Records are reproduced showing diffuse echoes from the F‐region of the ionosphere received continuously at night in equatorial regions over a wide range of wave‐frequency. Tney are interpreted as due to Rayleigh scattering by spatial irregularities in the distribution of electron‐density at or above a definite level in the F‐region. Because of the highly dispersive nature of the ionosphere, there is no marked dependence of Rayleigh scattering upon wave‐frequency such as there is for a non‐dispersive medium. According to this interpretation, variation of the maximum wave‐frequency to which diffuse echoes can be followed has nothing to do with variation of the maximum electron‐density of an ionospheric region, but merely indicates variation in the size of irregularities in electron‐density. Scattering of this type may have some bearing upon the phenomenon of persistence of E‐region echoes to wave‐frequencies greater than the critical penetration‐frequency of the E‐region.

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The edible mussel, Mytilus e dulls L., attaches to rocks, pilings, and other firm substrates, being particularly abundant in the lower intertidal zone, where it is a dominant organism in the community (Newcombe, 1935;Dexter, 1947). A temperate and boreal species, it is found on both American and European coasts of the North Atlantic as well as Asian and American coasts of the North Pacific Ocean. Its distribution has aroused the interest of ecologists as an example of

AbstractsFurther studies of the F-region, ordinarily composed of the F-layer at night which separates into the F•-and Fz-layers at small zenith-angles of theSun, have been continued at the Huancayo Magnetic Observatory of the Department of Terrestrial Magnetism of the Carnegie !nstituticn of Washington during the past year. The separation of the F•-and Fz-layers appears to occur because of a decrease of virtual height of the F•-layer and a marked rise of virtual height of the Fz-layer. To adequately represent this effect, iso-ionic charts showing the change of virtual height and ionization with time have been constructed. This construction is made possible by a convenient simplification of the virtual height versus frequency graphs obtained from the observed data. the iso-ionic curves of Figure 4 referring to the F•-layer can also be considered approximately as the ion-distribution along the 12th parallel of latitude for this date. To obtain a further idea of the geographical distribution of this layer it is of interest to refer to the evidence obtained at Washington by Kirby, Berkner, and Stuart [4]. The maximum F• critical-frequency of 4500 kc, reported for summer noon at Washington (zenith-angle about !4ø), corresponds very closely to the results given for Huancayo in Figure 7. This provides a powerful argument in view of the flatness of the diurnal characteristic at noon that the north-south distribution of ionization must be similar to the distribution along a parallel for equal zenith-angles. As the zenith-angle of the Sun increases with season to about 60 ø at Washington, the curves and description of the magnitude and character of the F• critical-frequency on the average agree quite well with the iso-ionic chart of Figure 4 for correspondingly greater zenith-angles of the Sun. It seems pr•fi)al)le, therefore, that isoionic curves for the F•-layer drawn along the meridian fr(•m a point directly under the Sun (that is perpendicular to Fig. 4 at 12 h 15 •") would appear approximately similar to Figure 4]J, at least up to about 60 ø (17•). ()n this basis Figure 9 gives the best conception of the geographical distrit)ution of this layer that can 1)e obtained from available data. . /50-IONIC CURVES GIVoeN IN

Observation of noon F2‐region ion‐density at stations in both the Northern and Southern hemispheres shows that variations in one are not predicted at the other with the hypotheses which have been advanced to explain them. Observations made at Washington by the National Bureau of Standards, and at Watheroo by the Department of Terrestrial Magnetism, Carnegie Institution of Washington, are analyzed. It is found that a variation‐component exists in the data from 1935–37 which is in the same phase at both stations. This term has a principal period which, over the period studied, is indistinguishable from one year. Its amplitude is about equal to that of the seasonal variation, maintaining a nearly constant ratio to the changing background ion‐density. This non‐seasonal variation is, therefore, of sufficient amplitude, if neglected, to vitiate quantitative calculations of seasonal effects based upon the observations from one hemisphere alone. While the average magnitude of the background ion‐density is closely related to annual averages of solar activity, departures of the non‐seasonal component from the mean show no sensible correlation with monthly sunspot‐averages. The data and methods are examined to determine whether inhomogeneity of data, or methods of analysis, could induce the effect artificially. It is concluded that the amplitude of such induced errors must be small compared to the amplitude of the non‐seasonal effect. Application of the heating hypothesis in its present form leads to a heating ratio more than 100 times as great at Washington as at Watheroo, which does not seem probable. Correction of the data for the non‐seasonal effect leads to a heating ratio for summer to winter of about 10 to 1 for both stations. Possible causes of the non‐seasonal term are considered. The paucity of data from widely separated locations through the 24 hours limits speculation. In reviewing the implications of the data, the writers are inclined to the view that the cause is associated with the Earth or its motion, though the arguments presented on the basis of existing data are not sufficient to establish this view unequivocally.