046 PHILOSOPHICAL TUANSACTIONS, [aNNO I79O. 



and acquaintance with the fixed stars are required to determine a path by them 

 so nearly. 



The direction of the arch here described, in that part near the zenith, is w. 

 18° s.; and if a line be drawn through Cambridge in this direction, Kimbolton 

 is 12.8 geographical miles north of it; and therefore, as the arch appeared 11° 

 more south at Kimbolton than at Cambridge, the height of the arch above the 

 surface of the earth must be 6 14. geographical or 71 statute miles. If we sup- 

 pose that the middle of the arch really passed through jS Aurigse, the height 

 comes out 52 statute miles. On the whole the height could hardly be less than 

 52 miles, and is not likely to have much exceeded 7J- 



The common aurora borealis has been supposed, with great reason, to consist 

 of parallel streams of light shooting upwards, which, by the laws of perspective, 

 appear to converge towards a point; and when any of these streams are over our 

 heads, they appear actually to come to a point, and form a corona. Hence, from 

 analogy, it seems not unlikely, that these luminous arches may consist of parallel 

 streams of light, disposed so as to form a long thin band, pretty broad in its up- 

 right direction, and stretched out horizontally to a great length one way, but 

 thin in the opposite direction. If this is the case, they will appear narrow and 

 well defined to an observer placed in the plane of the band; but to one placed at 

 a little distance from it, they will appear broader, fainter, and less well defined; 

 and when the observer is removed to a great distance from the plane, they will 

 vanish, or appear only as an obscure ill-defined light in the sky. 



There are two circumstances which rather confirm this conjecture: first, that 

 though we have an account of another arch besides this* having been seen at 

 great distances in the direction of the arch, we have none of any having been 

 seen in places much distant from each other in the contrary direction ; and 2dly, 

 that most of them have passed near the zenith, whereas otherwise they ought 

 frequently to appear in other situations; for if they appeared near the zenith to 

 an observer in one latitude, they should appear in a very different situation in a 

 latitude much different from that. Mr. C. however, does not offer this as a 

 theory of which he is convinced; but only as an hypothesis which has some pro- 

 bability in it, in hopes that by encouraging people to attend to these arches, it 

 may in time appear whether it is true or not. If it should hereafter be found, 

 that these arches are never seen at places much distant from each other in a di- 

 rection perpendicular to the arch, it would amount almost to a proof of the truth 

 of the hypothesis; but if they ever are seen at the same time at such places, it 

 would show that the hypothesis is not true. Supposing the hypothesis to be well 

 founded, the height above determined will answer to the middle part of the band, 

 * That of Feb, 15, 1750. Phil. Trans, vol. 46, p. 472 and 647.--Orig. 



