Marshall County Republican. (Plymouth, Ind.) 1856-1878, 1873-04-17

j
The secondary cause of a storm is a
reduced ~air-pressure over n particular
part of the earth's surface which is
indicated by a depression of the mer-
curry in the barometer within that region.
Hence the central district Of a storm is
technically called "an area of low bar
ometer." Towards this region the sur
rounding air rushes in to restore an equi-
Hbrium, and that rush of air is known as
wind. The resulting circular motion,
around the central point, is due to the
rotation of the earth upon her axis, from
west to east.
1t is easy to calculate that the velocity
of rotation at the equator, 24,900 miles
in "a hours, is 17.3 miles per minute,
while at the pole it is nothing. In the
latitude of 40 degrees it is 49, miles in
the latitude of As degrees it is 124 miles
per minute. The earth carries the at
mosphere along with it, and the velocity
of the atmosphere at any point is the We
locity of that point, which decreases with
an increase in the latitude. It is evident
that, If a mass of air at any point be
moved northward of southwark, from
any cause, it will preserve for a time the
eastward velocityof the point from which
it started just as a stone thrown from
the window of a moving railroad car is
carried forward by the impetus of the
train. Hence, a mass of air moving
southwark, in this hemisphere, will lag
behind in the rotation, while the relative
motion will be faster than that of the
surface if it move northward, because of
the change in velocity of the surface due
to the latitude.
Now suppose the top line of our dia-
gram to represent the parallel of 45 de-
grees of latitude, and the length of the
horizontal arrow indicate the motion per
minute on that parallel. Let the lower
line represent the parallel of 40 degrees {;
then the length of the arrow near it will
indicate the velocity of the air there per
minute. In each case the surface, and
its superintendent air, moves in the di-
rection of the arrow. If an area of low
barometer be at c., nearly midway be-
tween these two parallels, the air will
move towards that area from every point
of the compass and the result is indi-
Gated in the diagram, where the dotted
lines show the tendency, and the full
lines show the direction, of the actual
movement. A mass of air coming from
the north at n, moves eastward with a
velocity 124 miles per minute, while the
storm-center moves 124 miles per minute.
Hence, the moving air drags behind at
the rate of ] mile per minute, or 80 miles
{ per hour, and takes the direction of the
!, arrow, towards the circumference of the
: small circle, instead of its center at G.
Similarly a mass of air moving north
: ward, from S, has an initial velocity of
49, miles per hour greater than that OF
the center of the storm and moves In
the direction of the arrow from S, in
stead of the dotted line. Similarly, we
may show that the same deviation &C.
ours in the case of the air coming from
r every direction towards y; and the con,
sequence is that the air o round the
4 center of the area of low barometer, IL
a direction contrary to that of the hands
: of a watch that is laid with its face up
ward.
q The vertical motion resulting from
e all directions towards a storm-center il
r even more decided than indicated by tn,
e diagram, which only represents thu
t secondary tendency. The central rusl
e is opposed, or c~ounteracted, by the elias
s ITCHY of the several volumes of air a
they approach each other ; and thei
o impetus carries them around in the di
y rection of the movement already estab
lished, so that the whirl extends nearl
as far from the center as the distanc
from which the winds blow. ALI the al
e that tends towards the storm-centel
d moves round it, in an approximate circle
l. in just the same way that water course
round in a funnel, for a similar reason
le and in the same direction, the only differ
ence being that the central column c
air moves upward, while the centrf
le column of water in the funnel move
Is downward.
l. A knowledge of these facts enables U
h not only to tell the direction of the storm
D center, but often to know whether it
lie approaching towards US, or receding, {
N is passing us at a harmless distance.
y we face the wind the storm-center wi
is elways be on the right hand or if "
at turn so that the wind blows from left
y right we shall face the storm-center. l
at the same time, the mercury in the ba
Id ometer continues to fall, we may be SUI
that the vortex is approaching us
IN while, if the barometer rises, v
in may count on it's having passed b.
The general direction of the movemel