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 cm., the angular change being thus 28.4 minutes of arc. 

 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 

 scale 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 scale 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 



