1. Schminnann, R., J. Appl. Phys., 13, 235 (1942).
2. Schnurmann, R., Paper presented in Section III of the 6th Inter. Conf. of Appl. Mech., Paris, Sept. 25, 1946.
3. Schnurmann, R., Nature, 151, 420 (1943).
4. This was a home-made instrument of the Boerlage type (Engineering, 136, 46 ; 1933) using 3/8-in. steel balls. Experimental details will be given in the M.Sc. Thesis of H. J. Bezer.
5. Jones, H. A., Langmuir, I., and Mackay, G. M. J., Phys. Rev., 30, 201 (1927).