Year: 1870

-
and as such a column is, as nearly as
possible, that which the pressure Of a
column OF air will sUPpOrt-it results
Jnat the weight Of a column Of air Of
the helght OF our atmosphere, and of
surface equal to I inch superficial,
VIII also equal, as nearly as possible,


But our atmosphere, besides being a
heavy, Is also an elastic fluid-there-
fore compressible-s so the lower strata
is condensed by the weight of its su-
perincumbent mass. The density of
our atmosphere, therefore, cannot be
uniform, but must diminish and be-
come rarer as we approach its upper
limits; and this we can readily prove
TO be the case by taking flask, closed
air-tight at the foot Of a mountain, to
the summit; on opening it, air will
rush out Of the vessel. On again
closing it air-tight, removing it to the
base Of the mountain, and reOpening
it, air will rush in.


We have thus far learnt that the
mercury is sustained in Torricelli's
tube by the pressure Of the air on the
surface of the mercury in the cistern,
and that that pressure on a given sur
face Of mercury is proportional to the
height Of the cOlgmn.


The experimental tube Of TOrrieelli
Is, in fact, the instrument now uni
versally known as the barometer. its
name being derived from two Greek
words, signifying treigAt-medarcr,
though 1t is used as 4 measure OF at
mOspheric pressure in the same man
ner as we use a weight as a measure Of
mass, or an arc as a measure Of angle.


Experience teaches that the height
of the column Of mercury varies when
kept at the same level in the same
place. Such variations indicate cor
responding changes in the density and
elasticity Of the surrounding air; when
the airs density and elasticity are in
creased, the mercury rises; when
they are diminished, it falls. The
column, therefore, measures the in
tensity Of the atmospheric pressure,
and it is only necessary to add arec-
Ognized scale, to accurately measure
the height Of the barometer and the
variations In the instrument. In
Germany, the old Paris standard is
generally adopted; in France, the
decimal scale; and in England, the
English yard-measure is employed for
the barometric scale.


1t is commonly supposed that a
steady rise in the barometer indicates
the approach Of fine, and a steady fall
that Of rainy weather; a sudden and
great fall the approach Of a storm and
sudden rise the clearing up Of q
storm hence this instrument has pop
ularly obtained the name Of "the
Weather-glass." The common bar
Ometers are marked Fair"
Change" " Rainy" ETC., against GER
tain points Of the scale; but the soon
er these terms are disused, by public
consent, the better, as they have no
foundation on fact, and are useless
and misleading. Though in general
terms we may say that the rise and
fall Of the mercury indicates a change
Of weather, in reality the mere Obser-
walton of either of these effects, taken
alone, is not a sufficient guide. High
winds, electrical tension, and other
causes not yet thoroughly understood, 4,
exert their specific influences on the
pressure Of the atmosphere, either to
density or rarely it. In England, and
in this country, 80 inches is taken as
the mean or standard measure for the
pressure of the atmosphere, and the
variation ranges between 2S inches
and 81 inches. Other causes that in
fluence the height Of the mercury are:
l. The effect Of temperature upon this
metal, which, like others, expands
with heat and contracts with cold, so
that on hot day (Other things being
equal) the reading would be higher
than on cold day. ,. When the
mercury is rising, its surface is slight
ly convex; when falling, cOncave,
which may lead to q trifiing source Of
error in taking the readings on the
scale. 5. Should the tube Of the bar
Ometer be small, the capillary attrae-
tiOn between the glass and the liquid
metal would exert an action sufficient
ly great to create an error. When
great correctness of observation is de-
sired, corrections for temperature and
capillary action must be made, for
which purpose tables of corrections
have been drawn up by meteOrOlog-
lists.


Should the smallest quantity Of air
creep into and up the barometer tube,
il would create a serious deterioration
to the efficiency of the instrument, for
were but 1-IOO0th inch of air let into
the bottom of the tube, on reaching
the top or TOrricellean vacuum, sit
would expand to inch.


From these statements it will be OBJ
viOus that great care is required in the
manufacture Of the barometer, and the
following are the points of eonstruc
ton: A clean glass tube, over 80
inches long, of a diameter that will
render the column Of mercury dis
tinctly visible, and reduce the capilla-
" action to a minimum; mercury
pure, and freed from air
by boiling; an arrangement for pre-
venting air-bubbles passing up the
tube; an accurately graduated scale,
with some provision for correctly eX-
pressing the real height Of the column
from the level of the metal in the els
tern. Besides its use as an indicator
of present or coming weather, the
barometer is employed for deteTmin-
ing the height of mountains and 9TH
er elevations above the level Of the
sea, and is Of the greatest value to the
experimentalist in determining GER
tain physical properties Of gases, va
pOrs, etc.