1. Williams, J. C. Univ. Sheffield Fuel Soc. J. 14, 29–34 (1963).
2. Williams, J. C. Powder Technol. 2, 13–20 (1968).
3. Carson, J. W., Royal, T. A. & Goodwill, D. J. Bulk Solids Handling 6, 139–144 (1986).
4. Carson, J. W., Royal, T. A. & Goodwill, D. J. Chem. Eng. Prog. 92(11), 72-80 (1996).
5. Makse, H. A., Havlin, S., King, P. R. & Stanley, H. E. Nature 386, 379–382 (1997).