Nipher — Magnetic Disturbances. 159 



cottage. The observing station was a few rods to the 

 left of the view here shown. The path shown in the fore- 

 ground turns to the left past the station. 



Observations were made upon the effect of the fog- 

 horn during the entire summer. It soon became evident 

 that no effect could be expected upon days when the sky 

 was covered with clouds. The most marked effects were 

 observed in the afternoon on clear days and when the air 

 was quiet. 



The blast of the fog-horn was sometimes followed by 

 vibrations of the needle. Such a result is shown in 

 Plate XLII. Such vibrations occur which are due to other 

 causes, and it may well be that the air disturbance due 

 to the blast of the horn is the final agency, which, super- 

 posed upon other causes, brings about rhythmical varia- 

 tions in the earth's field. 



The usual effect upon the needle was a slow deviation 

 in a direction indicating a decrease in the intensity of 

 the field. In no case was the blast of the horn followed 

 by an apparent increase in the intensity. Plate XLI gives 

 a fair representation of the effect usually observed. Here 

 the drop from 11.1 to 10.8 cm. in the reading during a 

 two-minute interval indicates a change of 3x2.84 minutes 

 of arc in the position of the needle. On two occasions 

 the sunset disturbance followed at once. On Aug. 25 

 these oscillations, which represent this disturbance, began 

 about an hour before the time when they were usually 

 observed to begin. They are represented in Plate XLIII. 



The period of oscillation was nearly uniform and the 

 scale reading was taken at the extreme of each oscilla- 

 tion. If this diagram were plotted to the same scale as 

 those which precede, these oscillations would have five 

 times the amplitude shown in this diagram. At this 

 time the sky was clear, and there was no appreciable 

 wind. 



The effect of dynamite explosions at a great distance 

 from the explosions may be expected to resemble that 

 due to the fog-horn. 



