Year: 1873

The secondary cause of storm is a
reduced air-pressure over any 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 storm is
technically called can area of low bar
ometer." Towards this region the sur
rounding air rushes in to restore an equi-
ibrium, 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.


It is easy to calculate that the velocity
of rotation at the equator, "a, 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 time the
eastward velocity of the point from which
it started just as stone thrown from
the window of a moving railroad car is
carried forward by the impetus of the
train. Hence, o 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 viii


indicate the velocity of the air there per


minute. In each case the surface, and


it's superintendent air, moves in the di-


rection of the arrow. If an area of low


barometer be at @, 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 6


velocity 124 miles per minute, while the


storm-center moves 122 miles per minute


Hence, the moving air drags behind al


the rate of ] mile per minute, or 30 miles
per hour, and takes the direction of the
arrow, towards the circumference of the
small circle, instead of its center at 9
Similarly mass of air moving north
, ward, from S, has an initial velocity O.


134 miles per hour greater than that o


the center of the storm and moves il
the direction of the arrow from S, in
stead of the dotted line. Similarly, w
may show that the same deviation oc
, ours in the case of the air coming from
l every direction towards G; and the con
sequence is that the air moves round tn
5 center of the area of low barometer, i


a direction contrary to that of the hand
= of watch that is laid with its face up
: ward.


/ The vertical motion resulting fror
= all directions towards storm-center /
f even more decided than indicated by tn
r diagram, which only represents tn
D secondary tendency. The central rus
: is opposed, or counteracted, by the elias


ITCHY of the several volumes of air q


they approach each other and thei
impetus carries them around in the d


rection of the movement already estaI
: lished, so that the whirl extends nearl
I as far from the center as the distanc


from which the winds blow. All the a
: that tends towards the storm -cente
Il moves round it, in an approximate circle
u u in just the same way that water course
9 round in funnel, for similar reasol
/ and in the same direction, the only diffe
= ence being that the central column
/ air moves upward, while the centr


column of water i'll the funnel move
d downward.


e A knowledge of these facts enables I
Il not only to tell the direction of the storm
h center, but often to know whether it
n approaching towards US, or receding, I
l. is passing US at a harmless distance.
:, we face the wind the storm-center wi
l. always be on the right hand or if v
.r turn SO that the wind blows from left 1
y right we shall face the storm-center. I


at the same time, the mercury in the ba