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

Dec. 1831. VESSEL easy — compasses. 45 they close upon each other and receive vibratory undulations from each shore ; augmented perhaps by the peculiar formation of the bottom of that bay, the variation in depth, and the effects of currents, which, when running over uneven ground, or against the wind, alone cause a heavy swell; a striking exemplification of which may be seen on the bank of Lagullas, near the Cape of Good Hope. Though so deep in the water, our little vessel's movements were uncommonly easy, and all our best timekeepers being hung in particularly good jimbals, I had no fear of their rates being altered, except by the effect of a change of temperature. This was a point about which I was especially anxious, as so much would depend upon the going of our chronometers, and I did not then think that the motions of a ship affected those instruments so little : as I have since proved to be the case by trying them frequently in boats, or small craft of only a few tons burthen. In her previous voyage the Beagle was as easy a sea^boat as could be desired ; but, having raised her upper deck, altered her stowage and trim, loaded her more heavily, and sheathed her with two-inch plank, prepara- tory to this second expedition, I had abundant cause to feel anxious until the practical effects of such material changes were ascertained. A little alteration was required near the compasses, for owing to some ill-placed iron-work they did not quite agree but, after this change was made, we were gratified by finding four first-rate compasses, three fixed for steering, and one for bearings, agree precisely. Another source of satisfaction, connected with the compasses, was the knowledge that they were not affected, unless in a very trifling degree, by local attraction : for while lying in Barn-pool we swung the vessel in order to ascertain its quantity, but were agreeably surprised to find that none could be detected amounting even to one degree. This was attributed to her having only brass guns and to some very large iron davits for the quarter boats, which were placed rather closely abaft and above the com- passes, and perhaps counteracted the effect of iron in the hold, which was so much more distant.