436 Computed Height [Book V, 



merits, the air ought to have had, it was fufpected that 

 the upper parts of the atmofpherical regions were not 

 fubject to the fame laws with the lower ones. Philo- 

 fophers, therefore, had recourfe to another method for 

 determining the altitude of the atmofphere, viz. by a 

 calculation of the height from which the light of the 

 fun is refracted, fo as to become vifible to us before 

 he himfelf is feen in the heavens. By this method 

 it was determined, that at the height of forty-five 

 miles the atmofphere had no power of refraction ; and 

 confequently beyond that diftance was either a mere 

 vacuum, or the next thing to it, and not to be re- 

 garded. 



f This theory foon became very generally received; 

 and the height of the atmofphere was fpoken of as fa- 

 miliarly as the height of a mountain, and reckoned to 

 be as well afcertained, if not more fo, than the heights 

 of mod mountains are. Very great objections, how- 

 ever, which have never yet been removed, arife from 

 the appearances of fome meteors, like large globes of 

 fire, not unfrequently to be feen at vaft heights above 

 the earth. A very remarkable one of this kind was 

 obferved by Dr. Halley in the month of March 1719, 

 whofe altitude he computed to have been between 

 fixty-nine and feventy-three and a half Englifti miles ; 

 its diameter two thoufand fight hundred yards, or up- 

 wards of a mile, and a half, and its velocity about 

 three hundred and fifty miles in a minute. Others, 

 apparently of the fame kind, but whofe altitude and 

 velocity were ftill greater, have been obferved, parti- 

 cularly .that very remarkable one, Auguft i8th, 1783, 

 whofe diftance from the earth could not be lefs than 

 ninety miles; and its diameter not lefs than the former, 

 at the fame time that its velocity was certainly not lefs 

 than one thoufand miles in a minute. Fire-balls, in 



appearance 



