The Sun.
London, England, 1860-09-12 00:00:00

ROD, To PC )RETELL TVEATHE.R.

,______
The following rs ianua__ l of the barometer has been
compiled by Rear ,Admiral--Fitzroy.,
by the Board of Trade : and published
g, Familiar sas the pract
'
:cal use of weather-glasses
is, at sea as Nvell as o
long orate} led their . ;land, only those who have
indications and compared them
carefully are r Pjally able to conclude more than that
a fall
the risi• Ag e`ass a usually foretells less wind or rain,
, barometer more rain or wind, or both; a
hi.„,gu. one fine weather, and a low the contrary.
Drat, useful as these general conclusions are in
74anst cases, they are sometimes erroneous, and then
remarks may be rather hastily made, tending to
discourage the inexperienced.

" By ittention to the following observation (the
results of many years' practice and many persons'
experience) ealy one accustomed to use a barometer
may clo sc., without difficulty.
" 7:lae, barometer shows whether the air is get-
ting lighter or heavier, or is remaining in the same
relate. The quicksilver falls as the air becomes
righter, rises as it becomes heavier, and remains at
ent in the glass tube while the air is unchanged in
weight. Air presses en everything within about 40
miles of the world's surface like a much lighter
ocean, at the bottom of which we live, not feeling
its weight because our bodies are full of air,b but
.feeling its currents, the winds. Towards any place
from which the air has been drawn by suction s air
presses with a force or weight of nearly 151 b. on
a square inch of surface. Such a pressure holds
the limpet to the rock when, by contracting itself,
the fish ;has made a place without air d under its
shell. Another familiar instance is that of the fly,
which walks on the ceiling with feet that stick.
The barometer tube, emptied of air, and filled with
pure mercury, is turned down into a cup or cistern
containing the same fluid, which, feeling the weight
of air, is so pressed by it as to balance a column of
about 30 inches (more or less) in the tube, where no
air presses on the top of the column.
r' If a long pipe, closed at one end only, were
emptied of air, filled with water, the open end kept
ii water and the pipe held upright, the water
would rise in it more than 30 feet. In this way
water barometers have been made. A proof of this
effect is shown by any well with a sucking pump,
up which, as is commonly known, the water will
rise nearly 30 feet by what is called suction, which
is, in fact, the pressure of air towards an empty
place.
" The words on scales of barometers should not
be so much regarded for weather indications as the
rising or falling of the mercury, for if it stand at
changeable' and then rise towards fair' it pre-
sages a change of wind or weather, thourg'n not so
great as if the mercury had risen hipler ; and, on
the contrary, if the mercury stall:a dove fair' and
then fall it presages a cb_nge, though not to so
great a degree as if it had stood lower; besides
which, the direction and force of wind are not in
any way noticed. It is not from the point at which
the mercury may stand that we are alone to form
a judgment of the state of the weather, but from
its rising or falling, and from the movements of im-
mediately preceding days as well as hours, keeping
in mind effects of change of direction and dryness
or moisture, as well as alteration of tome or strength
of wind.
"In this part of the world, towards the higher
latitudes, the quicksilver ranges, or rises and falls,
nearly three inches—namely, between about 30
inches and 9-tenths (30.9) and less than 28 inches
(28.0) on extraordinary occasions • but the usual
range is from about 30 inches and a half (30.5) to
about 29 inches. Near the Line, or in equatorial
places, the range is but a few tenths, except in
storms, when it sometimes falls to 27 inches.
"The sliding-scale (vernier) divides the tenths
into 10 parts each, or hundredths of an inch. The
number of divisions on the vernier exceeds that in
an equal space of the fixed scale by one.
" By a thermometer the weight of air is not
shown. No air is within the tube. None can get
in. But the bulb of the tube is full of mercury,
which contracts by cold and swells by heat, accord-
ing to which effect the thread of metal in the small
tube is drawn down or pushed up so many degrees,
and thus shows the temperature.e
" If a thermometer have a piece of linen round
the bulb, wetted enough to keep it damp by a
thread or wick dipping into a cup of water, it will
show less heat than a dry one, in proportion to the
dryness of the air and quickness of drying. f In
very damp weather, with or before rain, fog, or
dew, tv:o such thermometers will be nearly alike.
" For ascertaining the dryness or moisture of
air, the readiest and surest method is the compari-
son of two thermometers, one dry, the other just
moistened and kept so. Cooled by evaporation as
much as the state of the air admits, the moist (or
wet) bulb thermometer shows a temperature nearly
equal to that of the other one when the atmosphere
is extremely damp or moist ; but lower at other
times, in proportion to the dryness of air and, con-
sequent evaporation.—as far as 12 or 15 degrees in
this climate, 20 or even more elsewhere. 'From 4to
3 degrees of difference is usual in England, and
about 7 is considered healthy for inhabited rooms.
" The thermometer fixed to a barometer intended
to be used only as •a weather-glass shows the tem-
perature of air about it nearly, but does not show
the temperature of mercury within exactly. It
does so, however, near enough for ordinary practi.
cal purposes, provided that no sun, nor fire, nor lamp
heat is allowed to act on the instrument partially.
" The mercury in the cistern and tube being
affected by cold or heat, makes it advisable to con-
sider this when endeavouring to foretell coming
weather by the length of the column.
" Briefly, the barometer shows weight or pres-
sure of the air ; the thermometer, heat and cold, or
temperature ; and the wet thermometer, compared
with a dry one, the degree of moisture or damp-
ness.g
" It should always be remembered that the state
of the air foretells coming weather, rather than
shows the weather that is present—an invaluable
fact too often overlooked ; that the longer the time
between the signs and the change foretold by them,
the longer such altered weather will last; and, on
the contrary, the less time between a warning and
a change, the shorter will be the continuance of
such foretold weather.
" To know the state of the air, not only barome-
ter and thermometer should be watched, but the
appearances of the sky should be vigilantly noticed.
"If the barometer has been about its ordinary
height, say near 30 inches (at sea level),A and is
steady, or rising, while the thermometer falls, and
dampness becomes less, north-westerly, northerly,
or north-easterly wind, or less wind, less rain or
snow may be expected.
On the contrary, if a fall takes place with a
rising thermometer and increased dampness, wind
and rain may be expected from the south-eastward,
southward, or south-westward.
" A fall with a low thermometer foretells snow.
" Exceptions to these rules occur when a north-
easterly wind, with wet (rain, hail, or snow) is
impending, before which the barometer often rises
(on account of the direction of the coming wind
alone) and deceives persons who, from that sign
only (the rising) expect fair weather.
" When the barometer is rather below its ordi-
nary height, say down to near 291 inches (at the
sea level), a rise foretells less wind, or a change in
its direction towards the northward, or less wet;
but when it has been very low, about 29 inches, the
first rising usually precedes or indicates strong
wind ; at times heavy squalls from the north-west-
ward, northward, or north-eastward,—after which
violence a gradually rising glass foretells improving
weather if the thermometer falls ; but, if the
warmth continue, probably the wind will back
(shift against the sun's course), and more southerly,
or south-westerly wind will follow, especially if the
barometer rise is sudden.

