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378
MAGNETISM, TERRESTRIAL


so on. Results are given for 1894 and 1895, the years which were on the whole the most favourable and the least favourable for Arrhenius's hypothesis, as well as for the whole eleven years. TABLE XL.

Day. n-4 n-3 n-2 n-1 n n+1

1894 +12 +9 +11 +12 +11 +6

Five daysof 1895 -16 -17 -15 -12 -'II -10

largest range I; yr5 4 9 ' 3 | 3 4. 7 4. 5 * 0.5 - 1 94 -15 -17 -19 -21 -21 -I9

F.;i'3:§ ;.°f +17 +10 + 1. - 2 -2 - 4

IIYYS- '4 -4-7 -7 -7-5

Takin the 11-year-means we have the sun-spot area practically normai on the day subsequent to the representative day of large magnetic range, but sensibly above its mean on that day and still more so on the four previous days. This suggests an emission from the sun taking a highly variable time to travel to the earth. The 11-year mean data for the five days of least range seem at first sight to point to the same conclusion, but the fact that the deficiency in sun-spot area is practically as prominent on the day after the representative day of small magnetic range as on that day itself, or the previous days, shows that the phenomenon is probably a secondary one. On the whole, taking into account the extraordinary differences between the results from individual years, we seem unable to come to any very positive conclusion, except that in the present state of our knowledge little if any clue is afforded by the extent of the sun's spotted area on any particular day as to the magnetic conditions on the earth on that or any individual subsequent day. Possibly some more definite information might be extracted by considering the extent of spotted area on different zones of the sun. On theories such as those of Arrhenius or Maunder, effective bombardment of the earth would be more or less confined to spotted areas in the zones nearest the centre of the visible hemisphere, whilst all spots on this hemisphere contribute to the total spotted area. Still the projected area of a spot rapidly diminishes as it approaches the edge of the visible hemisphere, i.e. as it recedes from the most effective position, so that the method employed above gives a preponderating weight to the central zones. One rather noteworthy feature in Table XL. is the tendency to a sequence in the figures in any one row. This seems to be due, at least in large part, to the fact that days of large and days of small sunspot area tend to occur in groups. The same is true to a certain extent of days of large and days of small magnetic range, but it is unusual for the range to be much above the average for more than 3 or 4 successive days.

§ 39. The records from ordinary magneto graphs, even when run at the usual rate and with normal sensitiveness, not infrequently show pulsations a. repetition of regular or nearly regular small rhythmic movements, lasting sometimes for hours. The amplitude and period on different occasions both vary widely. Periods of 2 to 4 minutes are the most common. W. van Bemmelen°' has made a minute examination of these movements from several years' traces at Batavia, comparing the results with corresponding statistics sent him from Zi-ka-wei and Kew. Table XLI. shows the diurnal variation in the frequency of occurrence of these small movements-called

pulsations by van Bemmelen-at these three stations. The Batavia results are from the years 1885 and record changes of magnetic induction through a coil traversed by the earth's lines of force, H. Ebert'4 has observed vibrations whose periods are but a small fraction of a second. The observations of Kohlrausch and Eschenhagen preceded the recent great development of applications of electrical power, while lon er period waves are shown in the Kew curves of 50 years ago, so that the existence of natural waves with periods of from a few seconds up to several minutes can hardly be doubted. Whether the much shorter period waves of Ebert are also natural is more open to doubt, as it is becoming exceedingly difficult in civilized countries to escape artificial disturbances.

TABLE XLI.-Diurnal Distribution of Pulsations. Hours. o-3. 3l6. 6-9. 9-Noon.|Noon-3. 3-6. 6-9. Batavia . 28 9 u 2 6 8 6 13 287

Zi~1<a-w@i(i> 33 5 2 7 4 4 10 35

(ii) 23 6 8 II 7 5 14 26

Kew . 4 8 19 14 22 18 111 4

§ 40. The fact that the moon exerts a small but sensible effect on the earth's magnetism seems to have been first discovered in 1841 by C. Kreil. Subsequently Sabine” investigated the nature of the lunar diurnal variation in declination L'""" at Kew, Toronto, Pekin, St Helena, Cape of Good Hope 1"”"""”° and Hobart. The data in Table XLII. are mostly due to Sabine. They represent the mean lunar diurnal inequality in declination for the whole year. The unit employed is Ol'0OI, and as in our previous tables -l- denotes movement to the west. By “ mean departure" is meant the arithmetic mean of the 24 hourly departures from the mean value for the lunar day; the range is the difference between the algebraically greatest and least of the hourly values. Not infrequently the mean departure gives the better idea of the importance of an inequality, especially when as in the present case two maxima and minima occur in the day. This double daily period is unusually prominent in the case of the lunar diurnal inequality, and is seen in the other elements as well as in the declination.

