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3. Covington, A. E.: 1959, J. Roy. Astron. Soc. Can. 53, 156.
4. Covington, A. E.: 1966, NRC-Monthly Rep. 228.
5. Covington, A. E.: 1968, Circular Letter to WG Members, Sep. 16; NRC-ERB 805, Jan., 1969.