C ni ] 
6. That after very great ftormes of Wind, when the 
Qjlickfilver has been low, it generally rifes again very faft. 
7. That the more Northerly places have greater altera- 
tions of the Barofcope than the more Southerly. 
8. That within the Tropicks and near them, thofeac-^ 
counts wee have had from others, and my own Oblervati- 
on at St. Helem make very little or no variation of the 
hight of the Mercury m all weatliers : Now that Theory that 
can well account for all theft Appearances, will in all pro- 
bability approach nearer the true Caule of the Barometers 
variations, than any thing hitherto offered ; and fuch an 
one I am bold to believe, is that which I here lay down, 
with fubmiflion to better Judgments. 
I conceive that the principal Caufe of the rife and fall of 
the Mercury y is from the Variable Winds, which are found 
in the Temper a,te Xones^ and whole great unconftancy here 
in Enghnoi is moft notorious : I fhall not at prefent inquire 
into the Caufe of its uncertainty, but the matter of Fad be- 
ing moft undoubted, the Legitimate Confequences thereof 
muft be allowed me \ let it proceq^ from what it will. 
A Second Caufe is the uncertam Exhalation and Pre- 
cipitation of the Vapours, lodging in the A/>, whereby it 
comes to be at one time much more crouded, than at ano- 
ther, and confequently heavier j but this latter in a great mea- 
fure depends upon the former. Now from thefe Principlep^ 
I fhall endeavour to Explicate the feveral Fh^nomenA of t^ 
Barometer^ taking them in the fame Order, I layd them 
down. 
I. Why in Calm Weather ^ the Air being inclined to Kain^ the 
Mercury is commonly low f I Anfwer that, the Mercury s be- 
ing low, enclines it to Rain, for the Kir being light, the 
Vapours are no longer fupported thereby, being become 
ipecifically heavier than the Medium wherein they floated ; 
fo that theydefcend towards the Earth, and in their fall 
meeting with other aqueous Particles, they incorporate to- 
gather and forme little drops of Rain j but the Mmurys be- 
