210 Mr Potter on the Aurora Borealis. 



made observations on 316 aurorae, and wrote a treatise wherein 

 these observations are recorded ; but which I have not yet 

 met with. 



The celebrated Mayer wrote also in the year 1726, pretty 

 fully on the aurora in the first volume of the Commentarii 

 Petropolitance ; and I was not a little surprised to find that 

 he had proposed a formula for finding the elements of an 

 arch from one observation of its altitude and its extent on the 

 horizon, bv considering it as a circle parallel to the equator. 

 The reader will see that this is similar to the method I have 

 proposed, only that he was ignorant of the relation of the 

 meteor with the earth's magnetism. When his formula is 

 adapted to this discovery, we may find the distance of an arch 

 by it in the first place, and knowing this, we easily find the 

 height. The results, as I have proved, are just the same as 

 by mine. He there gives only the formula without the de- 

 monstration; this last was, however, supplied by Maupertuis 

 in the Histoire de V Academic Royale des Sciences, for 1732. 

 Mayer laid his demonstration before the Petersburgh Aca- 

 demy in 1728, but, as their Transactions were not published 

 until some years afterwards, Maupertuis does not appear to 

 have been aware of it. 



In the year 1731 Mairan, who had previously written seve- 

 ral memoirs, published an extensive treatise on the aurora, 

 which stands as a supplementary part to the Hist, de VAcad. 

 for that year. He was much occupied with hypothetical 

 views, that the aurora was caused by the zodiacal light, and 

 that this latter was no other than the sun's atmosphere ex- 

 tendino- beyond the orbit of the earth. He gives an extensive 

 catalogue of aurora? which had been observed from very early 

 times, and several very good plates of the different appearance 

 of the meteor. He enters also very fully on the subject of its 

 height, and made calculations for several observations on the pa- 

 rallactic or trigonometrical method ; and in one case he made 

 use of the formula of Mayer. He found the height of the 

 aurora of the 19th October 1726, to be 70 French leagues 

 above the surface of the earth : but in general his calculations 

 brought out the height about 200 leagues, some few being as 

 low as 100, and some as high as 300 leagues. 



