--On arriving at the sea-level, make daily
observations with your boiling-point thermometer, barometer, and aneroid,
as they are all subject to changes in their index-errors. As soon as you
have an opportunity, compare them with a standard barometer, compare also
your ordinary thermometer and azimuth-compass with standard instruments,
and finally, have them carefully re-verified at the Kew observatory on
your return to England. A vast deal of labour has been wholly thrown away
by travellers owing to their neglecting to ascertain the index-errors of
these instruments at the close of their journey. A careful observer ought
to have eliminated the effects of instrumental errors from his sextant
observations; nevertheless it will be satisfactory to him, and it may
clear up some apparent anomalies, to have his entire instrumental outfit
re-verified at Kew.
Observations, to recalculate.--Send by post to England a complete copy
(always preserve the originals) of all your astronomical observations,
that they may be carefully recalculated before your return, otherwise a
long period may elapse before the longitudes are finally settled, and
your book may be delayed through the consequent impossibility of
preparing a correct map. The Royal Geographical Society has frequently
procured the re-calculation of observations made on important journeys,
at the Royal Greenwich Observatory and elsewhere.
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