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 em. in the reading during a 
two-minute interval indicates a change of 3x2.84 minutes 
of are 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 XLITI. 
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. 
