Observations on the permanency of the variation of the compass at Jamaica. In a letter from Mr. James Robertson to the Right Hon. Sir Joseph Banks, K. B. P. R. S. &c

Abstract

The object of Mr. Robertson, who resided in Jamaica, as a King’s Surveyor of Land, upwards of twenty years, is to show that no alteration has, for a considerable period, taken place there in the va­riation of the compass. In that island all grants of land have a dia­gram thereof annexed to the patent, which diagram is delineated from an actual survey of the land to be granted, and has a meridional line, according to the magnetical needle, laid down upon it; but no notice is taken of the true meridian. The boundary lines are marked upon the land; and in all disputes where the keeping up of these lines been neglected, surveyors are appointed to make actual re­surveys, which are compared with those preserved in the secretary of the island’s office; and it is expected that the lines and meridians of the former will coincide with those of the latter. It is evident, however, that this coincidence could not happen if any alteration in the variation had taken place in the interval between the two sur­veys. Mr. Robertson’s business, as a surveyor, having been very ex­tensive, he has had many opportunities of investigating the fact here treated of; and it appears from his observations, that the courses of the lines and meridians delineated on diagrams annexed to patents granted so long ago as the year 1660, coincide with, and are parallel to, the lines and meridians delineated on the re-surveys annexed to deeds, &c., or on the new diagrams, from recent surveys made by means of the magnetical needle, consequently no variation of the needle could have taken place, in Jamaica, during the above period of time. Our author subjoins to his paper a short history of the practice of surveying in Jamaica, from the Restoration to the present time, in order to obviate any doubt whether the quantity of the magnetical variation was not ascertained and allowed for in the first diagrams annexed to patents; and whether the present variation of 65 degrees east, might not then have agreed with the true meridian. He re­marks, that until the year 1700, when Dr. Halley published his theory of the variation of the compass, no observations to ascertain the quantity of the variation in the West Indies had (so far as he knows) been published; and the variation at Jamaica, as laid down by Dr. Halley, appears to have been the same as it is at present. Be­sides, had the first surveyors allowed for the variation, in delineating their diagrams, they would not have omitted to mention it; and the same system of surveying would have been continued, since a dif­ference of 65° would have so totally deranged all boundaries, as to have demanded legislative interference and correction. But no in­ stance of this kind has occurred.