ERRATIC CHANGES IN CLOCK RATES.—An interesting suggestion as to the cause of the sudden variations which are sometimes observed in the rate of the three standard clocks of the U.S. Naval Observatory has been made by Mr. W. A. Conrad (Popular Astronomy, vol. xxv., p. 522). It has long been noticed that the rates are subject to sudden fluctuations, and that the three clocks usually vary in the same direction at the same time, and by almost equal amounts. As the temperature and pressure controls appear to be beyond suspicion, such changes have hitherto been attributed to imperfect determination of instrumental constants. In seeking the cause of a very bad jump in the rates of the three clocks in February, 1917, it was found that many jumps were coincident with “cold waves,” and that on this occasion there was a very marked low-pressure area receding to the east and an abnormally high barometer to the west. It is suggested that the observations of the clock stars may have been affected by lateral refraction, and that a study of the weather map might possibly help to explain the anomalous results which have occasionally been obtained in determinations of the positions of stars.