Father Boscovich has determined ithe height of an aurora borealis, observed on the 16th of December, 1737, by the Mar quis of Poleni, to have been 825 miles ; and Mr. from a mean, of thirty computations, makes the average height of the aurora borealis to be 72 Swedish, or (supposing a Swedish mile to be about 6i English miles) 468 English miles. Eu ler supposes the height to be several thousands of miles ; and Mairan also as signs to these phenomena a very elevated region, the far greater number of them being, according to him,about 200 leagues above the surface of the earth. Dr. Blag den, speaking of the height of some fiery meteors, (Phil. Trans. vol. lxxiv. p. 227,) says, that" the aurora borealis appears to occupyas high,if not a high er,regionabove the surface of the earth, as may be judged from the very distant countries to which it has been visible at the same time;" he adds that " the great accumulation of electric matter seems to lie beyond the verge of our atmosphere, as estimated by the ces sation of twilight." However, the height of these meteors, none of which appear to have ascended so high as 100 miles, is trivial, compared with the elevations above ascribed to the aurora borealis.
rations on this phenomenon as are suffi cient to afford a just estimate of its aid. tude, they must be subject to a consider able variation, and to material error.
It is not improbable, that the highest regions of the aurora borealis are the same with those in which fire-balls move; more especially as Dr Blagden informs us, that instances are recorded, in which the northern lights have been seen to join, and form luminous balls, darting about with great velocity, and even leav ing a train behind like the common fire balls. Thisingenious author,however,con jecturing that distinct regions are allotted to the electrical phenomena of our atmo sphere, assigns the appearance of fire. balls to that region which lies beyond the limits of our crepuscular atmosphere ; and a greater elevation above the earth to that accumulation of electricity,in a light er and less condensed form, which pro duces the wonderfully diversified streams and coruscations of the aurora borealis.