SOUTH 
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 mnaged up several minutes. This will reduce 
the altitude observed, and corrections on this accomit 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 vivsible 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 pecuUarly difficult for 
getting really accurate observations, and it is necessary to reduce the 
probability of error in a single observation as much as possi^ le. 
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 re- 
sults showed an agi'eement between sextant and theodolite within 
the errors of the instrument if the temperature Avas above about 
20° Fahr. At lower temperatures there were frequently discrepa^ncies 
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 tim.e 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 Endur- 
ance about fifteen months elapsed during which no check on the chro- 
nometers could be obtained by the observation of known land, and had 
354 
