Mr Potter on the Aurora Borcalis. 217 



Therefore, taking e = trig. tang. 20°, /= trig, tang. 61°, 

 and the magnetic polar distance of the place of observation 

 being 41°, we have g — trig. tang. 49°. From these data I 

 find its distance to have been 412.5 miles, its height 159-4 

 miles. 



In looking over the history of the aurora, we see that the 

 calculations of Mairan, Cavendish, and Dalton, on various 

 trigonometrical bases, of from about thirteen miles to some 

 hundreds, agree with mine in placing the meteor always at a 

 great elevation, without a single opposing measurement from 

 good observations in favour of the contrary opinion. The 

 lowest altitude determined, namely, that for the aurora observ- 

 ed by Cavendish, had still a locality such that it is clear it 

 must be ascribed to other causes than the winds and clouds of 

 the denser part of the atmosphere. 



Though, from its position, we are hardly ever likely to be 

 able to demonstrate actually in what these causes consist, we 

 may yet, by comparing the different theories which can be 

 advanced, form our choice of the one which is most consonant 

 with our previous knowledge and the laws of nature hitherto 

 established. And as my own opinion, founded on observation 

 and comparison with other natural phenomena, is somewhat 

 different from any I have seen, I shall deem no apology neces- 

 sary in here submitting it. 



I have been struck with the similarity of the light general- 

 ly called electrical in the vacuum of a mercurial barometer and 

 that of the aurora. The exhibition of this light has general- 

 ly been considered a test of the goodness of the instrument. 

 But it may be seen very completely in the comparative baro- 

 meter attached to the condensing steam-engine, which general- 

 ly in a large long-stroke engine, oscillates between 1. and li 

 inches every stroke of the piston, and about two to three inches 

 of mercury below the atmospheric pressure. When the mer- 

 cury is falling in the tube there is seen a pale flickering light, 

 which ceases when the mercury commences to rise again. That 

 is, the light is seen only when the vacuum (at u.scd in coin- 

 tiion parlance) is becoming worse and the mercury falling. 



In whatever way we may explain this phenomenon, the 

 light of the aurora has a very great resemblance to it. 



