844 Prof. Riicker and Mr. Edser on the 



door-fasteners from a heavily weighted beam, which itsel 

 rested on india-rubber balls placed at a convenient height on 

 a double pair of wooden " steps." With these precautions it 

 was found that the bands remained tolerably steady even by 

 day when persons were moving about the building, and when 

 the traffic on the frost-bound road produced considerable mecha- 

 nical vibrations. Even under these conditions, we have satisfied 

 all who have seen the apparatus of the reality of the pheno- 

 mena. The experiments on which we rely, however, were 

 made on several occasions between midnight and 2 or 3 A.M. 

 The bands were then absolutely clear and steady. They were 

 undisturbed for many minutes at a time when the bellows 

 were being worked and the siren was sounding loudly. It 

 was only when a note of 64 vibrations per second was directly 

 or indirectly produced that they vanished, and there could 

 be no possible doubt or mistake as to whether the disturbance 

 was or was not produced by the sound or combination of 

 sounds under investigation. 



Up to the present we have used the fork above described 

 only. It was chosen because it was fairly stiff, and as re- 

 movable metal mirrors for the production of Lissajous' 

 figures were attached to its prongs, it was possible to replace 

 them by the glass mirror and square of wood without altering 

 its pitch. It would be quite possible to use properly made 

 forks of higher pitch as resonators, and the steadiness of the 

 bands at night is so remarkable, that we believe that if the 

 apparatus were set up in the country, on a stone isolated from 

 the rest of the room, the degree of sensitiveness we have 

 attained could be far surpassed. 



Tuning the Siren. 



Three methods were used for determining when the siren 

 was producing the required notes. When one of these was 

 fairly high, the beats given by it and a standard fork were 

 noticed, and the note could thus be kept hovering about the 

 required pitch for a considerable time. Although with the 

 aid of Konig's large forks we could apply this method to 

 vibration-frequencies of 48 per second and upwards, it was 

 difficult when the notes were very low to recognize the beats 

 with sufficient certainty. 



The prongs of various tuning-forks were therefore furnished 

 with two pieces of tin-foil, which opened and closed a slit 

 made in them twice in every complete vibration. They were 

 also compared with a standard by the aid of a revolving 

 cylinder, and were adjusted by weights to the required 

 frequencies. These were so selected as to make one of the 



