52 ELECTRICAL APPARATUS. 



ELECTRICAL APPARATUS. 



To ascertain whether any change takes place in the electrical 

 Btate of the atmosphere during the appearance of an aurora, the 

 end of a long insulated wire, suspended from two high masts or 

 two chimneys by means of silk threads, may be placed in con- 

 nection with a delicate gold leaf electrometer. Any change in 

 the electrical state of the atmosphere, simultaneous with the 

 aurora, will be indicated by the divergence of the leaves. Two 

 slips of gold-leaf attached by a little paste to the lower end of a 

 thick wire, passing through a cork in a four-ounce vial, will 

 answer for this purpose. The arrangement of the leaves will be 

 best made by a bookbinder, who is expert in the management 

 of gold-leaf. 



[A continuous series of photographic registers of the motion 

 of the magnetic needle is now kept up at the joint expense of 

 the Coast Survey and this Institution, which will serve for 

 comparison with any observations which may be made on the 

 aurora. 3 



Prof. Olmsted, in a recent paper published by the Smithsonian 

 Institution, classifies different auroras as follows : — 



"Class I. This is characterized by the presence of at least 

 three out of four of the most magnificent varieties of form, 

 namely, arches, streamers, corona, and waves. The distinct 

 formation of the corona is the most important characteristic of 

 this class ; yet, were the corona distinctly formed, without auroral 

 arches or waves, or crimson vapor, it could not be considered as 

 an aurora of the first class. 



"Class II. The combination of two or more of the leading 

 characteristics of the first class, but wanting in others, would 

 serve to mark class the second. Thus the exhibition of arches 

 and streamers, both of superior brilliancy, with a corona, while 

 the waves and crimson columns w^ere wanting, or of streamers 

 with a corona, or of arches without a corona, without streamers 

 or columns (if such a case ever occurs), we should designate as 

 an aurora of the second class. 



"Class III. The presence of only one of the more rare cha- 

 racteristics, either streamers or an arch, or irregular corusca- 

 tions, but without the formation of a corona, and with but a 



