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 



