The Portland gazette. [volume] (Portland, Me.) 1818-1824, 1823-07-01
PHILOSOPHICAL ESSAY.
OF THE BAROMETER.


PHILOSOPHICAL ESSAY.
OF THE BAROMETER.
On the pressure Of the air depends the con- |
struction and peculiar properties of the Baron |
eter, of which we have already mentioned. Of
which we shall proceed to give short account |
The engineers of the Count de Medici, Great |
Duke of Florence, having been requested to
raise water so or 60 feet, by means Of common
pump, perceived, when they made the attempt,
that water would raise only to certain height,
after which it appeared to them that nature was
reconciled to vacuum or at least suffered
these defects without producing those terrible
efiectsdescribed by the ancients. This was CON-
municated to Cables, who attended to the sub
ject. He was at length convinced by repeated
experiments that water would rise only to about
80 feet perpendicular in pumps, and that the re-
mainder OF the tube if it was longer would be
empty. He thus began to conceive that natures
adherence ola vacuum had its limits, and that
these phenomena might proceed from physical
cause, very different from that to which they
had hitherto been attributed.--His pupil Tori
cell, proved this by actual experiment. This
was In the year 1916, when he found that a COL
umn of air, as il exists In the atmosphere NAV be
placed in equl poise with a column of another
fluid which has the same base; at length to avoid
| the Inconvenience Of long pipe, instead DIVA
| ter, he made of mercury. He took glass
tube of about three feet in length, and two or
three lines in diameter, hermetically sealed al
one end and open at the other; be filled it with
pure mercury, and having stopped the orifce
with his finger reversed the tube and placed the
open end in vessel full of the same mercury
He had no sooner removed his finger than the
| column of mercury which was about SO inches
long, was reduced to the length of about % in
cEes. Now if we compare the experiment of
Calico with that Of Torricelli, we shall find that
fluids act in counterpoise to each other, exactly
in proportion to their respective densties; and
that the pressure of the air which elevates water
to the height Of 80 feet, cannot sustaina column
of mercury above the height of 90 or 80 inches.
Paschal added considerably to the proofs of
| this doctrine which Torricelh had afforded bis
reasoning was thus; if the air is the cause of
| this phenOmenon il is because it has weight and
fluidity; it must press, therefore, in the same
manner as liquid, and its pressure must be great
er or less according to its height; and every
column ofwhatever fluid is placed iD counter
| pose with IT, viii always be longer or shorter in
proportion TO its density. Hence he Inferred
that column of air must produce a pressure
greater or less, and was capable of sustaining a
column of any fluid higher or lower in proper
| ton to its own height consequently that column
of water or mercury at the bottom of moun-
tain. would rise higher in the Torrcelian vacu-
UNI than at the summit.
M. Perrier fixed tube ofTorricellis upon
| perpendicular plank graduated into inches and
lines and having observed to what height the
mercury was raised in the tube at the foot of the
mountain, be found that it fell gradually in pro-
/ portion as he ascended towards the summit, on
the contrary, it rose again 10 proportion as he
| descended; the difference was found to be three
| inches and one line between the height Of the
mercury at the summit aud at the base. This
experiment being often repeated, produced the
same result, whence it was concluded that mer-
curry was sustained above IFS level in the tube
by the pressure Of the atmosphere above the
reservoir, since 1.-THE mercury in the tube
was observed to fall when the column of air
which had the reservoir for its base was dimin-
ished in height.
! From these experiments the idea was sugges-
ted of making from the TorricelIian tube, new
meteorological instrument which is now COM
monty known by the name of the barometer.