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 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 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 rig-ht angles to 

 the observer. 



