Mr Potter on the Aurora Borealis. 209 



Such are the results I have been able to obtain, entirely con- 

 firming my former conclusions. The inquiry is certainly one 

 of a very troublesome nature, and where it is difficult to feel 

 perfectly satisfied. The principal obstacle is the want of a 

 thermometer at once sufficiently sensible and accurate. Those 

 who are practically conversant with air-thermometers, will pro- 

 bably be ready to admit that there is much uncertainty attend- 

 ing their indications. It is to the improvement of mercurial 

 thermometers that we must look for the extension of our know- 

 ledge on this and all questions connected with radiant heat. 

 To secure at once an instantaneous communication of the ef- 

 fect, a large range on the scale corresponding to a small degree 

 of heat, and a perfect freedom of motion to the liquid in the 

 tube, are the desiderata in instruments for these purposes, 

 whilst the graduation is wholly irrelevant. It is to be hoped 

 that some of our philosophical artists will direct their attention 

 to this point. 



I cannot conclude without repeating my earnest hope that 

 these experiments will be tried by others better qualified to do 

 justice to them. By the multiplication of observations, we can 

 alone hope to be able to speak with confidence as to investiga- 

 tions of so delicate a nature. — I remain, my Dear Sir, very 



truly yours, 



B. Powell, 



Oxford, July 24, 1831. 



Aut. IV. — On the General Equations for determining the Po- 

 sitions and Curvatures of Arches of Aurora Boreales, from 

 observations made in one place only ; with Calculations for 

 that if the 25th December 1830. By R. Potter Junior, 

 Esq. Communicated by the Author. 



Since writing my former essay on this subject, I bave taken 

 some little pains to learn what has been done by others. In 

 the beginning of the eighteenth century, the meteor appears 

 to have occurred very frequently, and attracted the attention 

 of many scientific men. Amongst these, the celebrated Swe- 

 dish philosopher Celsius, commencing about the year 17 Hi, 



