1. Strutt, J. W. (Lord Rayleigh), Theory of Sound (Dover Publications, New York, 1955), Vol.I, pp.103ff; also "General theorems relating to vibrations," Proceedings of the (London) Mathematical Society (June 1873).
2. Routh, E. J., Dynamics of a System of Rigid Bodies (Dover Publications, New York, 1955), Vol.II, pp.210ff; also "A treatise on the stability of a given state of motion," Adams Prize Essay (London, 1877).
3. Thomson, W. (Lord Kelvin) and Tait, P. G., Principles of Mechanics and Dynamics(Dover Publications,New York, 1962), Vol.I, pp.388ff; relevant material introduced in Treatise onNatural Philosophy (Cambridge Univ. Press, Cambridge, 1879),2nd ed.