Page:Proceedings of the Royal Society of London Vol 60.djvu/117

102 The irregularity in rB/rA may be due in part to the slightly unsteady character of the stand forming station B. The potentials at B were also much the lowest, so that errors of reading were there of most importance. At the highest station, F, the variations occurring in the potential sometimes made accurate measurements difficult. § 6. To give a clearer idea of the degree of uniformity shown by Table III, I give in Table IV the extreme and mean readings at the several stations, omitting, as in Table II, occasions of negative potential.

Series of observations. A.

B. A C. +( >r r~ Max. Min. Mean. Max. Min. Mean. Max. Min. Mean. i. 264 104 158 66 22 40 552 120 352 i i . 708 50 206 215 15 58 1320 81 364 m . 174 27 100 45 6 27 306 27 171 IY. 830 29 249 115 12 50 1452 52 476 Series of observations. D. E.

E. ( "A Max. Min. Mean. 648 152 385 1464 122 455 354 30 210 Max. Min. Mean. 776 264 524 1688 182 531 498 30 246 1785 93 662 r~ -*--- *' Max. Min. Mean. 2362 122 1032 i. i i . H I. IY.

On one exceptional day the potential at A varied from —1200 to + 1290 volts in less than forty minutes; at station F it varied from —2424 to over +4000 volts in about the same time. Constancy of Ratios during the Day.

§ 7. Table V gives the mean values of the ratios for the forenoon and afternoon observations, treated separately, during those days when there were readings at both 10.30 A.m. and 4.30 p.m. The days available numbered 17, 10, and 9 respectively in the second, third, and fourth series of observations. The headings “ a.m.” and “ p.m.” distinguish the forenoon and afternoon observations.

In each case treated in table V the mean value of the potential for the forenoon was considerably higher than that for the afternoon. Thus, at station A the ratio of the forenoon to the afternoon mean potential—for those days only on which there were both forenoon