Septejibeh 1, 1899.] 



KNOWLEDGE. 



193 



Founded by RICHARD A. PROCTOR. 



LONDON: SEPTEMBER 1, 1899. 



CONTENTS. 



m PAGF 



Sound Reflexion and Refraction. By the Rev. John M. 



Bacon. M.A.. KK..\.s 193 



The Mycetozoa. and some Questions wliich they 



Suggest. — V. By the Eight Hon. Sir Edwabd Fet, 



D.C.L., IL.D., F.B.S., and Agnes Fbt. {Illustrated) ... 194 



Fairy Rings. By A. B. Steele 197 



Ben Nevis and its Observatory.— I. By Willlsji S. 



BsrcE. F.E.s.a.s. {Illustrated) 198 



Some Suspected Variable Stars.— II. Bt J. E. Qobe, 



F.K.A.S .■ 200 



Clouds. By E. M. Antoniadi, p.e.a.s., and G-. MAiniEr. 



{Illustrated.) {Plate) ... 202 



Letters : Chables S. Pattebson ; W. H. S. Monck ; W. E. 



"Wilson; ])AyiD Flaneet 205 



Obituary 207 



Notices of Books 207 



Books Eeceited 208 



London Summers near Sunspot Minima. By Alex B, 



MAcDowiLL. >r.A. {Illustrated) " 209 



The Story of the Orchids.— II. By the Rev. Alex. S. 



Wilson, ma., b.sc. {Illustrated) ..'. 210 



Electricity as an Exact Science. — V. Electrical 



Reasoning and Incontrovertible Electrical Fact. 



By HowAED B. Little 211 



Notes on Comets and Meteors. Bv W. F. Denhino, 



J.B.A.S ." 213 



Microscopy. By John H. Cooke, f.l.s., f.o.s 214 



The Face of the Sky for September. By A. Fowleb, 



P.B.A.S 214 



Chess Column. By C. D. Locock, b.a. 215 



SOUND REFLEXION AND REFRACTION. 



M 



By the Rev. John M. Bacon, m.a., f.r.a.s. 



ORE than one recent disaster at sea, still 

 unexplained, has pointed to the necessity of 

 reconsidering certain accepted dogmas relating 

 to the transmission of sound waves, and official 

 reports of a disquieting nature from look-out 

 stations have called once and again for serious investigation 

 of the anomalous behaviour under special conditions of 

 such sound signals as are commonly in use at sea. In 

 particular it has been insisted on that the hearing of the 

 syren and fog-horn is apt to prove uncertain and that, on 

 occasions at least, there are to be found areas or zones of 

 silence where their waniing will unaccountably fade or 

 else cease altogether to be heard. 



It would appear, however, that this peculiarity is not by 

 any means confined to the signals of those instruments of 

 which the syren is the type. The writer has had special 

 opportunities of experimenting with explosive signals of 

 many kinds, and has on three separate occasions, and 

 under very different circumstances, carried out systematic 

 trials on the penetration of the service cotton-powder 



cartridge fired at different heights from balloons whilst 

 travelling over diversified open country as also over 

 populated districts including London itself. These trials 

 have drawn records from a large number of independent 

 observers, whose statements, when carefully analysed and 

 compared, have proved beyond all reasonable doubt that 

 even the most powerful and deservedly well trusted form 

 of modern explosive signal is sometimes fickle in its 

 character, failing or fading m unexpected quarters without 

 obvious cause. 



It will probably be readily conceded that the explanation 

 of these facts must be sought not so much in any 

 peculiarities in the sounds themselves as in the condition 

 of the medium through which they are propagated, and 

 it is here that the views expressed perhaps too confidently 

 a generation ago may need to be modified. Certain con- 

 clusions as to states of atmosphere commonly afiecting 

 the passage of sound waves have perhaps remained too 

 long unchallenged. 



Thus Professor Trndall states that while conducting 

 experiments with sound signals at the South foreland 

 there were present always and in all weathers invisible 

 acoustic clouds which returned echoes from the instru- 

 ments and cannon planted on the summit of the cliff 

 overhead. Hundreds of cannon-shots, he states, were 

 fired, and were always followed immediately by a rumbling 

 which the Professor asserted must have come only from 

 out of the empty air. If this were so, then we must 

 conceive that there was constantly present in the air some 

 form of obstruction that not only impeded but reflected 

 back the waves of sound that were being emitted. 



It is with regard to this point that some results recently 

 obtained may be deemed Instructive. In the first place, 

 although during the experiments in which I have been 

 concerned some scores of cotton-powder signals have been 

 fired from balloons under very difi'erent meteorological 

 conditions, nothing of the nature of an aerial echo has 

 ever been suspected, and the dead silence aloft has always 

 been absolutely unbroken after each report until, after an 

 interval of several seconds, the earth itself has replied 

 with a burst of sound which has reached the car even at 

 the height of a mile with all the intensity and reverbera- 

 tion of a thunder-clap. This striking result, invariably 

 the same, appears highly significant, and would point to 

 the conclusion that the initial report immediately con- 

 sequent on a lightning fiash is, like the fog-signal burst- 

 ing below the car, comparatively speaking only a trivial 

 sound, while the great uproar of sound must practically 

 be wholly due to echo, which is presumably largely off the 

 ground. In support of this I would state that when one 

 of the signal cartridges already described is fired, say, 

 150 feet above the ground, ui moderately open country, an 

 observer below hears a series of e.xtremely powerful echoes 

 which he can easily trace to each clump of trees or build- 

 ing around, and several seconds after this elapse before 

 the subsequent conflict of reverberations ceases. Further, 

 when such a rocket signal has been fired over a wide 

 extent of quiet common terminating at some distance in 

 an abrupt decline, it has awoke surprising and unexpected 

 echoes from woods lying m the valley, although these 

 have been completely out of sight and sheltered from the 

 ear by a considerable stretch of intervening level ground. 

 In this case both the incident and reflected waves of sound 

 had clearly been defracted or bent over the shoulder of the 

 hiU which hid the woods from view. 



I would then submit, first, that a condition of atmo- 

 sphere causing aerial echoes is by no means the normal 

 condition, and, secondly, that the return of sound which 

 Professor Tyndall, when stationed under the cliff, appeared 



