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APPENDIX.
the sea-work afterwards executed in the ship. I need hardly allude
to the facilities, afforded by heights, for making eye sketches of the
coast line, and other details, such as the ranges of hills, forms of
banks, &c. Ascending heights near the sea is advantageous in another
point of view ; for not a rock or a shallow escapes notice, if the
day is tolerably clear. While in harbour, every place m the vicinity
which could be examined in boats, or overland excursions, was explored,
as far as our means and time would allow.
Before I speak of the sea-work, it may be useful to say a word
about 'bases,' of four kinds, arranged according to their relative
value.
The first are those derived from good astronomical or chronometrical
observations, made at two stations several miles apart.
The second are deduced from angular measurements of small
spaces exactly known.
The third are obtained by actual measurement with a chain, with
rods, or with a line :—
And the fourth are the rather uncertain bases obtained by sound.
This statement of the relative value of bases, is only meant to refer
to their employment in sea-surveying. I need hardly remind the
reader of these notes, that the third description of bases, however
exact nominally, requires a host of minute precautions, in addition
to what I never found between Valdivia and Cape Horn, namely, a
nearly level and accessible space, of considerable length, on which to
measure.
To attain the utmost precision is a laudable endeavour, no doubt,
when carrying on extensive trigonometrical operations on land; but
it should be home in mind, that every hour employed in what is
commonly called • hair-sphtting'—^in minute details that do not aflfect
the chart or plan which is the result of a sea-siurey, is not only an
hour lost, but an hour taken away from useful employment.
The second kind of bases are so quicldy and easily measured, either
with a sextant or micrometer, across any kind of land or water, and
have been so repeatedly proved in every part of the Beagle's surveys,
that I consider them unobjectionable, when used for such Hmited
operations as making plans of harbours, or fixing the positions of
objects only a few miles distant. By multiplying bases, which with
such easy methods is soon eiFected; and by a frequent use of the
sextant, artificial horizon, and chronometer, material errors may be
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APPENDIX. 20 5
kept out of the work of a practised surveyor. With a sextant, horizon,
and chronometer (in a sheltered spot), a micrometer and board,*
a theodohte, and intelhgent assistants, much work may be done in a.
short time.
When ready to proceed, the chronometer rates being ascertained,
and the weather glasses affording reasonable hope of a day or two
without a gale of wind, we started at day-hght, and worked against
time. Those officers who were engaged particularly with the survey,
did not take part in the routine duties of the vessel. One attended
to the bearing compass, and usually wrote the various angles and
bearings, taken by others as well as himself, in a bearing-book.
Another officer took angles. A third attended to the ship's course, the
soundings, and the patent log. When many angles were required at
one time, or when observations for time, latitude, or true bearing,
were made while taking a round of angles, other officers assisted.
If the bearing compass was steady enough it was used, even
when true bearings were obtained ; or when, if cloudy, the triangulation
was carried on by points fixed from the last harbour. As the
compass was so placed as to be uninfluenced by local attraction,
the bearings it gave, when steady, were satisfactory; yet it was
never trusted imphcitly ; nor at all in matters of consequence. Its
use was as an auxiliary ; not as a principal. Bearings, or angles, of
the highest points, or of marks so well defined as not to be mistaken
in consequence of a change of the place of an observer, were,
of course, always selected, if such were visible : and vertical angles
of all notable heights were not omitted.
For the sake of perspicuity, we considered that positions, fixed
points, or marks, were separated into three classes. In the first
class, were observatories or places at which the latitude, longitude,
and true bearing, were accurately ascertained ; besides those high
peaks, or other well-defined objects which could be seen at a distance
of some leagues, and whose exact places were known by a
triangulation which connected them with an observatory ; and the
highest points of islands, which were neither low, nor small enough
for the eye to overlook them at the first glance.
* A board some feet long, painted black on one side, white on the
other ; exactly measured, and suspended horizontaHy, at right angles to
the observer.