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The needles used in these experiments were cylinders 0·16th of an inch in diameter, and 2·4 inches in length, pointed at the ends, and suspended by a silk fibre 5 inches long, over the centre of a graduated ivory circle. The needle, previous to beginning to count the vibrations, was drawn 50° or 60° from the magnetic meridian by another needle, and left to oscillate. When it had reduced its arc of vibration to 30°, the counting of its vibrations was commenced, and terminated at 5°. It usually took between 300 and 400 vibrations to reduce the arc of vibration to this limit, occupying from 12 to 16 minutes. Four of the needles, with an apparatus in duplicate, were sent to the author from Professor Hansteen of Christiana, to be employed in comparative experiments in various parts of Great Britain. They were vibrated in Edinburgh by Captain Basil Hall and Lieut. Robert Craigie, and the results are set down with the rest in this paper. The needles being returned, were also used in the experiments between Paris and London. The two remaining needles were made by Dollond, of the same size and form as Professor Hansteen’s. The author then relates his experiments, which were made on the 3rd of December, about seven weeks previous to his departure for Paris, in the garden of the Horticultural Society at Chiswick; and on the 15th of January, at Thornfold Park, near Tunbridge Wells; and on the 30th of January, in the garden of the Royal Observatory at Paris. An opportunity occurring, three of the needles were sent to England early in April, and, with one sent by Captain Hall from Edinburgh, were vibrated by Captain Chapman, R. A., in the garden of the Horticultural Society, and returned to Paris.

Experiments to ascertain the ratio of the magnetic forces acting on a needle suspended horizontally, in Paris and in London