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hypothecs about the formation of fuch bodies, which 
makes them to confiffc of certain fulphureous vapours 
arifing from the earth : for, befides what has been 
urged above, Dr. Halley has fhewn, “ That, at the 
te height of 41 miles, the air is fo rarified as to take 
<£ up 3000 times the fpace it occupies on the furface 
<c of the earth ; and that, at 5-3 miles high, it would 
“ be expanded above 30000 times : but thinks it is 
<c probable, that the utmoft power of its fpring cannot 
“ exert itfelf to fo great an extenfion ; and that no 
tc part of the atmofphere reaches above 45- miles 
This being the cafe, how can we fuppofe any fuch 
vapours to rife to the height of 90 or 100 miles, 
where the air muft be fo many millions of times rarer 
than what we breathe ? Again, we find that all 
vapours, in their afcent, mutually repel one other; 
fo little do they feem difpofed to run together into 
globes of fuch a fize. But Dr. Halley obferves (in 
treating of the great meteor of March 1719), “ That 
“ tho’ the aqueous, and moft of the other vapours, 
“ are foon condenfed by cold ; and wanting a certain 
“ degree of fpecific gravity in the air to buoy them 
“ up, afcend but to a fmall height;” yet he adds, 
in favour of the opinion of thefe bodies being formed 
of exhalations, “ That the inflammable fulphureous 
i( vapours, by an innate levity, have a fort of vis 
centrifuga , and not only have no need of the air 
“ to fupport them, but, being agitated by heat, will 
“ afcend in vacuo Boileano .” From hence he infers, 
“ That the fulphureous vapours, difengaged from all 
‘ £ other particles, may rife far above the reputed 
•* Phil. Tranf. N° 181. p. 104. Abridg. Vol. II. ch. 1. § 9. 
“ limits 
