December 27, 1901.] 



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and closed in rapid succession. This plan 

 was modified by Mach who caused the vi- 

 brating body to be illuminated by inter- 

 mittent light. 



If now we allow the stroboscopic images 

 of a movittg body to fall on a photographic 

 plate, giving the plate a movement of trans- 

 lation which is arrested before each appear- 

 ance of the image, we thereby obtain a 

 series of photographs of the successive posi- 

 tions assumed by the body. If, further, 

 matters are so arranged that the beginning 

 and duration of the phenomenon are traced 

 on the images, we have a new method, 

 which is called chronophotography. It was 

 M. Janssen who first conceived the idea 

 of taking automatically a series of photo- 

 graphic images in order to determine the 

 successive positions at diflerent times of the 

 planet Venus in its passage across the sun. 

 It was Janssen also who, in 1876, first sug- 

 gested the idea of applying successive pho- 

 tograms to the study of animal locomotion. 

 The analyzing of such movements was first 

 accomplished by Muybridge, of San Fran- 

 cisco. The method has been largely ex- 

 tended and perfected by M. Marey, who 

 has employed it in studying the locomotion 

 of all sorts of subjects, from men to in- 

 sects. 



The last of the methods to be noticed is 

 that employed by Rudolph Koenig in his 

 wave-siren. In this instrument a metal 

 band or disc with curvilineal edges passes 

 before a narrow slit from which issues a 

 current of compressed air. By means of 

 these discs we can produce either simple 

 sounds, or sounds of various timbres, con- 

 taining such harmonics as we please, the 

 intensities and phases of the latter being 

 varied at will. The first wave-siren was 

 constructed in 1867, and the account of the 

 first series of experiments was published in 

 1881. 



The mere enumeration of the methods of 

 acoustical research which have been de- 



vised since the days of Chladni is an indi- 

 cation of the enormous advances which 

 have been made in this branch of science. 

 It remains now to state more particularly 

 what these additions to our knowledge of 

 acoustical phenomena have been. This can 

 be most conveniently done under the fol- 

 lowing heads, viz. : The velocity of sound : 

 its pitch, intensity and timbre ; and the phe- 

 nomena produced by the coexistence of two 

 or more sounds. 



THE VELOCITY OF SOUND. 



Long before the beginning of the last 

 century it had been observed that the 

 propagation of sound was not instantane- 

 ous. Mersenne in fact had tried to esti- 

 mate the velocity by experiments on echoes, 

 and by counting the time which elapses be- 

 tween the flash of a gun and the report. 

 The latter experiments were also repeated 

 by Kircher as well as by the Academy of 

 Florence in 1660, The same experiments 

 were subsequently, in 1738, undertaken by 

 members of the Academy of Sciences at 

 Paris, by savants, such as Kaestner, Ben- 

 zenberg, Goldingham and others, but the 

 results obtained did not gain the confidence 

 of the scientific world. A new series of 

 experiments was accordingly undertaken 

 in 1822, on the suggestion of Laplace, by 

 members of the Bureau des Longitudes, to 

 determine the velocity in air and other 

 media. These experiments, which were 

 the beginning of truly scientific work in 

 this subject, were performed by Prony, 

 Arago, Mathieu, A. de Humboldt; Gay- 

 Lussac and Bouvard, between Montlhcry 

 and Villejuif, cannon being fired at both 

 stations. The result obtained was 331 m. 

 at zero temperature, with an increase of 

 0.6 m. for each degree above zero. In the 

 course of these experiments it was observed 

 that the cannon fired at Villejuif were all 

 distinctly heard at Montlhery, whilst the 

 reciprocal reports were so faint that only a 



