264 



CARNEGIE INSTITUTION OF WASHINGTON. 



Table 3. — Summary of results of preliminary analysis of the diurnal variation of the potential 

 gradient at the Apia Observatory for jjear May 1922 to April 1923, inclusive. 



The following features of table deserve brief mention thus : 



(a) The constancy, throughout the year, of <p2 and <f>i, for which the ranges, expressed in 

 time, amount to only 0.6 hour and 0.4 hour, respectively. 



(b) Except during the May-July quarter, the amplitude of the fourth harmonic, Ct, is 

 consistently greater than Cs- 



(c) Rather large differences, corresponding to 4 to 6 hours, are found, between the Apia 

 values of <pi and those obtained for the corresponding times of year from the observations 

 made in the Pacific Ocean, aboard the Carnegie. But these apparent differences may pos- 

 sibly be due, at least in part, to the rather large local effects indicated by the analysis. 



(d) The annual variation of the potential gradient for the 12 months in question appears 

 to have been very small; in fact, the variation here shown may practically disappear, or 

 even be reversed, when the effect of the removal of a tree in November 1922 from the vicinity 

 of the recording hut has been fully determined. 



Comparison of the above results with those previously obtained at Apia 

 must await the accumulation of data for a period of time more nearly com- 

 parable with the length of some of the earlier series. It is hoped also that 

 registrations of the potential gradient now being obtained at an observatory 

 erected on a reef at some distance from the island (see p. 242) may furnish 

 valuable data for the solution of some of the problems of atmospheric elec- 

 tricity as evidenced at Apia. 



The diurnal variation of atmospheric-electric conductivity and air-earth current from obser- 

 vations obtained on the Carnegie. S. J. Mauchly. 



During the months April to October 1921, fourteen 24-hour series of simul- 

 taneous diurnal-variation observations for positive and negative conductivity, 

 X-t- and X_, and potential gradient were obtained by Messrs. J. P. Ault and A. 

 Thomson on the Carnegie. The extreme latitudes represented by these obser- 

 vations were about 29° south and 34° north, and, with the exception of one 

 series in the Caribbean Sea, all were obtained in the Pacific Ocean. Half 

 the observations were made within 15° of the equator, average latitude 8°, 

 and the remainder near the tropics, with an average latitude of 26°. 



As found for X+ from earlier observations on the Carnegie (see p. 356 of the 

 1921 report), the diurnal variation for both X4. and X_ is less pronounced than 

 that of the potential gradient and progresses according to local time. Sep- 

 arate mean curves representing the region of the tropics and of the equator, 

 however, indicate some interesting differences. Although both curves show 

 maxima in the neighborhood of 8 to 10 a. m. and 8 to 10 p. m., the inter- 

 vening minimum appears from these observations to be decidedly secondary 

 for the region of the tropics, while it is the principal minimum for the region 

 of the equator. The mean values, expressed in unit of 10"* E. S. U., are as 

 follows: For the region of the tropics, X+ = 1.65 and X_ = 1.44, and for the 

 region of the equator, X+ = 1.63 and X_ = 1.38. 



The diurnal variation of the vertical air-earth current, as computed from 

 the potential gradient and total conductivity, resembles very closely that 

 of the potential gradient as regards its main features. That is, the curves 



