form a junotion with the northeast trades prevailing there at a certain season,
and to blow with little interruption from January until April.
The hypothesis of some is, that in passing over the deserts and lands of
Africa the “ Harmattan ” takes up an immense quantity of sand and dust,
sufficient to form a floating mass, producing an atmosphere so hazy as frequently
to obscure the sun, and prevent a sight of the land at the distance
of five miles only. There is no doubt of the wind, the dust, and the hazy
atmosphere, and possibly the latter may be occasioned entirely by the dust.
At the season of the “ Harmattan ” this peeuliar atmosphere may always be
seen at the Cape de Verd islands, four hundred miles from the continent, and
is constantly falling in quantities sufficient to cover the sails, rigging, and deck
of a ship. I t is also said to have been met with seven hundred miles further
westward.
Recent investigations, however, would seem to create some doubt as to
the source whence the dust is derived. I t was natural enough, as it was
found on the coast of Africa, to refer its origin to the nearest known desert
land; but the microscope, in the hands of Ehrenberg, would seem to intimate
the possibility of a more distant origin. This dust, from the Cape de Verds,
is found, upon examination, to oonsist of infusoria and organisms, the habitat
of which is not Africa, but South America, and in the southeast trade
wind region of that country. I t is, therefore, possible that the southeast
trades may have brought the dust, great as is the distance, from South
America. But if such be the fact, it must be confessed that there are
agencies in the philosophy of the winds, producing atmospheric phenomena,
which are not yet sufficiently understood by us to justify positive assertion.
A greater accumulation of facts is wanted. That stated by Ehrenberg is
very important, and quite sufficient to create doubt of the correctness of the
ordinary hypothesis.
Until the 30th of December the northeast trades continued, the ship
having then reached 6° 8' north latitude, and 16° 34' west longitude, when,
in a squall from the eastward the wind changed to the southward, and so continued,
though somewhat variable, until January 2, 1853, in latitude 1° 44/
north, and longitude 11° 37/ west, when the southeast trade was met, bringing
with it a swell, which retarded the ship’s progress considerably. Before this,
however, on the 29th of December, as the northeast trades had become light
and unsteady, with occasional calms, the floats were replaced on the wheels,
and the ship was put under steam, using the two after boilers only. With
light winds and a smooth sea, these proved sufficient to make a progress of
seven knots an hour; but when the southeast trades fairly set in, accompanied
as they were by a head sea, the speed was diminished to four and a half or
five knots. The use of two additional boilers, however, soon brought the
ship up to seven, at a daily consumption of twenty-six tons of coaL
I t had been the purpose of the Commodore, on leaving Madeira, to
make the entiro run to the Cape of Good Hope without stopping; as it was
supposed that with a proper use of the sails, and the supply of coal on board,
this might readily be done; but the northeast trades having ceased at a
point much further north than usual at this season, and the southeast winds
having also set in at a correspondingly early period, he > ordered the ship so
to be steered that she might touch at St. Helena, should it be deemed
desirable so to do, as a measure of prudence, to procure an additional supply
of coal.
In the observations made upon the currents since leaving Norfolk, the
Commodore was of opinion that such as he encountered were caused merely
by the winds acting on the surface of the ocean; and as a general rule,
though not perhaps universally true, it may be remarked that the current
will be found setting from the land in the direction of the prevailing winds;
at least such has been the opinion formed from the long observation and
experience at sea of the Commodore. There may be, however, and probably
are, currents other than those partial ones created on the surface by the
winds. These are caused by a difference in the specific gravity of the sea
water at different places and depths. This difference disturbs the equilibrium,
and the effort of the water to regain it must cause a current.
There is consequently, on and under the surface a system of currents
and counter currents constantly operating in a greater or less degree. They
are far from being yet perfectly known and understood, but the principle on
which they must exist is the law of hydrostatics, that when two fluids on
the same level differ in specific gravity, the one will not balance the other,
both must move; that motion is a current..
Various interesting experiments have been made on the subject of submarine
currents, counter to those on the surface, and their existence would
seem to be conclusively proved; their direction, however, cannot always be
ascertained. Practical seamen have also endeavored to find out the depth
of surface currents; this depth is not always the same, but there is, if not
impossibility, yet great difficulty, at times, in ascertaining the depth with
accuracy.
On Monday, January 3, 1853, the ship crossed the equator in longitude
11° 01/ west, and from that time up to the 7th had a moderately fresh
breeze directly ahead. The effect of this wind, instead of affording refreshment
by the motion of the vessel meeting it, (her course was due south,) was
to render the officers’ apartments, especially the cabin, and, indeed, all the
after part of the ship, particularly uncomfortable, as the wind brought much
of the heat and smoke directly aft. The wind, however, did not retard the
progress of the vessel, though it was directly ahead; for it must be remem-
bered that, in a steamer, motion is not so much retarded by adverse winds as
by the head sea which the wind produces. Indeed, a steamer will sometimes
go faster against a moderately fresh breeze, provided she be on a