It is a well-known fact, that the prevailing current of the atmosphere
in high latitudes is from the westward. The cause of this is ascribed by
Professor Dove to the transfer of the equatorial portions to a higher
latitude, by which the excess of its rotative velocity is made apparent,
by outstripping the slower moving surface in its progress eastward. No
doubt some effect is due to this, but still a difficulty remains. Let us
follow this current. The polar current reaches the surface on the
borders of the trades with less rotative velocity than the surface, and
is, therefore, met by the surface as a current partaking of both
motions. In the northern hemisphere it is north-east deflected to east
as it approaches the southern trades. By the same reasoning, coming from
the north before it readies the surface, it ought to be also a
north-east wind above the lower westerly currents. Now it is an observed
fact, that while in the latitude of New York, for instance, the lower
westerly winds are to the easterly, as 3 or 4 to 1, in the highest
regions of observed clouds, the ratio is much increased; and according
to our own observations in this place,[5] we have never seen the highest
cirrus clouds moving westward. How then is this continual interchange
kept up? Assuredly we cannot have a current from the poles without a
contrary current to the poles. If we go into the arctic circle, we again
find the westerly and northerly winds predominating. If the current from
the equator follows the surface, the westerly winds ought to be
south-west. If it be above the surface wind, then the surface wind is
the polar current, and ought to be north-east. Whereas, from the
testimony of all who have visited these regions, the prevailing winds
are north-west. How can this be?
Again, it is proved that the upper current near the equator is also from
the westward--as near due west as possible. Take the latitude of St
Vincent. The difference between the cosine of 13d and radius applied
to the circumference, is about 600 miles, which would give 25 miles per
hour to the eastward, in lat. 13d. But to do this, it is necessary to
transfer it suddenly from the equator; for by a slow motion the easterly
tendency would be lost. Give it 24 hours from the equator to lat. 13d,
without any loss of easterly tendency, and it comes to that latitude
with a velocity of 38 miles per hour to the northward, and only 25 to
the eastward; we have, therefore, a wind from south-west by south. Yet
it is known that in the tropics the highest visible clouds move from the
westward. But as no such case could occur as a transfer in twenty-four
hours without loss, and if we diminish the time, the wind is still more
southerly. Meteorologists usually cite the falling of ashes at Jamaica
during the eruption of Coseguina, in Guatamala, in February 1835, as
coming from south-west, whereas the true direction was about west
south-west, and the trade wind below was about north. But do we deny
that there is an interchange between the frigid and torrid zones? By no
means; but we would show that the great controlling power is external to
our atmosphere, and that the relative velocities of the earth and the
atmosphere is not alone adequate to account for it. By this view the
polar current is a north-west wind (which is impossible by Professor
Dove's theory), or is carried eastward by electric convection.