South: The Story of Shackleton's Last Expedition, 1914-1917
The greatest difficulty that was experienced was due to the
accumulation of rime on the instruments. In low temperatures everything
became covered with ice-crystals, deposited from the air, which
eventually grew into huge blocks. Sometimes these blocks became
dislodged and fell, making it dangerous to walk along the decks. The
rime collected on the thermometers, the glass bowl of the sunshine
recorder, and the bearings of the anemometer, necessitating the
frequent use of a brush to remove it, and sometimes effectively
preventing the instruments from recording at all.
One of our worst blizzards occurred on August 1, 1915, which was, for
the ship, the beginning of the end. It lasted for four days, with
cloudy and overcast weather for the three following days, and from that
time onwards we enjoyed very little sun.
The weather that we experienced on Elephant Island can only be
described as appalling. Situated as we were at the mouth of a gully,
down which a huge glacier was slowly moving, with the open sea in front
and to the left, and towering, snow-covered mountains on our right, the
air was hardly ever free from snowdrift, and the winds increased to
terrific violence through being forced over the glacier and through the
narrow gully. Huge blocks of ice were hurled about like pebbles, and
cases of clothing and cooking utensils were whisked out of our hands
and carried away to sea. For the first fortnight after our landing
there, the gale blew, at times, at over one hundred miles an hour.
Fortunately it never again quite reached that intensity, but on several
occasions violent squalls made us very fearful for the safety of our
hut. The island was almost continuously covered with a pall of fog and
snow, clear weather obtaining occasionally when pack-ice surrounded us.
Fortunately a series of south-westerly gales had blown all the ice away
to the north-east two days before the rescue ship arrived, leaving a
comparatively clear sea for her to approach the island.
Being one solitary moving station in the vast expanse of the Weddell
Sea, with no knowledge of what was happening anywhere around us,
forecasting was very difficult and at times impossible.
Great assistance in this direction was afforded by copies of Mr. R. C.
Mossmann’s researches and papers on Antarctic meteorology, which he
kindly supplied to us.
I have tried to make this very brief account of the meteorological side
of the Expedition rather more “popular” than scientific, since the
publication and scientific discussion of the observations will be
carried out elsewhere; but if, while showing the difficulties under
which we had to work, it emphasizes the value of Antarctic Expeditions
from a purely utilitarian point of view, and the need for further
continuous research into the conditions obtaining in the immediate
neighbourhood of the Pole, it will have achieved its object.
PHYSICS
By R. W. JAMES, M.A. (Cantab.), B.Sc. (Lond.), Capt. R.E.
Owing to the continued drift of the ship with the ice, the programme of
physical observations originally made out had to be considerably
modified. It had been intended to set up recording magnetic instruments
at the base, and to take a continuous series of records throughout the
whole period of residence there, absolute measurements of the earth’s
horizontal magnetic force, of the dip and declination being taken at
frequent intervals for purposes of calibration. With the ice
continually drifting, and the possibility of the floe cracking at any
time, it proved impracticable to set up the recording instruments, and
the magnetic observations were confined to a series of absolute
measurements taken whenever opportunity occurred. These measurements,
owing to the drift of the ship, extend over a considerable distance,
and give a chain of values along a line stretching, roughly from 77° S.
lat. to 69° S. lat. This is not the place to give the actual results;
it is quite enough to state that, as might have been expected from the
position of the magnetic pole, the values obtained correspond to a
comparatively low magnetic latitude, the value of the dip ranging from
63° to 68°.
So far as possible, continuous records of the electric potential
gradient in the atmosphere were taken, a form of quadrant electrometer
with a boom and ink recorder, made by the Cambridge Scientific
Instrument Company, being employed. Here again, the somewhat peculiar
conditions made work difficult, as the instrument was very susceptible
to small changes of level, such as occurred from time to time owing to
the pressure of the ice on the ship. An ionium collector, for which the
radioactive material was kindly supplied by Mr. F. H. Glew, was used.
The chief difficulty to contend with was the constant formation of
thick deposits of rime, which either grew over the insulation and
spoiled it, or covered up the collector so that it could no longer act.
Nevertheless, a considerable number of good records were obtained,
which have not yet been properly worked out. Conditions during the
Expedition were very favourable for observations on the physical
properties and natural history of sea-ice, and a considerable number of
results were obtained, which are, however, discussed elsewhere, mention
of them being made here since they really come under the heading of
physics.
