A Difeourfe of the Rule of the decreafe of the hight of 
the Mercury in the Barometer^ according as places 
are Elez^ated ahoz^e the Surface of the Earthy with 
an attempt to difcowr the true reafon of the Kiftng 
and Falling of the Mercury ^ upon change of Weather. 
By EDM. HAL LET. 
THe Elaftick property of the Air have been long 
fmce made out, by Experiments before the R, Socie- 
ty and elce where ; and the Refiftance of its Spring is found 
to be nearly equal to the Weight or Force that comprefles it \ 
as alfojthat the fpaces the fame Air occupies, under differing 
Preffures are Reciprocally as thofe PrelTures : it has been 
fiiown Hkewife by undoubted Experiment, that theSpeci- 
fick Gravity of the Air, near the Earths Surface to that of 
Water, was once as i to 840, again as i to 8 52, and a third 
time, in a very large Veffell holding ten Gallons, as i to 
860 \ all which, confidering the difficulty of the Experimejjt 
agree well enough, the Mercury ftanding at all thofe times 
about 29 Inches \ ; but by reafon twas Summer Weather and 
confequently the Air rarified when all thele were tryed, we 
may without fenfible Errour fay in round Numbers, that 
the Barometer ftanding at 30 Inches^ and m a mean ftate of 
Heat and Cold, the Specifick Gravity of the J/;^ to Water, 
is as I to 800: By the hk:e Tryals the weight of Mercury to 
W ater, is as I'j h. to i, or very near it, fo that the weight 
of Mera^ry to Air, is as 10800 to i, and a Cylinder of Air 
oi 1 0^00 Inches ov goo Feet, is equal to an hch of Mercury ^ 
and were the Air of an equal denlity like Water, the whole 
Atr/iofhere would be no more than 5, i Miles high, and in 
t!ie Afcent of every goo feet the Barcmeter would fink an 
Inch. But the expanfion of the Air encreafing in the fame 
proportion as the incumbent weight of the Atmoffhere de- 
creafes, that is as the Mercury in the Barometer finks, the up- 
per parts of the Air are much more rarified than the lower, 
and 
