420 



Reports on Special Researches 



Table 95. — Berndl's Results on the Daily 

 Variation of Ionic Content and Poten- 

 iial-Gradient Over the Atlantic Ocean. 



result which, however, would be expected if the diurnal variation were largely controlled by 

 the temperature. 



There are hardly any other ocean results with which to compare the present observa- 

 tions on the diurnal variation. Simpson and Wright,^ on the basis of certain ocean observa- 

 tions made within 40° of the equator, came to the con- 

 clusion that the potential-gradient has its chief minimum 

 soon after midday, a result in harmony with those of the 

 present cruise, as will be seen on reference to Figure 26. 

 Table 95 shows values obtained by G. Berndt'^ in the 

 Atlantic Ocean in 1913. The potential-gradient results 

 are here only relative, and X represents the means of 

 the potential-gradient observations on all days, while X' 

 represents the mean values as obtained on days labeled as 

 normal from an atmospheric-electric standpoint, and in which negative values of the 

 potential-gradient are omitted. These observations, as far as they go, are in agreement 

 with the present results on the diurnal variation, at any rate in the cases of X' and the 

 ionic content. 



ANNUAL VARIATION. 



In considering the type of annual variation to be expected from observations such as 

 those at present under discussion, one has to remember that it is the season, rather than the 

 time of the year, which controls the phenomena, so that it would not be reasonable to plot 

 the quantities against the time without due regard to the variation with latitude. Thus, 

 for example, account must be taken of the fact that the seasons are reversed in the southern 

 hemisphere. However, since the circumnavigation voyage in the sub-Antarctic Oceans 

 corresponds roughly to a constant latitude, it enables us to seek evidence on the annual 

 variation over the period of duration of this voyage (about four months). To this end the 

 10^2 values have been meaned for each of the four months, and the results plotted against 

 the corresponding mid-times for the months.' The results are shown in Figure 28. Remem- 

 bering that the seasons are reversed in the latitudes in question, we should, by analogy 

 with the results for land values, expect a minimum of the potential-gradient in January. 

 As a matter of fact, a maximum appears here, but this is followed so soon by a minimum 

 that it is difficult to draw any conclusions as to the normal type of variation, and, indeed, 

 it is probable that the mean of the observations over several years would be necessary to 

 satisfactorily settle the question of the annual variation. In the case of the ionic numbers, 

 conductivities, and air-earth current-densities, the curves suggest more definite conclusions 

 and indicate distinct minima in January. The land observations in the northern hemi- 

 sphere give minima for the ionic content and conductivity in the late winter months, and 

 maxima in the late summer and autumn. In view of the reversal of the seasons in the 

 southern hemisphere the results of the ocean observations are, therefore, somewhat sur- 

 prising. However, it is difficult to form a proper opinion of the general curve of the 

 aimual variation from observations extending over four months, and it is possible that the 

 minima which the present observations indicate for January are only depressions in the 

 main maxima of the curv^es. 



^Ptoc. R. Soc. a, vol. 85, p. 181, 1911. 

 'Met. Zeit., vol. 30, p. 606, 1913. 



•In the cases where there were no observations for certain periods of a month the time used is the mean time corre- 

 sponding to the observations secured. 



