, 1873-04-06

" T y
As stated in TIE TRIBUNE Of Sunday Inst, the
| secondary cause Of storm is d reduced air.
pressure over any particular part Of the earth'a
| surface; which is indicated BY a depression OF
the mercury in the baromster within that region.
Hence the central district OF storm is tech
nically called
| IAN AREA OF LOW BAROMETER"
! Towards this region, the surrounding air rush
ea in to restore an equilibrium and that rush Of
| air is known az wind. The resulting circular
motion, around ecentra1point, is due to the ro-
! tation Of the earth upon her axis, from west to
east,
It is easy to calculate that
THE VELOCITY OF ROTATION
at the equator, "a 800 miles in "A hours, is 17,3
miles per minute, while at the pole it is nothing
In the latitude Of 40 degrees 1t is 131 miles; in
the latitude Of IS degrees it is 125 miles, min
| ute. The earth carries the atmosphere along
| with it, and the velocity OF the atmosphere at
any point Is the velocity Of that point, which de-
| creases with an increase in the latitude It is
| evident that, If mass of air at any point be
| moved northward or southwark, from any cause,
| it will preserve for time the eastward velocity
OF the point from which it started just as q
| stone thrown from the window Of 3 moving rail-
road car is carried forward by the impetus Of
the train. Hence, mass OF air moving south
| ward, 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 st the chaugo in velocity Of the surface
due to the latitude. -
Now suppose the top line Of our diagram to
represent the parallel OF As degrees 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. meach
case the surface, and it's superincumbent air,
moves in the direction Of the arrow. II an area
OF low barometer be at G, nearly midway be-
tween these two parallels, the air will move to-
wards that area from every point Of the COM
pass; and the result is indicated In the diagram
where the dotted lines show the tendency, and
tho full lines show the direction, Of the actual
movement 4 mass OF air coming from the
north at N, moves eastward with velocity 12I
miles per minute, while the storn-centre moves
12N miles per minute. Hence,
TIE MOVING A1R DRA6s BEu1D
at the rate Of Is mile per minute, or so miles per
hour, and takes the direction Of the ar
TOW, towards the circumference Of the
small circle, instead Of its centre, at Q. simi-
1arly mass Of air moving northward, from s,
has an initial velocity of 13A miles per minute,
or 80 miles per hour greater than that Of the
centre Of the storm and moves in the direction Of
the a1ow from 8, instead Of the dotted line. simi-
1arly, We may show that the same deviation oc-
ours in the case OF the air coming from every dl-
rection towards G and the consequence is that
the air moves round the centre Of the area OF
low barometer, in direction contrary to that Of
the hands Of q watch that is lid with its face
upward:
TIE ToRIIoAL MOTION
resulting from this rush from all directions to-
wards storm-centre is even more decided than
indicated by the diagram, which only rep
resents the secondary tendency. The central
rush is opposed, OF counteracted, by the
elasticity Of the several volumes Of air
Is they approach each other; and their
impetus carries them around in the direction Of
the movement already established, 60 that the
whirl extends nearly IS far from the centre as
the distance from which the winds blow. AL
the air that tends towards the storm-cent1e !
moves round it, in an approximate circle; in |
Just the same way that water courses round in q
funnel, for similar reason, and In the same d.
rection, the only difference being that the cen
tra1 column Of air moves upward, while the GEN.
trM column Of water in the funnel moves down. |
ward
a knowledge Of these facts enables us rot |
only to tell the direction OF the storm-centre,
but = |
OFTEN TO Know WHETHER