Juniata sentinel and Republican. [volume] (Mifflintown, Juniata County, Pa.) 1873-1955, 1877-05-30

lW-tNte> OF MS NoNtlu7loN,t anu loF ever
exploding the time-honored belief 10
Natures abhorrenee Of a vacuum, a term
thai for ages had, under the plausible
gpise OF philosophy, covered mans ig-
norance of the real cause OF the phen-
omenon,
The escape OF part Of the mercury
len, in the upper part Of the tube, a
void, which has ever been called the
forricellian yaeEum. The ascent OF the
liquid is due To the existence OF this
void, for ii air occupied the space It
would exercise pressure on the top Of
the column sufficient 70 counterbalanoe
thar on the exposed metal in THE cup in
which its lower end terminates; hut
with vacuum AT enc end of the fluid
column, there i, nothing 10 resist its
yielding To the slightest variation oi
pressure OF the other.
The adherence at the mercury To the
glass, however, tends la beget an error
in estimating the true height of the
column, especially in instruments of
small bore, in which this source of in
accuracy is proportionately greater.
Water aud other fluids form a concave
surface al the top Of a small column by
climbing up the glass, through capillary
attractions, whereas, the top of mer-
curial column j. convex while rising
and concave while falling. hut in both
eases the centre Of the column indicates
the absolute height. Another cause of
possible error l. due tO the expansion
and contraction of the mercury accord
ing 10 the temperature of the air, indie
pendent of pressure, so that the height
Of the column varies with every change
in the former a. well a. in the latter.
A. the amount of mercurial expansion,
however, i accurately known. by oh
serving the height OF the mercury in the
thermemeter which is usually attached
to the barometer. proper corrections on
this 0ccOunt can always be- made.
The chief reason why mercury has
been selected for barometrical purposes.
is that the required weight OF it occu-
pies the least space. Water harometers,
have, however, been constructed. and a
each vertical, inch of mercury j. equiva-
lent to a trifle over thirteen and one
half inches of water. the length of the
column renders il 80 extremely sensitive
to slight atmospheric changes that in
tempestuouS weather IL Is continually
vibrating up and down under moment
airy iluctuation, Of pressure,
Observation has cOnclusively shown
that our own sensations are untrust-
worthy criterions of atmospheric pres
sure. ON dry, bracing days. when the
spirits are high and the body buoyant.
one il apt to imagine that the air itself
presses lightly upon him; but just on
such days high barometer usually
indicates q heavy atmospheric pressure.
and hints that our exhilaration j. some
what akin 10 that experienced by those
undergoing the ''air-cure," when con-
fined IL q chamber where the aerial
density has been artifcislly increa -ed.
In foul, moist, and foggy weather, on
the other hand. when -oul and body are
alike -luggi-h, and the very air -eem-
AN oppressive burden, iOW barometer
generally proclaim- light atmospheric
pressure, aud the hedgeling scientist 4
APT ~ attribute hi- inertnes- partly to
lack Of the normal amount 0f oxygen in
the attenHated air.-Rac0I Ne,e T"rtsr.