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D. C. E. Speller, R. Y. Cartwright, E. G. V. Evans, R. J. Hay, D. W. R. Mackenzie, L. J. R. Milne, F. C. Odds, D. W. Warnock, H. J. Wingfield, Report of a Worki
Speller et al. (Sun,) studied this question.