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

64 ABROLHOS BANKS AND ISLETS. Marcll again steered westward, till a heavy pull upon the line, and a sudden jerk, showed that we had hooked the bank. The ship was hove-to, and the necessary observations made on the spot. The grapnel, when hauled up, was found to be straightened, a proof, in addition to that afforded by the lead, that the bottom was rocky. Our soundings at this time were thirty-eight fathoms, and thence to the Abrolhos islets we carried a line of soundings, no where exceeding that depth, but extremely irregular, between thirty-six and four fathoms. As far as we had time to examine, the chart of these islands, by the Baron Roussin, appeared to be satisfactory ; but the soundings are so very irregular in the vicinity of the Abrolhos, that little dependence could be placed on the lead. More than once we had four or five fathoms under one side of the vessel, and from fifteen to twenty under the other. These sudden and startling changes, called by the French, ' Sauts de sonde,' are very unpleasant and perplexing. The tide, or rather current, which we found when lying at anchor near the islets, set continually to the southward, vary- ing in strength from half a mile to a mile and a half an hour ; but we had only three days' experience. I had imagined, from what I had heard, that the rock of which these islets were chiefly composed was coral ; but was sur- prized to find only coralline growing upon gneiss or sandstone. While sounding near the Abi'olhos we made a great num- ber of experiments with Massey's lead, in order to verify its qualities ; and found it agree remarkably well with the com- mon lead, while in less than forty fathoms, but differ from it frequently when the depth of water exceeded seventy fathoms ; and wholly fail when used in upwards of one hun- dred and twenty fathoms. The failure, in great depths, was in consequence of the small hollow cylinder, to which the vanes were attached, bursting, or rather, being compressed by the weight of water. Some more remarks upon this instrument will be found in the Appendix. We anchored near the islets, at dusk, on the 28th, after being in frequent anxiety, owing to sudden changes in the