The sun. [volume] (New York [N.Y.]) 1833-1916, 1888-01-15
THE SUN, SUNDAY, JANUARY is, 1ssay-sIxTEEN PAGES.

charttng has given the results upon which q real
science OF the weather is founded. It wa
necessary, before any progress could be made,
to work back to general views, and such views
are now attainable. Asynoptic chart shows
MS at q glance tho condition OF the atmos-
phere at any prescribed moment over the
whole OF Europe or the United States.
The magician's wand In this case IL the
fsobar. An isobar is an imaginary line In the
atmosphere, or q line drawn upon . chart,
which passes through all places having the
same atmospheric pressure rs shown by tho
barometer. Having got his telegraphic reports
from every part Of the territory to be covered,
the meteoroloRist draws his isobars upon the
chart, and Immediately the cyclone stands re-
vealed, II cyclone there be.
Now let US see what a cyclone is. Suppose
that over a broad region, several hundred
miles across. the afr, through causes into
which we need not here inquire, becomes
warmer than the surrounding atmosphere. 1t
will manitestly expand and tend to rise. The
barometer gives immediate notice Of this con-
dition by a fall OF tho mercury. since the
pressure OF the expanded air Is diminished.
Hero We have. then, what is called . region Of
low barometer. The moteorologist at once
proceeds to ascertain the condition. as to
pressure. OF the surrounding atmosphere. He
draws his isobaric lines through all the points
who're the pressure Is equal, and finds that the
lines surround the region Of low barometer ap-
proximately as circles, or ovals. one with
In another. Suppose. for instance. that
the lowest baromotric pressure is 20
inches. The line. or isobar. Of 30 inches
will then form the innermost circle. A hun
dred miles further out. let US suppose, q SEQ
ond circle indicates where the barometer
stands at thirty inches. Between those two
| Nnn0nnfrin foNho 10 thA rnans,rn rrrud,,o nu In
cresses from the centre toward the circumfer-
once. forming a slope that may be likened to
the side Of a dish. II the isobars were closer
together 1t is clear that the slope would be
sleeper; on the other hand. II the isobars were
wider apart tho slope would be more gentle.
This slope Is called the barometric gradient.
Upon 1t the force Of tho wind depends. We
shall readily understand this IL we consider
what the slope signifies. 1t simply means a
gradual decrease of bsrometric pressure to-
ward the centre Of the territory included in the
isobars; that is to say, the air decreases in
density toward that centre. 1t must, therefore.
tend to rise, and as 1t rises the heavier sur
rounding air will rush In to I'LL UP tho gap.
The moist air from the surface thus rising in
great quantities, becomes cooled below the
dew point 30 that its vapor condenses. and so
over our area Of low barometer we shall behold
the formation Of cloud and rain. The faster
the density increases toward the centre. or the
sleeper the slope becomes. the faster the up
rush and thrush will be. or, in other words, the
stronger the winds.
These winds. caused BY the air seeking to
fill the depression. will blow from all direc-
tons toward tho centro Of the area Of low
barometer. Since the diameter Of this area is
several hundred miles, the reader will readily
see. upon a little consideration. that the winds
cannot blow straight to the centre, but in con-
sequence OF the DETECTING force due to the
oarth's rotation. they will approach the centre
spirally, and so a rotary motion originates
having the centre Of the depression for it's axis.
The direction Of this rotation, In consequence
Of tho defective force we have already de-
scribed. will always be from right to left. or
opposite to the motion Of the hands Of a watch.
in tho northern hemisphere. and from left to
right in the southern hemisphere.
The great circular area Of barometic depres-
sion We have Just described, with its clouds
and rain and its system Of ~spirally revolving
winds. Is . CYCLONE. In addition to its spiral
currents q cyclone has a motion of translation
as a whole. In this hemisphere cyclones gem
orally advance In q northsasterly direction,
though they sometimes move toward other
points Of the compass.
Lot us inquire a little moro closely what Of
fects a cyclone produces. We have already
seen that rain and winds are due to 1t. but 1t
must not be imagined that these c~yclonic winds
appear as whirling or revolving currents to an
observer situated anywhere in the cvclone's
path. The breadth Of the cvclone "a too great
for that. All that the solitary observer can
perceive ia hat as the cyclone passes over him
the wind changes its direction in a nerfectIv
definite way, Suppose he Is In the path of an
approaching cyclone. but a little south of the
centre, As its front edge moves over him ha
gets tho wind from ths southeast, and striking
into the downward alone. his barometer bapins
to fall. The sky ia overcast. the air becomes
damp and oppressive. the wind shifts toward
the south. and presently the rain area. which
occupies the central region Of the cvclope ar-
riyes. The wind gets around into tn, sonth-
west. and the barometer continues IN fall untu
the central part Of the cvclone bss nassod vben
the baromstrie slope being reversad. r. hecins
To rise. With the rising of the haromatar tho