Nipher — Variations in the Earth's Magnetic Field. 113 



the movement was apparently uniform in one direc- 

 tion, records were made at intervals of five or ten min- 

 utes, although the needle was under almost constant ob- 

 servation. 



During the oscillations above referred to, the watch 

 reading was taken at as frequent intervals as was pos- 

 sible. In these oscillations the time of one to and fro 

 vibration was about 50 seconds, as nearly as it could be 

 determined. The time covered by consecutive vibrations 

 frequently varied so greatly on other occasions that no 

 precise value could be given for the time of vibration. 

 The effect of the damping liquid was such that the needle 

 would come practically to rest in three or four semi- 

 vibrations when deflected 90'^. 



These vibrations above described are graphically 

 shown in Plate XXI. The instant when the blast 

 of wind reached the observing station is indicated 

 by the arrow, located near the middle of the 

 group of vibrations representing this local magnetic 

 storm. The ordinates are in scale divisions. The great- 

 est amplitude of swing was about 20 scale divisions. The 

 hour of the day is laid off upon the horizontal axis. The 

 fact that the needle was affected by this air disturbance 

 south of the station at least nine minutes before it 

 reached the station, is in exact harmony with the sug- 

 gestion given in the former paper in these Transactions 

 referred to above. As was there pointed out, a brush 

 which disturbs iron filings at any point on a plate of 

 glass above a bar magnet, is producing an ether disturb- 

 ance in the field of that magnet. This observation also 

 shows clearly, why it is that on stormy da5^s, when gusts 

 of wind follow each other at frequent intervals, the effect 

 due to any one gust cannot be identified. At any instant, 

 the needle responds to a summation of these disturb- 

 ances. 



It could hardly be expected that such a wind disturb- 

 ance could produce a magnetic storm of more than a 

 local character. But it is not at all improbable that tor- 



