Mr William Sturgeon on the Aurora Borealis. 233 



tare below, that succeeds these phenomena, as well as the immense 

 hail-storms that attend lightning, infer that the change of tempera- 

 ture commences far above the earth's surface, and that it progresses 

 downwards with various degrees of speed. 



The observations which I have myself made on the predisposition 

 of clouds for a display of the aurora borealis, are similar to the ob- 

 servations of Captain Back at Fort Reliance, (north latitude 62° 46', 

 and west longitude 109°.) "The aurora was frequently seen at 

 twilight, and as often to the eastward as the westward. Clouds, also, 

 were often perceived in the daytime, in form and disposition very 

 much resembling the aurora." 



The same scientific officer observed also, that " a dense fog, in 

 conjunction with an active aurora, was uniformly favourable to the 

 disturbance of the needle ;" and, " when seen through a hazy at- 

 mosphere, and exhibiting the prismatic colours, almost invariably 

 affected the needle." These observations are of great interest, both 

 as regards the decomposition of the electrical light, and the produc- 

 tion of magnetic disturbance : shewing, in my opinion, that the fog 

 was essential in the pi-oduction of colours, as well as to the transit of 

 a portion of the electrical fluid at no great distance from the needle ; 

 and this view is strongly supported by the opposite effects of the 

 aurora when no fog or haze was present. " On the contrary," says 

 Captain Back, " a very bright aurora, though attended by motion, 

 and even tinged with a dullish red-yellow, in a clear blue sky, seldom 

 produced any sensible change (of the needle) beyond, at most, a tre- 

 mulous motion."* 



There is a great difFei-ence in the character of aui'oral displays, 

 scarcely any two being alike : some of them appear to be of such a 

 complex and mysterious character as to bid defiance to scientific in- 

 vestigation : whilst others develop a peculiarity of features that can 

 hardly be misunderstood ; and may, with propriety, be considered as 

 keys of admission to the whole. Amongst the latter may be enume- 

 rated those in which are observed a predisposition of nubiferous mat- 

 ter during daylight, — the luminous streaks or bands of vapour, — the 

 waves of light that shine across the eye of the spectator, — the hazy 

 character of the atmosphere, — and also the transcolourations of the 

 light; all of which have appeared in unusual abundance during the 

 past season, or since the commencement of last autumn. 



Dr Halley gives a very precise account of the appearance of lu- 

 minous vapour, haze, and streaks of light, in the aurora of 16th March 

 1716. This eminent philosopher tells us, that he did not see the 



* Mr Dancer, oj)ticia:i, of Manchester, whilst observing the white light of 

 one of the aurorae described in this paper, breathed upon a pane of glass through 

 wliicli lie was looking, and immediately the prismatic colours appeared. The 

 game effect is produced to passengers travelling in a close coach, and looking at 

 the gas-liglits through a window covered with aqueous particle.s from breath- 

 ing ; a beautiful prismatic iris is seen around the burning gas. 



