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

206 APPENDIX. In the second class we considered all the minor fixed points wliich were included in the triangulation, excepting the details of the coasts and ' boundary lines,'* which belonged to the tliird class. We supposed that the ship had sailed from the ' first' harbour. f At six, in the morning, (marked 6) the position of the vessel was fixed by two or more angles between marks, already fixed, upon the land. At 6'30, and at 7 similar means were used to fix the ship's place. Soundings were taken, and laid down by proportioning to the times of each sounding, the portions of distances run between the two stations, as shewn by patent log, by bearings of a mark, such as 6 (in the figure) while sailing from 6 30 to 7 : by independent double angles, (two angles between three marks), orliy simple cross- bearings. Transit bearings were always sought for, by compass as well as by noting when marks were ' on ;' or, in a line, one with another, with- out reference to the compass. We endeavoured to ascertaiii (or fix) the ship's position at the same moment, by the most available of the numerous methods so readily occurring Mobile the log was going, the time noted carefully (as often as angles or bearings were taken), and several first class marks in sight. Transit bearings were useful in the details of the coast line, as may be seen by the lines dra^vn from 6-30, 7 and 8 ; and they corroborated the correctness of the triangulation, when applied to first and second class marks. By a judicious selection of objects, and a clever application of transit bearings, I have seen extensive and correct triangulations carried on from data, obtained at sea, which appeared uttei-ly in- adequate. | I do not imply that the absolute position of any one point was independently correct, because all depended at first upon observations at sea ; but tkat the points of the triangulation were all so correct, relatively, that, upon after examination, when the regular routine of harbour work had been combined with the data obtained afloat, and their truth ascertained by connection with the previous observatory, no alteration was found necessary. Perhaps I should explain, that in the plan of the ' first ' harbour all depended upon the base Ab ; by which also were fixed B and C. From the summits B and C ; — G, D, E, F, were fixed, as well as a * By ' boundary lines ' [ mean limiting outlines of shoals, or rocky place?. f See figure, t By Mr. Stokes.