In addition to these main lines of work, many observations of a
miscellaneous character were made, including those on the occurrence
and nature of parhelia or “mock suns,” which were very common, and
generally finely developed, and observations of the auroral displays,
which were few and rather poor owing to the comparatively low magnetic
latitude. Since most of the observations made are of little value
without a knowledge of the place where they were made, and since a very
complete set of soundings were also taken, the daily determination of
the ship’s position was a matter of some importance. The drift of the
ship throws considerable light on at least one geographical problem,
that of the existence of Morrell Land. The remainder of this appendix
will therefore be devoted to a discussion of the methods used to
determine the positions of the ship from day to day.
The latitude and longitude were determined astronomically every day
when the sun or stars were visible, the position thus determined
serving as the fixed points between which the position on days when the
sky was overcast could be interpolated by the process known as “dead
reckoning,” that is to say, by estimating the speed and course of the
ship, taking into account the various causes affecting it. The sky was
often overcast for several days at a stretch, and it was worth while to
take a certain amount of care in the matter. Captain Worsley
constructed an apparatus which gave a good idea of the direction of
drift at any time. This consisted of an iron rod, which passed through
an iron tube, frozen vertically into the ice, into the water below. At
the lower end of the rod, in the water, was a vane. The rod being free
to turn, the vane took up the direction of the current, the direction
being shown by an indicator attached to the top of the rod. The
direction shown depended, of course, on the drift of the ice relative
to the water, and did not take into account any actual current which
may have been carrying the ice with it, but the true current seems
never to have been large, and the direction of the vane probably gave
fairly accurately the direction of the drift of the ice. No exact idea
of the rate of drift could be obtained from the apparatus, although one
could get an estimate of it by displacing the vane from its position of
rest and noticing how quickly it returned to it, the speed of return
being greater the more rapid the drift. Another means of estimating the
speed and direction of the drift was from the trend of the wire when a
sounding was being taken. The rate and direction of drift appeared to
depend almost entirely on the wind-velocity and direction at the time.
If any true current-effect existed, it is not obvious from a rough
comparison of the drift with the prevailing wind, but a closer
investigation of the figures may show some outstanding effect due to
current.[1] The drift was always to the left of the actual
wind-direction. This effect is due to the rotation of the earth, a
corresponding deviation to the right of the wind direction being noted
by Nansen during the drift of the _Fram_. A change in the direction of
the wind was often preceded by some hours by a change in the reading of
the drift vane. This is no doubt due to the ice to windward being set
in motion, the resulting disturbance travelling through the ice more
rapidly than the approaching wind.
[1] Cf. “Scientific results of Norwegian North Polar Expedition,
1893–96,” vol. iii, p. 357.
For the astronomical observations either the sextant or a theodolite
was used. The theodolite employed was a light 3´´ Vernier instrument by
Carey Porter, intended for sledging work. This instrument was fairly
satisfactory, although possibly rigidity had been sacrificed to
lightness to rather too great an extent. Another point which appears
worth mentioning is the following: The foot-screws were of brass, the
tribrach, into which they fitted, was made of aluminium for the sake of
lightness. The two metals have a different coefficient of expansion,
and while the feet fitted the tribrach at ordinary temperatures, they
were quite loose at temperatures in the region of 20° Fahr. below zero.
In any instrument designed for use at low temperatures, care should be
taken that parts which have to fit together are made of the same
material.
For determining the position in drifting pack-ice, the theodolite
proved to be a more generally useful instrument than the sextant. The
ice-floes are quite steady in really thick pack-ice, and the theodolite
can be set up and levelled as well as on dry land. The observations,
both for latitude and longitude, consist in measuring altitude of the
sun or of a star. The chief uncertainty in this measurement is that
introduced by the refraction of light by the air. At very low
temperatures, the correction to be applied on this account is
uncertain, and, if possible, observations should always be made in
pairs with a north star and a south star for a latitude, and an east
star and a west star for a longitude. The refraction error will then
usually mean out. This error affects observations both with the
theodolite and the sextant, but in the case of the sextant another
cause of error occurs. In using the sextant, the angle between the
heavenly body and the visible horizon is measured directly. Even in
dense pack-ice, if the observations are taken from the deck of the ship
or from a hummock or a low berg, the apparent horizon is usually sharp
enough for the purpose. In very cold weather, however, and particularly
if there are open leads and pools between the observer and the horizon,
there is frequently a great deal of mirage, and the visible horizon may
be miraged up several minutes. This will reduce the altitude observed,
and corrections on this account are practically impossible to apply.
This error may be counterbalanced to some extent by pairing
observations as described above, but it by no means follows that the
mirage effect will be the same in the two directions. Then again,
during the summer months, no stars will be visible, and observations
for latitude will have to depend on a single noon sight of the sun. If
the sun is visible at midnight its altitude will be too low for
accurate observations, and in any case atmospheric conditions will be
quite different from those prevailing at noon. In the Antarctic,