[ ] 
arrive at their greateft height, which mui^ always 
be in proportion to their firft velocity j and then they 
mud: defcend by the laws of fpedfic gravity, as all 
other bodies do. The velocity, with which we gene- 
rally fee vapour afcend, is not fufficient to carry it a 
few yards in height j and therefore, if there were no 
other caufe of afcent but impulfe, the vapours and 
exhalations mud be pretty equably diffufed over the 
furface of the earth, afcending to a fmall height, and 
immediately defcending again ; or if it were poffible 
for them to float at that height, the confequence mud 
be a perpetual fog. How far the great ufes of viflon 
would be obdruded or lod in this cafe, I need not 
mention; nor need I fay, how detrimental it would 
prove to refpiration, fince mankind have agreed to 
call fogs unwholfome. However, when I come to 
fpeak of the ufes of eledrical fire in animal life, I 
diall fay fomething on this fubjecl. 
However necelTary an impulfive power may be in 
the fird emifiion of vapour and exhalation from their 
maflTes, I think it evident, from the flownefs of their 
afcent, that it cannot carry them to that height, 
where clouds are ufually formed ; much lefs is it 
poflible, that it diould be the caufe of their floating 
there for many days, as we fee they do. This can 
only be done by an alteration of their fpecific gravity, 
till they are brought into an aequilibrium with part of 
the atmofphere where they float. 
It now remains to inquire, by what means this 
may be done ; fince neither impulfe, rarefadlion of 
the air, or any formation of their parts by expanfion, 
( which I know of) feem fufficient for the purpofe. 
S 2 
There 
