Nipher—Magnetic Disturbances. 155 
ture rested upon six posts, which were set two feet into 
the ground. It was held together by means of brass bolts 
and copper nails. It was so rigid that the impact of the 
body of a man against it produced no appreciable effect 
upon the needle, 
When ignited in open air, the black powder burned 
rather slowly. The best results were obtained when 25 
or 30 lbs. of the powder, spread uniformly over a plat- 
form of 20 square feet, was ignited directly east or west 
of the needle, and at a distance of a foot from the side 
of the tent. Boards were placed on end between the tent 
and the platform in order to protect the tent. The fly 
above the roof of the tent was moistened by a spray of 
water. In order to protect the magnets from any possi- 
bility of heat effects two curtains, formed of heavy blank- 
ets padded with cotton, were hung within the tent. They 
were so placed that an air-space of five or six inches was 
between the two curtains. The chair of the observer was 
inside of the middle of the inner curtain and between the 
flame and the magnetic needle. There was no explosive 
effect due to the burning of the powder. It required a time 
of about 30 seconds for 30 Ibs. of powder to burn. The 
flame shot upward to a height of about 20 feet. 
On the day when the results to be given were obtained, 
there was a very faint wind from the north. The sky 
was clear. A huge column of smoke drifted slowly up- 
ward. It practically coincided in direction with the lines 
of the earth’s magnetic field. The effects produced were 
of the same order of magnitude as were often observed, 
due to other and unknown causes. 
Plates XXXVIII and XXXIX are fairly representa- 
tive of the results. The hour of the day is laid off on the 
horizontal axis. On the original diagram 5 em. represent 
one minute of time. 
On the vertical axis the reading of the position of the 
needle is represented, 5 cm. representing 10 scale di- 
visions or 28.4 minutes of are. The zero reading was at 
the middle point of the scale. 
