 

 make sure, by slightly raising or lowering the head, that the 

 maximum sound was being heard. 



When the point of observation deviated (in the horizontal 

 plane) from the axis, the difference due to rotation w^as soon 

 apparent. At 30° obliquity the sound appears greatly in- 

 creased as AC passes from the horizontal to the vertical 

 position. At higher obliquities with AC horizontal the sound 

 falls off greatly, but recovers when AC is made vertical. 

 Altogether the effects are very striking, and carry conviction 

 to the mind more fully than experiments with sensitive flames 

 where one is more or less in doubt as to the tnaqnitude of the 



