A consideration of the fluctuations in the angular momentum of the earth's mantle, indicated by changes in the length of the day, and an estimate of possible rotations in the earth's fluid metallic core that would compensate for them. Measured at 5-year intervals from 1885 to 1965, the length of the day has varied by about 5 milliseconds total. The slowing of rotation (longest days) noted in 1910 and 1965 is correlated with decreases in the westward drift of the earth's magnetic eccentric dipole. After 1950 the surface estimates do not agree with the astronomical data, and two careful estimates for 1965 show glaring disagreement: Leaton's 7.15 km/yr and Cain's 16.70 km/yr. The fit to main field data is about equally good for both. Estimates of westward drift in the core based on higher-degree harmonics give a lower drift. These discrepancies are unexplained. 18 pp. Refs.
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