Letter from Captain Basil Hall, R.N. to Captain Kater, Communicating the Details of Experiments made by him and Mr. Henry Foster, with an Invariable Pendulum, in London; at the Galapagos Islands in the Pacific Ocean, near the Equator; at San Blas de California on the N.W. Coast of Mexico; and at Rio de Janeiro in Brazil. With an Appendix, Containing the Second Series of Experiments in London, on the Return