At Ben Heynish, in Tiree, one of the westerly isles of Scotland, the sergeant had to struggle in watching and taking a few observations between the almost incessant storms. Next he was employed in the remeasurement of the latitude of Dunnose, the southern extremity of a British arc of meridian, to verify its result as determined with Ramsden’s zenith sector in 1802, and also to test the value of Professor Airy’s sector. The observations for this purpose were carried on both at Week Down and Dunnose. “The near agreement in the results of the comparison proved very satisfactory as regards the work performed with both instruments; but to endeavour to trace the extent and amount of the disturbance that evidently affected the inclination of the plumbline at Dunnose,” the sergeant afterwards made observations both at Boniface Down and Port Valley, in the Isle of Wight, by which “the difference in the geodetic and astronomical amplitudes between Greenwich and Port Valley were found to be almost insensible, and the comparison with Boniface Down and Week Down tolerably good.” The discovery, however, of singular disagreements in the observations at “one of the stations in the Isle of Wight, which had hitherto been looked on as the southern extremity of the longest of the British arcs of meridian, and where no sensible deviation could _à priori_ have been anticipated, led to the re-examination of the northern extremity of the same arc, situated in Balta Island, by revisiting it, and by observing also from two other stations in the Shetland Islands contiguous to it, viz., Gerth of Scaw and Saxavord.”[58] The disturbance alluded to—the effect of local attraction—caused the plumbline and level to be deflected or acted upon, as a loadstone would influence the needle of a mariner’s compass, and thus, when the levels indicated that the instrument was pointed at zenith, it was in fact directed to a point nearly four seconds to the north of it.