282 CARNEGIE INSTITUTION OF WASHINGTON. 



Table 7. — Mean values of atmospheric-electric elements obtained on the fourth cruise of the Carnegie. 



The new values are in very good general agreement with those for the 

 period through March 1917, and, indeed, they suggest nothing to be added 

 to or taken from the conclusions already published and based upon the 

 former data. 



In working up the diurnal-variation data, some of the material from the 

 period up to March 1916, has been included, in order to obtain the mean diurnal- 

 variation curve for a whole year. There is naturally some indefiniteness to 

 the meaning to be attached to such a result on account of the great variation 

 in latitude concerned. The diurnal variation has been investigated for the 

 potential gradient, positive ionic content, and penetrating radiation. A 

 Fourier analysis has been made for the first two, but for the purposes of 

 abstract it is better to discuss the curves in general terms. 



The diurnal variation of the potential gradient has been obtained from 20 

 sets of observations, ranging from February 25, 1916, to February 20, 1917. 

 The curve shows its principal maximum about 9 a. m., in good agi'eement with 

 the results through March 1916. A secondary maximum occurs about mid- 

 night, or a little earlier, but it is less pronounced than in the earlier observa- 

 tions, and this naturally results in the early-morning minimum being less 

 pronounced than in the earlier observations. The afternoon minimum for- 

 merly found about 3 p. m. now appears somewhat later, about 6 p. m., and is 

 less marked than before. In fact, speaking generally, the effect of the 12-hours 

 Fourier wave is less important in the present curves than in those already pub- 

 lished. 



The diurnal variation of the ionic content has been obtained from 18 sets 

 of observations ranging from February 25, 1916, to February 20, 1917. The 

 curve shows a fairly regular variation with a maximum at midday and a 

 minimum somewhat later than midnight, but which would have occurred 

 almost exactly at midnight except for the interposition of a small secondary 

 maximum about that hour, which interfered with the development of the 

 principal minimum. In the observations already published there appeared 

 a flat maximum, ranging from 6 a. m. to 2 p. m., and a minimum at midnight. 



