April 13, 1900.] 



SCIENCE. 



583 



Langley in the same direction. In a fur- 

 tlier paper tlie tlieory of the vena contracta 

 and of colliding jets is subjected to analysis, in 

 ■whicti (following Maxwell) the inferences are 

 drawn directly from the principle of the con- 

 servation of linear momentum. To the ques- 

 tion of jets, Rayleigh returns in succeeding 

 papers. In the iirst of these the conditions of 

 instability are discussed, both for capillary or 

 statical instability, and for dynamic insta- 

 bility, such as occurs, for instance, in waves or 

 surface of water under the influence of the 

 wind. The other paper examines the capil- 

 lary phenomena observed in jets issuing from 

 an orifice which is not circular, but elliptic, 

 triangular, etc. Apart from form, the wave 

 lengths of the issuing stream have a close rela- 

 tion to the square root of pressure. Disposing of 

 this case, Rayleigh then passes to the dismem- 

 berment of the circular jet into drops, or of an 

 oblique jet into sheathes, using the experimen- 

 tal (shadow) method of Butf. A curious result 

 of the analysis may be mentioned, viz, that 

 the radius of the sphere which vibrates capil- 

 larly in seconds is about one inch. Bayleigh's 

 more recent work with jets and ripples is not 

 included in this volume, but interspersed 

 among other hydrodynamic researches is the 

 fascinating and well-known paper on the influ- 

 ence of electricity on colliding water drops, 

 proving that whereas unelectrifled drops re- 

 bound on collision, electrified drops coalesce. 

 The conclusions are made more striking by the 

 examination of paired jets, and an important 

 inference is drawn relative to the growth of 

 rain-drops stimulated by thunder storms. A 

 farther paper on the instability of fluid motions 

 (the preceding cases being chiefly of interest in 

 their relation to sensitive flames and smoke 

 jets) reopeus the whole question, obviating the 

 preceding hypothesis of discontinuous fluid mo- 

 tion and admitting only such gradual changes 

 of velocity as must inevitably occur in viscous 

 liquids. A final paper is devoted to progres- 

 sive waves, treating the case frequently ob- 

 sferved that the group velocity of waves ad- 

 vancing into still water is often below the 

 velocity of the constituent members of the 

 group. The investigations are largely em- 

 bodied in 'Sound.' They are referred to the 



case of two infinite wave trains of the same 

 amplitude and nearly the same wave-length, 

 superposed. 



Meanwhile Lord Rayleigh has not lost inter- 

 est in acoustical subjects. In a paper on our 

 perception of the direction of a source of sound 

 there occurs a humorous passage which is rare 

 in his writings. " The efiicient action of a lens" 

 (for the jiurpose in question) "depends on its 

 diameter being at least many times greater than 

 the wave-length of light, and for the purposes 

 of sight there is no difliculty in satisfying the 

 requirement. The wave-length of the rays by 

 which we see is not much more than a ten- 

 thousandth part of the diameter of the pupil of 

 the eye. * * * The waves of sound issuing 

 from a man's mouth are about eight feet long, 

 whereas the diameter of the passage of the ear 

 is quite small, and could not well have been 

 made a large multiple of eight feet." Usually 

 the imputation of ears longer than 8 inches is 

 regarded sufficiently undignified to be resented. 



A similar paper with acoustical observations 

 relating to binaural audition, reflection and in- 

 terference of sound, pitch, etc., follows. Ray- 

 leigh then contributes to the few data then 

 known of the amplitude* of the audible sound 

 wave, by computing it roughly from the energy 

 needed to blow a whistle and the distance 

 of audibility, using a straightforward method 

 which, like many others in the volume ought 

 to flnd its way into our text-books. He finds 

 the observed amplitude to have been of the 

 order of 1/10' centim., but believes the l/40th 

 part of it to be audible under favorable con- 

 ditions. A paper on absolute pitch is con- 

 cerned witli the discrepancy observed between 

 Konig's and Apunn's tonometers, which Ray- 

 leigh attributes to the mutual influence of 

 simultaneously sounding reeds. He proposes 

 a tuning fork clock method of his own. Another 

 paper relating to Mayer's phenomenon of 

 acoustic repulsion shows the pressure within 

 a resonator to be in excess of atmospheric pres- 

 sure, which is equivalent to a force at the 

 mouth of the resonator directed normally in- 

 ward. Then comes an original explanation of 



* Witli the inTention of Professor Webster's inter- 

 ferential apparatus this dearth has already become 

 fruitful. 



