1895.] on the Physical WorJc of von Eehnholtz. 489 



In some respects the apparatus requires careful handling. Of 

 course if you blow down the connecting cone the fork may be dis- 

 turbed, and sometimes a particular note of the siren appears to affect 

 the fork for no very obvious reason. Probably the resonance of the 

 air in the cone, or the vibrations of the wooden disc, may at times be 

 the causes of such effects. We have, however, found that whatever 

 they may be due to, they differ in appearance from those produced by 

 vibrations synchronous with the periodic time of the fork, and they 

 can in general be got rid of by a very slight readjustment of the 

 apparatus. The fact that our main conclusions do not depend on any 

 such nicety, is proved by the fact that the instrument has been set up 

 twice in the laboratory, and once in the lecture room in the College. 

 In each case all the experiments have been successful, and on one oc- 

 casion only were we troubled by a disturbance due to a note (of about 

 253 vibrations) when sounded alone. A slight readjustment of the 

 cone, however, eliminated this effect entirely. 



Such difficulties make it no easy matter to set up the apparatus in 

 a hurry, and the most I can hope to do this evening is to demonstrate 

 to you the methods of using it. I cannot undertake to make the 

 actual measurements before you. 



It is, however, desirable to illustrate the sensitiveness of the 

 apparatus to vibrations of 64 per second, and its insensitiveness to 

 other sounds. 



Provided the current of air does not travel directly down the cone, 

 organ pipes may be blown just outside it without producing any 

 effect. One of Konig's large tuning-forks may be bowed strongly 

 without effect. 



If, however, the exciting fork be tuned to 64 vibrations per second, 

 and if it be struck as lightly as possible with the handle of a small 

 gimlet, used as a hammer, the handle having been previously covered 

 with india-rubber, the bands will imemdiately vanish, though the note 

 produced is often quite inaudible, even to a person whose ear is 

 placed close to the fork. 



Let the weights on the fork be shifted so that it makes 63-5 

 vibrations per second, then the resonating fork beats, and the bands 

 regularly ajjpear and disappear every two seconds. 



Having thus explained the construction and working of the appa- 

 ratus, let me show you how we have tested whether it responds to a 

 different tone. When the proper rows of holes are opened, the siren 

 will give simultaneously the c' of 256 and the e' of 320 vibrations. 

 The interval is a major third, the difference tone is 64 vibrations. 

 The pitch is determined by the beats between the upper note and a 

 standard tuning-fork which gives e'. Sounding the upper note alone 

 no effect is produced on the interference bands, as the beats first 

 appear, then die out, and are finally heard again when the note given 

 by the siren is too high. 



It could be shown in like manner that the 256-note alone pro- 

 duces no effect, but if, when the standard fork of 320 vibrations and 



