Two Magnetic Surveys of the British Isles have been made previous to that of which an account is given in this paper. The observations necessary for these were taken between the years 1834-38 and 1857-62, and the results were reduced to the epoch 1842-5 by Sir E. Sabine, in a paper published in the ‘Philosophical Transactions’ for 1870 (‘Phil. Trans.,’ vol. 160, 1870, p. 265). As a full account of both surveys is given in that paper, it is unnecessary to describe them in detail here. The first was made by five observers, viz., Sir E. Sabine, Captain J. C. Ross, Mr. R. W. Fox, and Professors Lloyd and John Phillips. In the second survey (1857-58), Mr. Welsh, Superintendent of the Kew Observatory, made an admirable series of observations in Scotland, though, unfortunately, the exposure to which he was subjected brought on an illness which terminated in his death. Sir Edward Sabine made observations on the Force and Dip at 24 stations in England, and some declinations determined by several naval officers between the years 1855 and 1861, were utilized. Altogether observations were made at 243 stations. (‘Phil. Trans.,’ vol. 162, 1872, p. 319.) It has, we believe, for some time been thought by those interested in terrestrial magnetism, that another survey of the United Kingdom should be undertaken, and we ourselves drew attention to the matter in a paper “On the Irregularities in Magnetic Inclination on the West Coast of Scotland” (‘Roy. Soc. Proc.,’ vol. 36,1884, p. 10). Not only was this desirable in order that the secular changes in the direction of the lines of equal Inclination, Force, and Declination might be re-determined, but also because the earlier surveys left much to be desired with regard to the distribution of the stations and the number of the Declination observations. Thus the Declination was determined about the epochs 1836 and 1857 at 84 stations only, of which 11 were common to the two surveys. The maps given by Sir E. Sabine in the paper already referred to show that different districts have received very different degrees of attention. Stations where the Dip has been determined cluster thickly about the coast of Scotland to the south of Oban, about the English lakes, and the south coast of England, and are thinly distributed in the North of Scotland, in the eastern counties of England, and in the centre of Ireland. In like manner, while (owing chiefly to the labours of Mr. Welsh) the Declination had been measured at 40 places in Scotland, it had been observed with adequate instruments at only 28 stations in England, and 16 in Ireland. We have therefore undertaken, and, in the course of the five years 1884-88, both inclusive, have completed a new magnetic survey of the British Isles.