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 lbs. 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 cm. 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 arc. The zero reading was at 

 the middle point of the scale. 



