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The Editors do not hold themselves responsible jO'I' opinions expressed by their cO'l'Tespondents. No notice is taken of anonymous communications. Variation in theSpeed of Rotation of the Earth since June 1955 TIME and frequency comparisons have. been ma.de since June 1955 between the National Physical Laboratory, Teddington, and the U.S. Naval Observatory, Washington, with the ultimate purpose of establishing the value of the transition frequency of cresiuro, {4,0) ~ {3,0), in terms of the second of Ephemeris Time based on the orbital motion of the Moon 1 , 2 • The second of Ephemeris Time is being determined at the Naval Observatory from photographic observations of the Moon, and will be reported on later. 9192631950 vu 900

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The U. S. Naval Observatory (USNO) uses clock averaging of about 16 selected cesium beam clocks for the derivation of an extremely uniform clock time scale. Frequency stabilities of better than 2 x 10-14 for very long intervals are being achieved together with utmost reliability of the time scale. Individual clocks differ widely in their performance and are carefully selected for service in the USNO clock time scale.I n this set which is selected according to frequency stability, the standard deviation of a clocks average frequency difference from the mean is 1.7 x the standard deviation of the uncorrected ensemble mean frequency is 4.4 x 10-13. The day to day frequency variation of individual clocks is 2.2 x 10-13 (rms) and the best clock observed had a frequency stability of 3 x for 40 day averaging. P (mean)= 6.2.An uncorrected mean will, therefore, approach 1 x lO-1* for z > 30 days. This is very likely the case

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