42 



NATURE 



[September 8, 192 1 



far towards solving many of our chief ditliculties. It 

 is perhaps somewhat revolutionary, but, whether we 

 like it or not, even the scientific times move forward, 

 fVnd we can no longer maintain — most of us cannot 

 afTord to maintain — our pre-war usages. One would 

 like opinions on this matter, for action of some kind 

 will need to be taken in the not distant future. 



William B. Brierley. 

 Rothamsted Experimental Station, 

 Harpenden, August 29. 



Whispering-Gallery Phenomena at St. Paul's 

 Cathedral. 



The very curious and interesting acoustical effects 

 observed in the Whispering Gallery under the dome 

 of St. Paul's Cathedral have, as is well known, been 

 explained by the late Lord Rayleigh as due to the 

 curvilinear propagation of sound, the waves which 

 proceed from a source placed close to the wall of 

 the gallery clinging to its surface and creeping tan- 

 gentially along it. This view was developed mathe- 

 matically by Lord Rayleigh ("Scientific Papers," 

 vol. 5, p^ 617), the theoretical conclusions arrived at 

 being (a) that the sound-waves travel in a compara- 

 tively narrow belt skirting the wall, the thickness of 

 this belt decreasing with the wave-length of the 

 sound ; (b) that in this belt the intensity is a maxi- 

 mum near the wall and decreases rapidly and con- 

 tinuously as we proceed radially away from it ; and 

 (c) that the intensity does not fluctuate markedly as 

 we proceed circumferentially parallel to the wall. 



We were much interested in the subject, and by the 

 courtesy of the authorities of the cathedral have been 

 enabled to carry out an extended series of observa- 

 tions in the gallery with the view of making a precise 

 test of Lord Rayleigh 's theory. Our experiments 

 show conclusively that while the indication of theory 

 as expressed in (a) is substantiallv accurate, neither 

 of the conclusions (&) and (c) is in accordance with 

 actual facts. Using a steady source of sound placed 

 close to the wall at one point, we found that else- 

 where the Intensity of the sound showed pronounced 

 oscillations In proceeding Inwards radially from the 

 wall, the ear of the observer passing several times 

 through alternate zones of great intensity and of com- 

 parative silence. In the latter some of the overtones 

 of the source could be heard clearly, while the funda- 

 mental was practically inaudible. These alternations 

 of Intensity could be demonstrated in the gallery, 

 using a fairly high-pitched source and a sensitive 

 flame as Indicator. The distance between the succes- 

 sive zones of silence was about the same as the half- 

 wave-length of the source. There were also distinct 

 periodic fluctuations of Intensity In proceeding cir- 

 cumferentially — that is, parallel to the wall. The latter 

 were not equallv distinct in all parts of the gallery, 

 being most marked at the other end of the diameter 

 containing the source. 



The circumferential fluctuations of intensity might 

 be interpreted as being, at least in part, due to the 

 stationary interferences of waves which meet after 

 passing In opposite directions round the gallery. But 

 the radial fluctuations are less easily explained, and 

 must be regarded as fundamental In anv satisfactory 

 theorv of the Whispering Gallery. We find that 

 effects similar to those we observed at St. Paul's may 

 be demonstrated In the laboratory with anv large cir- 

 cular reflecting surface, using a bird-call with a sensi- 

 tive flame as sound-detector. 



The exoeriments thus show that, while the explana- 

 tion put forward by Lord Ravlelgh is at least on the 



NO. 2706, VOL. 108] 



right lines, it is far from being a completely satis- 

 factory theory of the Whispering Gallery. We pro- 

 pose at an early opportunity to go more fully elsewhere 

 into the question of the revision necessary in the 

 theor}^ C. V. Raman. 



G. A. Sutherland. 

 22 Oxford Road, Putney, S.W. 15, August 26. 



Ceratium furca and Pedalion mirum. 



In describing the specific characteristics of Ceratium 

 furca, one of the Peridiniae or Dinoflagellates, Saville 

 Kent in his " Manual of the Infusoria " gives the 

 habitat as salt-water, and he appends a note to the 

 effect that, "although usually regarded as entirely 

 marine, JNl. Werneck has reported the occurrence of 

 an apparently identical species in fresh-water in the 

 vicinity of Salzburg." 



It may be of interest to students of the Protozoa to 

 state that on August 19 and 27 I discovered this 

 species in two separate bodies of fresh-water in this 

 district. It may be the case that other workers have 

 found the form at other points in Great Britain, and 

 that, by reason of my not having access to the scat- 

 tered literature on Protozoa, I am only reporting an 

 already well-established fact ; but it will be most 

 interesting to know whether Ceratium furca has been 

 found elsewhere In this country, and I shall be glad 

 to have the views of those who have given attention 

 to this matter. 



Kent gives the entire length of C. furca as 1/ 120th 

 of an inch, say 212 microns. I find that very few 

 of my specimens are so small as this, and I have 

 measured several up to 256 microns, which is 21 per 

 cent, larger than the recorded length. 



The remarkable and Interesting Rotifer, Pedalion 

 mirum, discovered by Dr. Hudson in 1871 Is described 

 In Hudson and Gosse's classical work, "The Rotl- 

 fera," as being very rare, and up to the date of the 

 publication of that work, twelve jears later, it had 

 been recorded only from three places. Others have 

 doubtless since been added, but Pedalion may 

 probably still be regarded as a rare species. I have 

 found it In both the waters in which Ceratium 

 furca occurred, and In the first gathering on August 19 

 it was fairly numerous. A list of the known 

 habitats of this Rotifer would be most Interesting. 



Alfred E. Harris. 



44 Partridge Road, Roath, Cardiff, August 29. 



illumination of Plankton. 



In avoiding compressed-air illness by the excellent 

 method which we owe to Dr. J. S. Haldane, divers 

 ascending from deep water often have to spend 

 periods of half an hour or so in idleness suspended 

 on a rope in mid-water 20-30 ft. below the surface. 

 The blank tedium of such occasions can be relieved 

 by watching the ebb and flow of plankton past the 

 face-glass of the helmet and musing on the remark- 

 able variation in its apparent quality and quantity 

 from day to day. 



As the diver looks upwards and inwards towards 

 the black, shadowy keel of the salvage' vessel he sees 

 the individuals of the plankton standing out vividly 

 by a sort of dark-ground illumination. Turning his 

 head outwards and looking into unrelieved water 

 space, again using a simile from microscopy, the 

 field Is overflooded with light, contrast is absent, 

 and the teeming multitudes blend into granular hazi- 

 ness. In August, after our salvage ship has been 

 moored for an hour or two in the open sea, mackerel 

 find her out, and daily a compact shoal of them con- 



