152 



DE. C. CHREE : ATMOSPHERIC ELECTRICITY POTENTIAL GRADIENT 



When the sign is plus the surface density is increasing numerically. The total 

 increase in the negative charge per sq. cm. from 4 a.m. to 10 a.m. amounts to 

 153xlO~ 18 coulombs, and the most rapid hourly increase occurring between 

 7 and 8 a.m. is 49 x 10~ 15 coulombs. This is equivalent to a steady current of 

 l'36xlO~ 17 amperes per sq. cm. The average value of the air-earth current, 

 as measured at various stations, is in the neighbourhood of 2xlO~ 18 amperes 

 per sq. cm., or about 15 times as big as the above current. Thus in general the 

 changes in surface density must represent the balance between a much larger air-earth 

 current and earth currents whether vertical or horizontal. 



11. Table V. gives the amplitudes and phase angles of the 24-, 12-, 8- and 6-hour 

 Fourier "waves" in the diurnal inequalities derived from the whole 15 years, and 



TABLE V. Diurnal Inequality 18 ( J8 to 1912. Amplitudes and Phase Angles. 



Table VI. does the same for the period 1905 to 1912. The inequalities are supposed 

 to be represented by the formula 



Cj sin (t + a,) + c a sin (2t + 2 ) + . . . , 



t being counted from local midnight, and 15 being taken as the equivalent of one 

 hour. The inequalities in Tables II. and III. referred to G.M.T., and the phase angles 

 were first calculated in terms of G.M.T., and then corrected to local mean time by 



