Year: 1873

The secondary cause of storm is &


reduced alr-pressnre over any particular


part of the earth's surface which is


indicated by 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


equilibrium, 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 1871 miles in


the latitude of As degrees it is 123 miles


per minute. The earth carries the at


mosphere along with it, and the velocity
of the atmosphere at any point is the ve-
locity of that point, which decreases with
an increase in the latitude. 1t is evident
that, if mass of air at any point be
moved northward Of southwark, from
any cause, it will preserve for a time the
eastward velocity of the point from which


it started; just as a stone thrown from
the window of moving railroad car is
carried forward by the impetus of the
train. Hence, q 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 As de-
grees of latitude, and the length of the
horizontal arrow indicate the motion per
minute on the 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
direction of the arrow. If an area of
low barometer be at G, nearly midway
between these two parallels, the air will
move towards that area from every
point of the compass; and the result
is indicated 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 II miles per
minute, while the storm centre moves
II; miles per minute. Hence, the NOV
ing air drags behind at the rate Nf
mile per minute, or 80 miles per hodr,
and takes the direction of the arrow,
towards the circumference of the small
circle, instead of its centre at G. SINH
larly mass of air moving northward,
from s, has an initial velocity of 13,
miles per hour greater than that of the
centre of the storm; and moves in the
of the dotted line. Similarly, we may
show that the same deviation occurs in
the case of the air coming from every
direction towards c.; and the conse-
quence is that the air moves round the
centre of the area of low barometer, in
a direction contrary to that of the hands
of watch that is laid with its face
upward.


The vertical motion resulting from
all directions towards a storm center is
even more decided than indicated by
the diagram, which only represents the
secondary tendency. The central rush
is opposed, or counteracted, by the
elasticity of the several volumes Of air
as they approach each other and their
impetus carries them around in the
direction of the movement already
established, so that the whirl extends
nearly as far from the center as the
distance from which the winds blow.
All the air that tends towards the
moves round it in an ap-
proximate circle in Just the same way
that water courses round in a funnel,
for a similar reason. and in the same
direction, the only difference being that
the central column of air moves up
ward, while the central column of water
in the funnel moves downward.


A knowledge of these facts enables
us not only to tell the direction of the |
storm-center, but often to know whether |
It Is approaching towards us, or re-
,eding, or is passing us at harmless
distance. If we face the wind the
'torm-center will always be on the |
right hand or if we turn so that the
wind blows from left to right We shall /