1. Frith, R., Met. Mag., 77, 241 (1948).
2. Langwell, P. A., J. Met., 5, 243 (1948).
3. Megaw, E. C. S. (see following communication).
4. Scott, J. M. C., unpublished memorandum (1944). (I am indebted to the Chief Scientist, Ministry of Supply, and to Mr. Scott for permission to refer to this unpublished paper.)
5. Hartridge, H., and Weale, R., Nature, 164, 999 (1949); and subsequent discussion: ibid., 165, 146, 663, and 1019 (1950); 166 151 (1950).