1850-60] ROYAL ASTRONOMICAL SOCIETY 117 Observer, who in an excellent address paid full tribute to the medallist's unexampled perseverance and industry. In 1852, R. Wolf announced a corrected period of ii-n years, and in the same year the connection between sun-spots and the magnetic elements on the earth was shown independently by three men : Gen. Sabine in England, Wolf at Berne, and A. Gautier at Geneva. We must also claim for this decade the actual completion and publication of a gigantic work, not perhaps strictly astronomical, but with an intimate connection with astronomy, the Principal Triangulation of the United Kingdom. The scientific direction and control of the calculations were in the hands of A. R. Clarke, who was elected a Fellow on 1850 March 8, and continued in the Society until his death a few years ago. He was never awarded the Gold Medal, an omission which some of the geodesists in the Society regret, but they admit at the same time that there is no instance of an award for geodetic work pure and simple. Clarke's Geodesy still remains the best English book on the science, though the great activity on the Continent in recent years has produced publications which now leave it, in certain directions, somewhat out of date. In 1852 there occurred the death of one of the founders of the Ordnance Survey, Thomas Colby, who was concerned with the triangulation of England from its beginning in the early years of the century and took a leading part in it up to the year 1847. His obituary notice covers nearly fourteen pages of the Report of the Council to the Annual Meeting on 1853 February 1 1 . Among many other activities, he was the designer of the compensation bars used in the measurement of the Salisbury Plain and other bases. The publication and discussion of these geodetic results drew renewed attention to an old problem, the determination of the mean density of the earth. Three methods were available for its solution. Firstly, the comparison of the earth's attraction with the horizontal attraction exercised by a mountain mass. This method had been used by the Ordnance Survey, and an account of it was presented by Col. James, the Director-General, to the Royal Society in 1856 February. A second possible method was a comparison of the attraction at the surface with that found at a definite dis- tance below the surface. This was tried by the Astronomer Royal at Harton Colliery in 1854, with every precaution that his skill and experience could suggest. The actual result was disappointing, the value found, 6-566, being now known to be very considerably in excess of the truth. The third method, and the only one of these three which would now be reckoned of any value, is the Cavendish experiment, wherein the earth's attraction is directly compared, by means of a torsion balance, which can be made of almost any required degree of sensitiveness, with the attraction
