1. Crook, A. W., Phil. Trans. Roy. Soc., A, 250, 387 (1958).
2. Grubin, A. N., Central Scientific Research Institute for Technology and Mechanical Engineering, Book No. 30 (Moscow, 1949). (D.S.I.R. Translation No. 337.)
3. Dowson, D., and Higginson, G. R., J. Mech. Eng. Sci., 1, 6 (1959); 2, 188 (1960).
4. Archard, G. D., Gair, F., and Hirst, W., Proc. Roy. Soc. (in the press).