Narrative of the surveying voyages of his majesty's ships Adventure and Beagle (vol.2- Appendix): between the years 1826 and 1836 : describing their examination of the southern shores of South America, and the Beagles's circumnavigation of the globe

APPENDIX. 205 kept out of the work of a practised surveyor. With a sextant, hori- zon, and chronometer (in a sheltered spot), a micrometer and board,* a theodolite, and intelligent 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-light, 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 triangu- lation 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 mis- taken 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 obsen^atories 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 dis- tance 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 horizontally, at right angles to the observer.