226 Mr William Sturgeon on the Aurora Borealis. 



spring from some definite speck in the heavens, and wax gradually 

 to their full dimensions, and then as gradually fade away. Some 

 streamers, it is true, shoot rapidly to their full growth, and almost 

 as suddenly disappear ; but in all cases, they can be seen expanding 

 lengthwise, whatever may be the rapidity of their growth ; they some- 

 times lengthen in both directions, but most frequently in one direc- 

 tion only, a circumstance more favourable to the idea of the nubifer- 

 ous bands being the media of transit, than in the capacity of trans- 

 parent screens, permeated by an electric light from above. 



In many displays of the aurora, floods of streamers appear to flow 

 upwards, from every part of a luminous bow, which crosses the 

 meridian in the north, and stretch to various angles of altitude to- 

 wards the spectator ; some of them reaching to his zenith, whilst 

 others terminate their career before they arrive midway, but in no 

 instance do streamers spring into existence mature or full grown. 

 This fact also gives countenance to the idea of their consisting of 

 streaks or bands of thin aqueous vapour, gradually, though rapidly 

 in some cases, illuminated longitudinally, by transmissions of the elec- 

 tric fluid. This view is still further supported by the fact that, in 

 whatever direction streamers may be elongated, the point from which 

 they spring is the most intensely luminous of the whole, and becomes 

 the base of the group ; from this base or starting point, the light be- 

 comes more and more attenuated, until at last it softens gradually 

 and melts into the normal light of the sky and is lost. This gradual 

 decay of brilliancy during the progress of streamers, from their birth- 

 place to their terminal points, has every appearance of a gradual 

 dispersion, and consequent attenuation of the electric fluid, as it flows 

 along the aqueous conductors, until eventually it becomes so enfeebled 

 as to be incapable of displaying a sufficiency of light to be traced by 

 the eye, any further in its progress towards its destination. Hence 

 also, the different distances to which streamers reach from their re- 

 spective birth-places ; some fade away and are lost within a range of 

 a iew degrees, whilst others progress through an immense span in 

 the heavens, but in all cases terminating in nearly the same manner. 



Nor are these the only indications of the aurora being within the 

 limits of the atmosphere. The colour of the meteor is not that of 

 an electrical light in a vacuum, nor in very highly attenuated air ; 

 but such as would be produced by floods of the electric fluid amongst 

 attenuated aqueous vapour. The auroral light is that of a pure 

 candle flame, and sometimes of a silvery white, neither of which can 

 be imitated by electrical transmissions through a vacuum. 



Philosophers have long been endeavouring to ascertain the height 

 of the auroral arch, when displayed in the northern heavens, but 

 hitherto, no two of them have arrived at similar conclusions. Some 

 have supposed it to be only a ^evf miles above the earth's surface, and 

 others have given it a height of above a thousand miles. From some 

 calculations made by the late Dr Dalton, he infers that " the height 



