Lord Rayleigh's Acoustical Observations, 151 



and of maximum pressure-variation are usually called nodes; 

 and the places of zero pressure-variation and of maximum 

 velocity are called loops. If we retain this nomenclature, we 

 may say that silences as investigated by the ear occur at 

 loops, and that the maximum sound is found at nodes; but in 

 Savart's papers the silences are identified with nodes. More- 

 over the difference is not one of words merely; for Savart con- 

 siders that (apart from the effects of obstacles) the silences 

 are to be found at distances from the wall which are even 

 multiples of the quarter wave-length. A large part of his 

 work is thus an endeavour to bring the facts into accordance 

 with a mistaken theoretical view. 



When the median plane is parallel to the wall, the obstruction 

 presented by the head displaces considerably the positions of 

 the silences. In his first paper Savart proposes to add 27 

 mm. to the measured distances between the external ear 

 nearer to the wall and the wall itself, in order to take account 

 of the interval between the external ear and the sentient ap- 

 paratus. In the case of the ear further from the wall a similar 

 distance is to be subtracted. I am at a loss to understand how 

 the situation of the sentient apparatus can be supposed to be an 

 element in the question at all. Every thing must surely de- 

 pend upon whether there is or is not a variation of pressure at 

 the outer end of the ear-passage. In the second paper Sa- 

 vart takes (as it appears to me) a further step in the wrong- 

 direction. He states that the positions of the silences are the 

 same, whether they be observed with the ear nearer to the 

 wall, or with the ear further from it, and draws the conclusion 

 that the part of the head with which we have to deal is that 

 situated in the median plane midway between the ears. Having 

 already added 27 mm. to his measurements (in the case of the 

 ear nearer to the wall), to take account of the distance between 

 the external ear and the labyrinth, he now adds 50 mm. 

 more. By this artificial treatment the distances of the silences 

 from the wall are made to agree with the series of even multi- 

 ples of the quarter wave-length, though considerable anoma- 

 lies remain unexplained. 



There can be no doubt, I imagine, that Savart's theoretical 

 views are quite erroneous, and that what has to be explained 

 by the action of the head as an obstacle is the displacement of 

 the silences from the loops, and not from the nodes. An exact 

 theoretical investigation of this subject is of course out of the 

 question ; but some information bearing upon it may be ob- 

 tained from a calculation given in my ' Theory of Sound/ 

 § 328, relating to the character of the obstruction to sound pre- 

 sented by rigid spheres. It appears that if a source of sound 



N2 



