TO THE COLLECTION OF BRITISH BIRDS IN THE MUSEUM OF THE CAMBRIDGE PHILOSOPHICAL SOCIETY. Querquedula glocitans. Bimaculated Duck. Somateria molissima. Eider Duck. fusca. Velvet Duck. Podiceps auritus. Eared Grebe. rubricollis. Red-necked Grebe. Adult. Colymbus glacialis. Northern Diver. arcticus. Black-throated Diver. septentrionalis. Red-throated Diver. Alca impennis. Great Auk. Carbo cristatus. Crested Shag. Sterna Dougallii. Roseate Tern. anglica. Gull-billed Tern. Larus glaucus. Glaucous Gull. Adult. islandicus. Iceland Gull. capistratus, (Temm.) atricilla, (Temm.) minutus. Little Gull. Lestris pomarinus. Pomarine Gull. Procellaria glacialis. Fulmar. Puffinus Anglorum. Shearwater. N. B. Mr. LEADBEATER, No. 19, Brewer Street, Golden Square, is employed by the Society for stuffing their Birds, and will prepare any of the above for them, if sent to him. At the same time, notice of any Bird presented should be addressed to the Secretary of the Society at Cambridge. XIII. On a Correction requisite to be applied to the Length of a Pendulum consisting of a Ball suspended by a fine Wire. BY GEORGE BIDDELL AIRY, M.A. MEMBER OF THE ASTRONOMICAL SOCIETY, FELLOW OF TRINITY COLLEGE, AND OF THE OF CAMBRIDGE. [Read Nov. 16, 1829.] IN the deduction of the length of the simple pendulum vibrating seconds from the time of vibration of a compound pendulum, consisting of a metallic ball supported by a wire, it has always been supposed that the diameter of the ball which in its position of rest was vertical, continues during the whole vibration to be in the same straight line as the wire. This at least was tacitly assumed in the experiments by Borda, Cassini, Arago, and Biot, (the corrections in the Base du Systeme Metrique, Tome III. p. 358, and Biot, Astronomie Physique, Tome III. Additions p. 173, are calculated on that supposition,) and in the only account which I have seen of the experiments lately made by Mr. Bessel, the same thing is tacitly supposed to be correct. Yet it is perfectly certain that, during the vibration, the wire and the diameter which was vertical will make an angle, except in one position which will in practice be the same as the position of rest. I propose in this paper to investigate the motion of such a pendulum, |