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minimum in summer arises from causes peculiar to stations near the ground, and that at moderate heights the winter type of inequality applies in all latitudes throughout the entire year. It should be remarked, however, that some of the results in Tables I. and II. hardly seem to fit this theory. Some of the irregularities in Tables I. and II. would probably disappear if more years’ data were employed; this is especially true in cases where the range is small, as at Cape Thorsden in winter. In this instance, according to the data, the inequality is almost the opposite of that usually met with, the potential being highest in the early morning hours, and at about its lowest value between 9 and 11 p.m. In summer the morning minimum is as well marked as at other places. It will Noon a6m N0 n be noticed that Night & ™ Night ° ™ Nigm . the f evening maximum is decidedly less prominent at Greenwich than at Kew, especially in winter, and that the afternoon minimum in winter is distinctly present at Kew though hardly visible at Greenwich.

Cape Thorsden, 1882-83 Sodankyla, 1882-83 Kew,’ 1880 fpos.daonly ys •. Greenwich, 1893-94, 1896 Florence, 1883-86. Perpignan, 1886-88 Lisbon, 1884-86. Batavia (2 m.), 1887-90 Batavia (7‘8 m.), 1890-95

ELECTRICITY In considering such discrepancies it would be well to bear in mind the possibility, or rather probability, that atmospheric potential may, like thunderstorms and auroras, vary much from one year to the next. At Lisbon the summer inequality presents three fairly equal maxima. These are possibly fictitious, as the range is exceptionally small, but the phenomenon was exhibited by each of the three years included, and there is a suggestion of more than two maxima at some of the other stations. Narrowness of diurnal range is clearly no general attribute of low latitudes, for that at Batavia is most conspicuous. Some of the irregularities in the tables may be due to the interaction of conflicting agencies, of which one may be dominant at one time, another at another. Thus C. R. Andre (Comptes Rendus, 112 (1891), p. 1509), on separating quiet bright days at Lyons into two sets, according as the wind was northerly or southerly, concluded that the characteristics of the diurnal inequality were conspicuously different in the two cases. Andre’s diagrams show two well-marked maxima and minima (with small doubtful ones) in each instance, but with southerly wind the morning maximum is much the larger, whereas with the wind northerly it is somewhat the smaller.

§ 8. The mean of the hourly ordinates of electrograms (electric potential curves) for a month gives the monthly mean, and the intercomparison of the monthly means gives the annual inequality. For reasons already indicated, Table III. gives relative not absolute values for the twelve monthly means. The data are from the same sources as those utilized in Tables I. and II. ♦ Table III. . Annual Inequality; Percentages of Mean Potential for the Year. Jan. Feb. March. April. May. June. July. August. Sept. Oct. Nov. Dec. 153 86 63 178 65 147 209 79 13 7 94 133 148 93 186 53 155 77 47 72 71 71 165 140 62 157 60 106 57 43 54 96 127 134 161 145 149 59 62 61 105 44 53 102 123 136 110 112 76 127 83 71 84 83 107 104 104 139 132 110 98 81 84 86 77 90 89 99 129 125 121 112 92 108 89 91 82 89 74 99 122 121 104 105 104 93 91 92 87 100 99 115 117 97 115 105 155 129 79 62 69 127 79 90 93 100 89 103 103 85 120 98 99 73 101 117 112

At Sodankyla the year employed began with September 1882, at Cape Thorsden with November 1882. At the latter station there were only ten months’ observations, and the results for November and December seem hardly credible. The figures suggest that the rapid fall in potential observed at Kew and Greenwich in April and May takes place later in the far north, and somewhat earlier in Florence, Perpignan, and Lisbon. At the higher level at Batavia the annual inequality seems very irregular, notwithstanding that it is a mean from six years’ results. § 9.. Diurnal and annual inequalities have been published for a variety of places, which are based on eye observations with Exner s electroscope. Such results depend only on observations taken at certain hours of the day, sometimes only on bright days ; consequently they are not directly comparable with inequalities based on the continuous records of self-recording instruments Two examples are given in Table IY. The Wolfenbiittel data are due to Elster and Geitel (Wien. Sitz. ci. Abth. ii., 1892, p. 703); the data for Ladenburg were obtained by Gockel (Met. Zeit for 1897, p. 281) during the years 1892-95. The figures are the monthly means in volts of the potential at a fixed point in the atmosphere. Table IY. (Annual Inequality). Jan. Feb. Mch. April. May June. July. Aug. Sept. Oct. Wolfenbllttel 391 339 294 138 110 102 123 121 188 Ladenburg 397 537 148 66 62 60 60 73 116 § 10. Table Y. gives some data bearing on the question of how the amplitude of the diurnal inequality is influenced by the height of the water-dropper above the ground. The heights are

in metres, the maximum and minimum values of the potential are in volts. The data for Paris and Trappes are from the paper by Chauveau already mentioned ; the others from the same sources as Tables I. and II. London is the mean of Kew (1880) and Greenwich. Table V. Paris. Station. London. Bureau Eiffel Trappes Perpignan. Batavia. Central. Tower. Height 285 8-4 SumSum- SumYear. mer. SumSeason mer. mer. mer. Year. Year. Year. Maximum 172 2490 372 72-3 125 1496 Minimum 102 1740 213 24 401 39-0 Max./mean 1-24 1-34 1-29 1T3 1-23 1-31 1'5S 1-55 Min./mean •85 ’76 •77 •70 •71 •30 •42 Perhaps all that can be said is that the results are not unfavourable to the view that the relative importance of the diurnal inequality diminishes as the height above the increases. Nov. Dec. ground § 11. The diurnal inequality near the ground on 260 470 the top of a mountain has no necessary relationship 106 134 to that above a plain at the same height above the sea. Elster and Geitel (Met. Zeit. for 1891, p. 321) believe that on the top of a mountain the diurnal variation is small, basing their view partly on some observations of their own on the top of the Sonnblick (3100 metres). The mean of these

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