[ *45 3 
cent vapour, and make room at the top for other 
vapours to flow in from all fides, whi.h, by their 
meeting, may alfo coalefce and fubfide to form this 
central defcending cloud, and fo alfo of like fucceed- 
ing vapours. And thus, I think, clouds of any den- 
fity may be formed in a quiet air (and thunder gene- 
rally happens in calm weather), till the denfity is too 
great for the buoyant eledrical fire to fuftain the 
weight, which flying off, in part, from the largefl 
drops, lets them fall almofl: in fpouts. 
Why the mercury falls and rifes in the barometer, 
will appear from what I have been faying j for in 
long continued fair dry weather, there is a great 
quantity of vapour railed, and kept afloat in the 
air ; whieh muft neceflarily add a column to the 
air at leaf! a thoufuid times greater than the vapour 
taken up j and often much greater than that ; which 
muft as neceflarily increafe the weight of the air, 
and make the mercury rife proportional to the addi- 
tional column ; and when rains defeend, they fub- 
ftrad a column from the air, near a thoufand times 
as great as the water fallen, whieh mud neceflarily 
leffen the weight of the air, and make the mercury 
fall proportional to fuch fubtradion. As to the 
winds, they affed: not the mercury, but as they are 
generally, in thefe climates, a confequence of de- 
fcending clouds and rains, blowing into thofe parts, 
where the column of air has been leffened, and while 
they blow, the mercury will continue low in the ba- 
rometer, till the equilibrium of the atmofphere be 
reflored. But, I believe, if it were inquired into, 
whether the mercury defeends, when the wind begins 
to blow every morning between the tropics, and the 
VoL. 45>. , U vapours 