"The most dangerous shifts of wind, or the
heaviest northerly gales, happen soon after the
barometer first rises from a very low point ; or, if
the wind veers gradually, at some time afterwards.
" Indication of approaching changes of weather
and the direction and force of winds are shown less
by the height of the barometer than by its falling
or rising. Nevertheless, a height of more than
thirty (30 0) inches (at the level of the sea) is indi-
cative of fine weather and moderate winds; except
from east to north, occasionally.
" The barometer is said to be falling when the
mercury in the tube is sinking, at which time its
upper surface is sometimes concave or hollow; or
when the hand (see note in page 2) moves to the
left. The barometer is rising when the mercurial
column 18 lengthening ; its upper surface being
convex or rounded, or when the hand moves to the
right (see note in age 2).
A rapid rise of the barometer indicates unsettled
weather; a slow movement the contrary ; as like-
wise a steady barometer, which, when continued,
and with
dryness, foretells very fine weather.
s, A rapid and considerable fall is a sign of stormy
weather and rain (or snow). Alternate rising and
sinking indicates unsettled andthreatening weather.
The greatest depressions of the barometer are

quicksilver, or
" acolumn,ass, barometer,
b Or atmosphere, or the atmospheric fluid which we
breathe.
6' e Or exhaustion.
d A vacuum.
" e Thirty-two degrees is the point at which water begins
to freeze, or ice to thaw.
" f Evaporation.
" 9 The two thus combined making a hygrometer ; for
shift.
which
some kinds of hair, grass, cr seaweed may be a make-
"h It or stands lower, about a tenth of an inch
for each hundred feet of height directly upwards, or verti-
cally/ above the sea • its average height being 29.94 inches
44 the mean sea level in England. Allowances must, there-
pro, be slide for barometers on high land or in buildings.