62 Trans. Acad. Sci. of St. Louis. 



electrification of the magnet had no appreciable effect 

 upon its deflecting effect. The fibers of cotton then 

 appear to determine the lines along which the ionized 

 air-filaments shall form. 



While observing the needle during a wind storm in 

 which sudden and violent gusts of wind occurred, 

 it was observed that the vibrations were affected in 

 a marked way at the beginning of a wind-gust. 

 The velocity of the needle would be suddenly and 

 greatly changed. Several times it was observed that 

 when it had come to rest at the extreme of a vibration, 

 it would suddenly start into motion and in the direction 

 in which it had been moving. Sometimes the velocity 

 of swing would be greatly diminished, the motion would 

 be arrested prematurely, and in the return swing, the 

 velocity would be increased to a marked degree. These 

 sudden changes in the motion of the needle occurred at 

 the beginning of a gust of wind of unusual sever- 

 ity. In one case the unusual velocity of the wind 

 persisted apparently unchanged for a couple of 

 minutes, but the change in the motion of the needle 

 occurred at the beginning of the gust. The amplitude 

 of successive semi-vibrations would sometimes change 

 from five minutes to half a degree, or the reverse. The 

 amplitude was sometimes observed to gradually increase 

 from zero to a maximum and then diminish again to 

 zero. The maximum amplitudes were greatest when 

 the wind was most violent and when it came in sudden 

 gusts. On days when the wind was mild the maximum am- 

 plitude would not exceed one degree. On days of violent 

 winds the amplitude has often risen to eight degrees. 

 Notwithstanding the fact that the gusts of wind occur at 

 irregular intervals, the variations in the amplitude of the 

 needle resemble an irregular series of beats, as will be 

 seen by reference to Plates XIX and XX. 



The observations represented in these two plates were 

 made on consecutive days, when the wind was unusually 



