, 1875-10-09

trical readings are lower over any area
than over those adjacent, the air will
sweep round that area as a centre, and the
direction OF the motion will be opposite
to that of the hands OF a watch. Converse.
~ the air wall sweep round an area of
relatively high barometrical readings in
the direction in which the hands of a
watch move.
The existence of a deficiency of atmos-
pherlcal pressure, or what is termed a
b'rometrIcal depression, over any district
IS accompanied by the movement of the
air IN the neighboring districts. The
velocity of the air depends-at least in a
great measure, though not absolutely
on the difference of barometrical readings
over a given distance, or on what is termed
the barometrical gradient" The gra-
dents are expressed in hundredths of an
inch of mercury per fifty geographical
miles. Among observations that have
passed into proverbs in the Meteorological
Department are that when the barometer
falls suddenly in the western. part of
New England it rises at the same time
in the Valley of the Mississippi and also
at st, Johns, Newfoundland. In great
storms the wind for several hundred miles
on both stiles of the line of minimum
pressure blows toward that line directly
or obliquely. The force of the wind is in
proportion to the suddenness and greatness
Of the depression of the barometer. In
all great and sudden depressions of the
barometer there is much rain or snow.
Most storms il the star west,'
but some commence in the United
States. The fluctuations of the barometer
are generally greater in the eastern than in
the western parts Of the United States.
During the passage Of storms the wind
generally changes from the eastward to
the westward by the south. The northern
part OF the storm generally travels more
rapidly toward the east than the southern
part. During the high barometer on the
day preceding the storm it is generally
clear and mild in temperature, especially
II very cold weather preceded. The. tenn.
perature generally falls suddenly on the
passage OF the centre of great storms ; so
that sometimes, when a storm is in the
middle of the United States, the lowest
temperature of the month Will be in the
west on the same day that the highess
temperature is in the east.
People imagine when we have a north
easterIy wind that we are to have a storm
from the northeast, whereas the storm
must necessarily come from the south west,
or the very opposite direction, by reason
of low barometer being there. In the
West Indies the cyclone season is from
July to October. They originate near the
equatorial limit of the trade winds, where
these winds are irregular and travel west.
The air in cyclones has not merely a
movement of translation, but also a gyra-
tory motion about the centre of the storm.
The motion of the air is spiraily inward,
as has been shown in the storms of the
United States /; but over the ocean the
whlrllng motion is usually more decided
than it is over the land.
North of the equator this gyratory mo-
ton is from right to left, or in a direction
contrary to that of the hands o&a watch
south OF the equator the motion is from
left to right, or in the same direction. as
that of the hands Of a watch. Near the
centre OF the hurricane there is generally
a great fall of rain. Storms are caused bv
9 strong and extensive upward motion Of
the air. by which means its vapor is con-
densed by the cold elevation.