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

204 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 lulls, forms of banks, &c. Ascending heights near the sea is advantageous in ano- ther point of view ; for not a rock or a shallow escapes notice, if the day is tolerably clear. While in harbour, every place in the vicinity which could be examined in boats, or overland excursions, was ex- plored, 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 chronome- trical observations, made at two stations several mUes apart. The second are deduced from angular measurements of small spaces exactly known. The third are obtamed 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 borne in mind, that every hour employed in what is commonly called ' hair-sphtting' — in minute details that do not affect the chart or plan which is the result of a sea-survey, is not only an hour lost, but an hour taken away from useful employment. The second kind of bases are so quickly 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 limited •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 effected ; and by a frequent use of the sextant, artificial horizon, and chronometer, material errors may be