The action of the polar current of the ether, therefore, tends to cause
a depression of the barometer, and an elevation to the _northward_ and
southward, and there is a general set of the wind below to the point of
greatest depression. The action of the tangential current works the
outer surface of the atmosphere into great ridges and hollows, whose
distances apart as well as actual dimensions, are continually changing
under the influences of causes not yet alluded to, and it is in the
hollows where the action of the polar current will be principally
expended. Luckily for the earth, the axis of the vortex is never long in
passing over any particular place. In this latitude, whose natural
cosine is three-fourths, the velocity _westward_ is over 700 miles per
hour; but at its extreme limits north, the motion is much slower, and is
repeated for two or three days in nearly the same latitude, for then it
begins to return to the south; thus oscillating in about one sidereal
period of the moon. At its southern limit, the vortex varies but slowly
in latitude for the same time, but the velocity is much greater. The
extreme latitudes vary at different times with the eccentricity of the
lunar orbit, with the place or longitude of the perigee, and with the
longitude of the moon's ascending node, but in no case can the _central
vortex_ reach within 5d of the equator, or higher than about 75d of
latitude north or south. Hence there are no storms strictly speaking
beyond 88d[7] of latitude; although a storm may be raging close by, at
the turning point south, and draw in a very strong gale from the
northward with a clear sky above. So also, although rains and short
squalls may be frequent in the vapor-loaded atmosphere of the equator,
yet the hurricane does not reach there, owing to the adjustment of the
mass and distance of the moon, and the inclination of the axes of the
vortices to the axis of the earth. If the temperature of the upper limit
or highest latitude of the vortex, was equal to the temperature which
obtains at its lowest limit, and the daily extremes of the solar
influence as great, the hurricanes would be as violent at the one as the
other, and even more so on account of the smaller velocity. But the
deficiency of temperature and moisture, (which last is all-important,)
prevents the full development of the effect. And even in the tropics,
the progress of the sun, by its power in directing the great annual
currents of the atmosphere, only conspires in the summer and autumn
months, to bring an atmosphere in the track of the vortices, possessing
the full degree of moisture and deficiency of electric tension, to
produce the derangement necessary to call forth the hurricane in its
greatest activity.