Nipher—Magnetic Disturbances. 157 
although it had not been exposed to any explosion. The 
effect due to an explosion of 21 sticks, 200 ft. to the north 
of the needle, is shown in Plate XL. The arrow indicates 
the instant at which the explosion took place. The in- 
tensity of the field was increased at first. The change 
in position caused the scale reading to change by about 
1 em., the angular change being thus 28.4 minutes of are. 
The highest reading was reached about one minute after 
the explosion. This was followed during the next min- 
ute by a decrease in H, to a value less than that just 
preceding the explosion. At the end of two minutes the 
value of H had returned to that which it had at the time 
of the explosion. This disturbance was attended by to 
and fro vibrations of the needle, which did not exist be- 
fore the explosion. 
On the same day, shortly after 11 a. m., when the nee- 
dle had been indicating for half an hour a steady rise in 
the value of H, an explosion of 20 sticks was followed 
by a series of vibrations which continued for twelve min- 
utes. The amplitude increased for four minutes, and the 
Vibrations gradually ceased at the end of twelve min- 
utes. The maximum angle of oscillation, four minutes 
after the explosion, was 114 minutes of arc, or forty 
Seale divisions. The mean reading for each oscillation 
Showed a rise in the value of H, during the first four 
minutes, the average reading changing by twenty scale 
divisions. When the vibrations ceased the reading dif- 
fered from that at the time of the explosion by four scale 
divisions, the field being slightly weaker. 
When ten sticks of dynamite were exploded there was 
no appreciable effect upon the needle. It did not exceed 
two seale divisions. When six sticks were exploded, the 
needle having been very quiet, the needle continued to 
move uniformly, and without oscillations. The field was, 
however, made weaker. Two minutes after the explo- 
Sion, the change in reading was eight scale divisions. The 
field then began to increase in strength, and five minutes 
after the explosion it had regained the value which it 
