September 1, 1896.] 



KNOWLEDGE. 



193 



LONDON: SEPTEMBER 1, 1896. 



CONTENTS. 



By VAroHAy 



Waves.— IX. Notes on Sound Waves. 



COBXISH, M.Sc. [lUttitrateJ) 



Linoleum. Bv Dr. (Ieokgk Mc-Go-n-Ax 



The Sooty or Brown Albatross. {Plate) 



The Causes of Colour. By J. J. Stewabt, B.A.Cantab., 



B.Sc.Lond. ... ... ... ... . . 



Microscopy. By A. 15. Steele 



A Quarter of a Century s Work on Respiration. Bv 



C. F. lo-n-y-END, F.C.S. (Illustrateil) !. 



The Rev. Francis Wollaston. Amateur Astronomer. 



By W. T. Lynx, B.A., F.R A.S 



The Total Eclipse of August 9, 1896. Bv E. Waltee 



Mai-.m.ee, F.R.A..S ; 



Extended Nebulosity round Antares. Bv Prof E. E 



B.iBXABD, F.E.A.S. (Flaif) '. ' 



Notices of Books. {Ilha/rated) 



Letters :— G. H. Hill; H. A. Cooesox ; E. J. D. Saie 



Some Curious Facts in Plant Distribution. — IV. 



W. BoiTiSG IIemslet, F.B.S 



Emery. By Kichabd Bevnox 



A Crocodile Mummy in the British Museum. 



H. Spencer 

 Some Notes on Spiders. By Rev. Samuel Babrer 

 The Affinities of Flowers. — The Harebell and 



Daisy. By Felix Oswalh. B.A. I.ond. {Illustrated) ... 

 The Face of the Sky for September. By Herbeet 



Sadleb, F.R.A.S 



Chess Column. By 0. D. IiOOOCE, !Bjl.Oxon. 



By 



Bv 



the 



WAYES.-IX. 



NOTES ON SOUND WAVES. 

 By Vaughan Cornish, M.Sc. 



WE leave for the present the waves of water in 

 order to apply our study of the wave pattei-us 

 giveu by moving bodies to the study of the 

 physical properties of sound waves in air. 

 By means of the electric spark it is possible 

 to see a noise. The arrow-headed wave front in Fig. 1 

 is the photograph of the hum of a bullet. The bullet 

 completes an electric circuit when it brushes by the two 

 wires shown in the ligure, a spark illuminates the chamber 

 through which the bullet is passing, 

 and the phenomena may either be 

 photographed by the light of the 

 spark, or the whole thing can be seen 

 if the eye be placed in the position 

 of the photographic plate. The wave 

 pattern is simpler than that formed 

 in front of a small floating body ; it 

 is as if all the ridges and hollows of 

 the water pattern had been omitted 

 except the ridge of the wave of minimum velocity — 

 that nearest the body. The single wave formed in 

 front of a flying bullet is a wave of compressed air, as 



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196 



197 



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20) 



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203 



2f5 

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207 



208 

 210 



210 

 211 



212 



215 

 215 



is shown by the fact that it appears dark. The light 

 comes from the further side of the bullet, and where it 

 meets this denser air at a grazing angle is deflected from 

 its path, leaving a dark line. The bright line behind it is 

 believed to be due simply to the fact that this part of the 

 photographic plate is illuminated both by the light which 

 passes straight through the place and by the light deflected 

 from the condensed air. The velocity of the free wave of 

 compressed air can be determined from the angle of the 

 cone, the velocity of the bullet being known. With a 

 Martini bullet (Eig. 1) the apes of the cone advances 

 one thousand three hundred feet, while the wave has 

 spread laterally one thousand one hundred feet on each 

 side. The velocity of the free wave is therefore not one 

 thousand three hundred feet per second, the velocity of 

 the bullet, but one thousand one hundred feet per second, 

 which is the velocity of sound. The bullet from a Lee- 

 Metford rifle travels at two thousand feet per second, and 

 consequently the wave front makes a sharper arrow-head 

 (Fig. -2). On the other hand, when a bullet is moving at 

 less than one thousand one hundred feet per second there 

 can be no arrow-headed wave track, for the sound wave 

 would run beyond the bullet. The figures show clearly 

 that when the hum of a rifle bullet is heard overhead the 

 bullet has already gone by, though everyone sympathises 

 with the impulse to " duck.' Meteors, which move much 

 faster than bullets, are accompanied by a sound wave of 





Fio. 1.— Photograph of Martini Bullet in Flight. 



{By |>('riiii.<sioi( o) Mti-.rs. Nfxtton X Co.) 



extremely acute angle. The wave front does not reach 

 the observer until the meteor has passed far beyond his 

 station, or it may be not until some time after the meteor 

 has burst. Therefore, as Prof. Boys has pointed out, 

 the noise often heard after the bursting of a meteor may 

 not even be due to the shock of its disruption, but must 

 frequently, if not always, be caused by the wave track 



.y 



Enlarged Drawing of above, showing Form of Air Waves. 



crossing the observer's position, a consideration which 

 may avert erroneous calculations of the altitude and 

 position of the bursting points of meteors. 



