OF ARTS AND SCIENCES. 11 



room and the other outside of the window, to prevent their tones 

 from mixing before reaching the ears. No differential tone could 

 be heard. We also tried to use Koenig's forks for illustrating 

 beat-notes, but their very rapid vibrations were so deadened by 

 the tubing that they could only be heard very faintly. (3) By 

 the same process as above, with the exception that the ends of the 

 rubber tube were connected with cylindrical resonators which were 

 blown as already described. This gave almost deafening tones in 

 each ear, but not the slightest trace of a differential. But in this 

 case, as also in (2), if the two tubes were connected to two branches 

 of a Y tube, and a third rubber tube leading to the ear was con- 

 nected to the third branch, the differential tone came out at once. 

 (4) The most striking proof was the following. The ears being 

 closed with wax, a brass rod about five feet long was held lightly 

 against the wax in each. When the stems of forks struck by two 

 assistants were pressed against the farther end of the rods, very 

 loud tones were heard in the ears, unaccompanied by any differential 

 tone. If, however, one of the rods was removed from the ear and 

 pressed tightly against the head, or, better, against the teeth, a loud 

 differential tone was heard at once in the ear against which the rod 

 was placed. If both rods were held against the teeth or head, the 

 differential tone was heard in both ears. 



This apparent impossibility of producing differential tones when 

 the primary tones are caused to actuate separate ears would seem 

 conclusive of the point under discussion except for the fact that it 

 might reasonably be argued that, if sound conduction through the 

 skull occurred from one ear to the other, the vibrations thus con- 

 ducted might act directly on the inner ear, and not indirectly 

 through the membrana tympani and chain of bones, in which case 

 no differential tones should arise, although beats could still take 

 place in the manner assumed by Helmholtz. But our exjieriment 

 with the brass rods, described above, shows that such tones are in 

 fact readily produced when conduction to the ear takes place through 

 the bones of the head. And if there were any material conduction 

 of this kind, it certainly seems as if the very loud sounds used 

 by us would have given evidence of it at least by the occasional 

 production of differential tones when the sounds actuated separate 

 ears. 



The production of beats without the simultaneous action of the 

 beating sounds upon the same ear seems, therefore, to be clearly 

 shown from these facts : (1) Beats are produced by sounds falling 



