158 PHILOSOPHICAL TRANSACTIONS. [aNNO 1726. 



Hence, 1. It plainly appears, from the position of the arches, that they could 

 not owe their figures to the sun : they seem to have been partly optical, and 

 partly to have depended on the different lieights of the luminous vapours ; but 

 for want of sufficient data, it will be no easy matter to determine how far each 

 of these causes concurred. 2. The prismatic colours, wherever they appeared, 

 seem to have been caused by the sun. 3. None of the streams proceeded 

 directly from the horizon. They were nearest it towards the north, where 

 there were some weak irregular lights in the confused parts of the arch before 

 described. 



An Account of the same Phenomenon at Plymouth. By Dr. Huxham, Physician 

 at Plymouth. N° SQa, p. 137, 



As the particulars of this description are nearly the same as in the foregoing 

 articles, it is needless to repeat them. 



Dr. Huxham at last adds, as to the weather, the morning was fair, though 

 the air was thick, and we had a great dew: the mercury was at 30 inches, 

 Hauksbee's thermometer at 50, little or no wind. The day was pleasant and 

 warm, and the air grew much thinner. The evening was serene ; a very soft 

 breeze from n. and by w. About 5 the next morning, there were several 

 clouds formed, and the air was very thick and hazy, at 7 it was all cloudy, and 

 a few drops fell. 



Though I had before seen several faint appearances of the aurora boreatis ; 

 yet this, for beauty, lustre and duration, vastly exceeded any thing of that na- 

 ture I had ever seen. Indeed, I saw not that of March 6th, 1716, not being 

 then in England. 



On the same Subject. By Dr. Hallet, Physician at Exeter. N° 395, p. 143. 



Exon, Oct. 12, 1716. 



Oct. 3, 1726, at 9 in the evening, I saw an aurora borealis, in which there 

 was nothing different from former appearances, excepting that from the lumi 

 nous arch which appeared in the north, were frequently shot oft' parts of arches 

 towards the zenith, which vanished there. 



After describing the appearances more particularly. Dr. Hallet concludes with 

 the following reflections as to the cause of the phaenomena. 



The best account I can give of this phenomenon is this : I imagine a thin 

 cloud composed of a sulphureous exhalation, hanging over us in the air, at a 

 considerable height, parallel to the horizon ; the length of it being very great 

 from east to west nearly ; the breadth of it at (irst not so great, but that we 



