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. 207 number of inferior marks ; by a ' round ' of angles taken with a theodolite, at each station (B and C), the instrument being in each case set to A. From A, the true bearings of B and C were ascertained astronomically. When the position of L was exactly determined by latitude, and distance from the meridian of A, the long and accu- rate base AL became known. With that, as a foundation, the work was laid down : and by that base, if necessary, the former positions of G, D, E, and F, were corrected. But so well did angular mea- sures answer, that it was scarcely ever requisite to make such cor- rections. It has been shewn that the log served only to place soundings, or help to fill up a space, while clouds obscured marks. I should add that it was serviceable in ascertaining the direction and strength of currents. Currents altering in strength, as well as in direction, prevented our applying the patent log to other uses, although we had every reason to put implicit confidence in its indications, and have often proved their value in the still waters of a deep sound, where no stream of tide^ or current, existed : as well as in harbours, where angular bases were measured for the special purpose. Views of the lands, both in plan and profile, were very frequently taken. When boats could be lowered, and a sufficient object demanded their employment (as at 11, of the first day, and at 10 of the second day) they did not hang idly at their davits. In the example I have given, circumstances conspired in our favour, a rare event in Tierra del Fuego, or on any similar coast, exposed to the prevailing westerly winds of high latitudes ; but when we failed to find anchorage, the triangulation was carried on by first class marks ; and by the ship's positions, when fixed by good observa- tions at sea. But however well such a method may answer in a fine climate— on that coast it was in general unsatisfactory, and very inferior to that of going from one harbour to another. Among many kinds of notation useful in svirveying, the annexed sketch shews a method used by Mr. Stokes, which I had not then seen adopted. It is very convenient, and assists the memory more than any other. In figure 2, A and B are stations, at which the angles specified were taken right and left of a mark, whose bearing was ascertained. Or, the angles only were taken, and the triangles afterwards calculated, or protracted, by reference to the base upon which they depended, such as AB. A sketch on this principle, how-