30i Lord Riiyleigb 



The factor, corresponding to (27), is therefore simply 



'^^'- " (30) 



No work at all is done if c be such that kc is a multiple of 

 ^, ol* 2c a multiple of X. 



By the method of the ' Theory of Sound ^ {loc. cit.) we 

 might in like manner investigate the effect of distributing 

 ii source of sound uniformly throughout the volume of a 

 sphere, but the above examples will suffice for our purpose. 



Experimental Illustrations* 



There is no difficulty in illustrating upon a small scale the 

 results above deduced from theory. The simplest experi- 

 ment is with an ordinary open organ-pipe, gently blown, 

 so as to exclude overtones as much as may be. The 

 open ends act as two equal sources of sound in the same 

 phase. Connected by a long flexible tube with a well- 

 regulated bellows, the pipe can be held in any position and 

 be observed in the open air from a moderate distance. When 

 the length of the pipe is perpendicular to the line of 

 observation, the two sources are at the same distance and 

 the effects conspire. But if the pipe point toward the 

 observer, the two sources, being at about JX apart, are in 

 antagonism and the sound is much enfeebled. 



In order to exemplify the principle further, a multiple 

 pipe was constructed (fig. 2). This consisted of a straight lead 

 tube 31 inches long and '35 inch bore*, open at the ends 

 A, B. At four points D, E, F, G, distant 6^ inches from 

 the ends and 6 inches from one another, the tube was per- 

 forated, and the holes were blown by four streams of wind 

 from the branched supply-tube C. The whole was cemented 

 to a framework of wood, so that it could be turned round 

 without relative displacement. The intention was that all 

 six apertures A, D, E, F, G, B should act as sources of 

 sound in the same phase, but one could hardly be sure 

 a prio7'i that this behaviour would be observed. Would the 

 simultaneous motions of the air-column on the two sides of 

 E (for example) be both towards E or both from E ? 

 Might it not rather be that the motions would take the 

 same absolute direction, in which case E, F, &c., would fail 

 to act as sources ? A little observation, however, sufficed to 

 prove that the apparatus really acted as intended. By listen- 

 ing through a rubber tube whose outer end was brought into 

 proximity with the apertures, it was easy to satisfy oneself 



* Inch = 2-54 cms. 