Lunar action has been specially studied in connexion with observations from India and ]ava. Broun” at Trivandrum and C. Chambers 57 at Kolaba investigated lunar action from a variety of aspects. At Batavia van der Stok5S and more recently S. Figee” have carried out investigations involving an enormous amount of computation. Table XLIII. gives a summary of Figee's results for the mean lunar diurnal inequality at Batavia, for the two half yearly periods April to September (Winter or W.), and October to March (S.). The + sign denotes movement to the west in the case of declination, but numerical increase in the case of the other elements. In the case of H and T (total force) the results for the two seasons present comparatively small differences, but in the case of D, I and V the amplitude and phase both differ widely. Consequently a mean lunar diurnal variation derived from all the months of the year gives at Batavia, and presumably at other TABLE XLII.-Lunar Diurnal Inequality of Declination (unit 0'~o01).i Lunar Kew. Toronto. Batavia. St Helena. Cape. Hobart. V Hour. 1858-1862. 1843-1848. 1883-1899. 1843-1847. 1842-1846. 1841-1848. 1892 to 1898. Of the two sets of data for Zi-kawei (i) answers to the years 1897, 1898 and 1900, 0 ' +123 1215:go I 43:148 92 as given by van Bemmelen, while (ii) answers to I +I 0 7% 3 5 IO7 PZ the P€riod 1900-1905, as given in the Zi-ka-wei 2 +140 +15 33 + 37 ' 35 4 Bulletin for IQOS. The Kew data are for 1897. The 3 + 33 + 2; 8 + go * 4g -107 results are expressed as percentages of the total for 4 + go Ig-5 2 + 5 +10 45 the 24 hours. There is a remarkable contrast be- 2 7 2 5 183 + 7; +140 + tween Batavia and Zi-ka-wei on the one hand and lgg 3°2 7 + 4 ' I§ 2 + Kew on the other, pulsations being much more g 16 255 +77 + 5 2 +122 numerous by night than by day at the two former I 3 137 +40 gs 'L' g +I§ O stations, whereas at Kew the exact reverse holds. 9 7 + g 4 2 7 + 2 Van Bemmelen decided that almost all the occasions 10 + 2 +Ig8 g5 IO; 143 + X7 of Dulsation at Zi-ka-wei were also occasions of H + 92 +2 80 9 IE7 57 pulsations at Batavia. The hours of commence- 12 +160 +323 6; 73 I 5 120 ment at the two places usually differed a little, oc- 13 +188 +27§ 32 H2 152 casionally by as much as 20 minutes; but this he 14 +158 +14 4§ + 13 33 “'47 ascribed to the fact that the earliest oscillations were 15 + 9°;7 + 52 + '5 105 too small at one or other of the stations to be visible 16 + 10 I O +g0 + 73 +132 35 on the trace. Remarkable coincidence between 17 85 297 + 2 + 73 +I;§ + 45 pulsations at Potsdam and in the north of Norway 18 142 337 +;§ + 52 +I +112 has been noted by Kr. Birkeland.“9 19 163 290 + + 17 +122 +152 With magneto graphs of greater sensitiveness and 2° '147 170 +5§ 2g + 45 +x52 more open time scales, waves of shorter period be- 21 123 7 + 8 5 40 +113 come visible. In 1882 F. Kohlrausch” detected 22 40 +125 26 II2 + 47 waves with a period of about 12 seconds. Eschen- 23 + 27 +2 5 5 153 32 hagen” observed a great variety of short period waves, 30 seconds being amongst the most common. Mean De' 105 200 50 54 104 93 Some of the gecordi he obtained suggest the Parture super osition o regu ar sine waves of different perio<i)s. Employingavery sensitive galvanometer to .Bangs ' 376 66° 174 187 304