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physical observations originally made out had to be considerably modified.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.*
* Cf. "Scientific results of Norwegian North Polar Expedition,
1893–96," vol. iii, p. 357.
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.
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, therefore,
conditions are peculiarly difficult for getting really accurate
observations, and it is necessary to reduce the probability of error in a
single observation as much as possible. When possible, observations of the
altitude of a star or of the sun should be taken with the theodolite,
since the altitude is referred to the spirit-level of the instrument, and
is independent of any apparent horizon. During the drift of the Endurance
both means of observation were generally employed. A comparison of the
results showed an agreement between sextant and theodolite, within the
errors of the instrument if the temperature was above about 20° Fahr. At
lower temperatures there were frequently discrepancies which could
generally be attributed to the mirage effects described above.
As the Endurance was carried by the ice-drift well to the west
of the Weddell Sea, towards the position of the supposed Morrell Land, the
accurate determination of longitude became a matter of moment in view of
the controversy as to the existence of this land. During a long voyage
latitude can always be determined with about the same accuracy, the
accuracy merely depending on the closeness with which altitudes can be
measured. In the case of longitude matters are rather different. The usual
method employed consists in the determination of the local time by
astronomical observations, and the comparison of this time with Greenwich
time, as shown by the ship's chronometer, an accurate knowledge of the
errors and rate of the chronometer being required. During the voyage of
the Endurance about fifteen months elapsed during which no check on
the chronometers could be obtained by the observation of known land, and
had no other check been applied there would have been the probability of
large errors in the longitudes. For the purpose of checking the
chronometers a number of observations of occultations were observed during
the winter of 1915. An occultation is really the eclipse of a star by the
moon. A number of such eclipses occur monthly, and are tabulated in the
"Nautical Almanac." From the data given there it is possible to
compute the Greenwich time at which the phenomenon ought to occur for an
observer situated at any place on the earth, provided his position is
known within a few miles, which will always be the case. The time of
disappearance of the star by the chronometer to be corrected is noted. The
actual Greenwich time of the occurrence is calculated, and the error of
the chronometer is thus determined. With ordinary care the chronometer
error can be determined in this way to within a few seconds, which is
accurate enough for purposes of navigation. The principal difficulties of
this method lie in the fact that comparatively few occultations occur, and
those which do occur are usually of stars of the fifth magnitude or lower.
In the Antarctic, conditions for observing occultation are rather
favourable during the winter, since, fifth-magnitude stars can be seen
with a small telescope at any time during the twenty-four hours if the sky
is clear, and the moon is also often above the horizon for a large
fraction of the time. In the summer, however, the method is quite
impossible, since, for some months, stars are not to be seen.
No chronometer check could be applied until June 1915. On June 24 a
series of four occultations were observed; and the results of the
observations showed an error in longitude of a whole degree. In July,
August, and September further occultations were observed, and a fairly
reliable rate was worked out for the chronometers and watches. After the
crushing of the ship on October 27, 1915, no further occultations were
observed, but the calculated rates for the watches were employed, and the
longitude deduced, using these rates on March 23, 1916, was only about 10´
of arc in error, judging by the observations of Joinville Land made on
that day. It is thus fairly certain that no large error can have been made
in the determination of the position of the Endurance at any time
during the drift, and her course can be taken as known with greater
certainty than is usually the case in a voyage of such length.
APPENDIX I:
SOUTH ATLANTIC
WHALES AND WHALING |
|
Free world delivery
2001