194 



KNOWLEDGE. 



[September 1, 1899. 



to hear almost instantly from the offing may in actual fact 

 have been defracted from cliffs and headlands hard by but 

 out of sight. 



That aerial echoes are sometimes heard I would, as the 

 result of my own experiences, very readily admit, and 1 

 would regard the condition of atmosphere favouring this 

 phenomenon as one probable cause of that failure of sound 

 at certain spots which has been already referred to. 



But some other dogmatic statements which have already 

 done service full long respecting the reflection of sound 

 waves may well be questioned. Take the well-known 

 example attributed to the Whispering Gallery of St. Paul's. 

 Here, as I have pointed out elsewhere, the transmission 

 of a whisper around the circular wall is obviously not due 

 to reflection in the ordinary sense, as anyone may convince 

 himself by testing the phenomenon in quiet night hours. 

 Or, again, if on a calm day a smooth crescent-shaped sur- 

 face similar to a segment of the Whispering Gallery be 

 constructed in the open — as may be readily done by means, 

 say, of a length of continuous stout brown paper stretched 

 on battens — an experimenter will find that the Whispering 

 Gallery effect can be produced under circumstances where 

 reflection from an opposite surface is impossible. A 

 whisper communicated either by the mouth or suitable 

 instrument against such a curved surface appears to course 

 round in close contact with the surface. 



I would note that the description given by Sir John 

 Herschel of the peculiar phenomenon of the Whispering 

 Gallery, which has been copied and re-copied into every 

 text-book, is obviously incorrect, and so also, as I am pre- 

 pared with due permission to point out, is that relating to 

 the echoes at Woodstock. From which I would gather 

 that neither Herschel nor those who have quoted his 

 words have ever investigated the phenomena in question. 



In connection with echoes a very noteworthy result has 

 presented itself in the course of the acoustic ballooning 

 experiments in which I have been engaged — namely, that 

 the echo from earth of exploding signals has been always 

 and uniformly retarded. This result has been obtained 

 and verified very many times by independent observers 

 using carefully corrected instruments, and no doubt has 

 been entertained of its truth. Whether, however, this 

 retardation be due to a diminution in the rate of sound 

 travelling vertically through the atmosphere or to any 

 "dwell" or "lag" in the actual reflection of sound I 

 am not at present prepared to state. 



As suggesting a further cause possibly operative in the 

 occasional local failure of sound signals, I would call in 

 evidence certain experiences of my own with regard to 

 the effect on sound of fog or cloud. Professor Tyntlall 

 insists that fog has no sengible power to obstruct sound, 

 while air associated with fog being, as a general rule, highly 

 homogeneous is favourable to the transmission of sound . 



I am able to assert that this view is very far from being 

 generally accepted by practical men employed on look-out 

 stations, and it is entirely opposed to the statements of 

 Stevenson and other eminent authorities. My own observa- 

 tions gathered in balloon voyages, and also during a 

 sojourn of several days and nights, generously granted me 

 by Trinity House, on the Maplin Lighthouse in thick 

 weather, go to show that whereas a condition of still and 

 settled fog may aid the travel of sound, compacted cloud- 

 heaps or wreaths, and masses of rolling mist are capable 

 of refracting and diverting sound waves in a manner that 

 will deceive the most practised ears. For example, the 

 warning of neighbouring light-ships' foghorns as heard 

 from the Maplin are influenced in a far greater measure 

 by the circumstances and qualities of intervening fog than 

 by a gentle wind ; moreover, a horn which had been nearly 



quenched by interposing mist has been observed to sound 

 with far more than normal intensity as soon as the fog 

 had rolled away behind, forming a background to gather 

 and reflect the sound wave. 



I would call attention to the fact that many accepted 

 statements that have done duty in the past with regard 

 to the travel of sound waves seem to have been based on 

 the results of experiments conducted in the laboratory, 

 and I would submit that a more unfavourable place for 

 satisfactory experiments on sound could hardly be chosen, 

 inasmuch as the six bounding surfaces must surely cause 

 reflections seriously endangering the truth of results 

 obtained. Conducting experiments of this kind must 

 often rather resemble such a feat as trying to project 

 lantern pictures in a room where walls, floor, and ceiling 

 are all faced with brilliant mirrors. 



THE MYCETOZOA, AND SOME QUESTIONS 



WHICH THEY SUGGEST.-V. 



By the Right Hon. Sir Edwasd Fby, d.c.l., ll.d., f.r.s., 



and Agnes Fry. 



ABERRANT FORMS.— Having thus given some 

 description of the various parts of the Endo- 

 sporous Myxies, we shall now revert to the 

 aberrant forms which have hitherto been left 

 out of consideration— viz., the Exosporous 

 Myxies and the Acrasieee, the position of which in the 

 classification may be learned by again referring to the 

 table given in an earlier paragraph. 



ExospORE.E. — The Exosporese, or Myxies which carry 

 their spores on the surface and not in the inside of the 

 sporangium, consist of one genus — Ceratomyxa — and of 

 two species, or, according to other authorities, of one 

 species only with one variety. Of this small organism a 

 drawing will be found in Fig. 15. Its first describer, 



Micheli, called ii Pucrinia 

 /•awosrt (in 1729). In 1805 

 it was called Ceratium 

 hydnoides by Albertini and 

 Schweinitz. It was de- 

 scribed as Ceratomyxa 

 Dutcida by Schrater in 

 1889, and, as that name 

 is adopted by Mr. Lister, 

 whose works are the most 

 convenient for the English 

 reader, we have thought 

 it best to follow him. 

 But we have given the 

 synonyms to prevent our 

 readers from being 

 misled by the puzzling 

 and lamentable variety of names. 



The Ceratomyxa mucida is by no means uncommon on 

 rotten wood, and might at first sight be mistaken for a 

 white or pale-coloured fungus. It consists of an aggrega- 

 tion of finger-like projections from a common base, and 

 presents somewhat the appearance of a minute piece of 

 white coral. When the surface of these projections is 

 examined, it is found to be marked off by delicate lines 

 into polygonal spaces, from the centre of each of which 

 rises a delicate white stalk, and on the summit of this an 

 equally delicate and white egg-shaped spore. 



The development of this little organism has been 

 elaborately studied by two Russian botanists, and it is 

 sufiiciently interestmg to demand a few minutes' attention. 



Fl8. 15. — Ceratomi/xa mueida, 

 Magui6ed. (After Famintzin and 

 Woronin.) 



