The Cavendish experiment for determining the constant of gravitation, from which the density of the earth may be calculated, is so well known that there is no occasion to describe it. This experiment, devised by the Rev. John Mitchell, F. R. S., was first carried out by Cavendish, and has been since performed by Reich, Baily, and Cornu and Bailie, who have all followed very closely the arrangement of Cavendish.