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377
MAGNETISM, TERRESTRIAL


on prearranged term days. During March 1879 and August 1880 some large magnetic storms occurred, and the magnetic curves showing these at a number of stations fitted with Kew pattern magneto graphs were compared by W. G. Adams.” He found the more characteristic movements to be, so far as could be judged, simultaneous at all the stations. At comparatively near stations TABLE XXXIX.-Direction of First Decided Movement. Place. Declination. Horizontal Force. Vertical Force. Pavlovsk Vest -i- +

Potsdam West -le-Greenwich

West + +

Zi-ka-wei East -1—Kolaba

East -l- -

Batavia West -|—Mauritius

East -|- +

Cape Horn West -1-such

as Stonyhurst and Kew, or Coimbra and Lisbon, the curves were in general almost duplicates., At Kew and St Petersburg there were usually considerable differences in detail, and the movements were occasionally in opposite directions. The differences between Toronto, Melbourne or Zi-ka-Wei and the European stations were still more pronounced. In 1896, on the initiative of M. Eschenhagenf” eye observations of declination and horizontal force were taken at 5-second intervals during prearranged hours at Batavia, Manila, Melbourne and nine European stations. The data from one of these occasions when appreciable disturbance prevailed were Published by Eschenhagen, and were subsequently analysed by Ad. Schmidt." Taking the stations in western Europe, Schmidt drew several series of lines, each series representing the disturbing forces at one instant of time as deduced from the departure of the elements at the several stations from their undisturbed value. The lines answering to any one instant had a general sameness of direction with more or less divergence or convergence, but their general trend varied in a way which suggested to Schmidt the passage of a species of vortex with large but finite velocit . The conclusion that magnetic disturbances tend to fbllow one another at nearly equal intervals of time has been reached by several independent observers. ]. A. Broun 45 pronounced for a period of about 26 days, and expressed a belief that a certain zone, or zones, of the sun's surface might exert a prepotent influence on the earth's magnetism during several solar rotations. Very similar views were advanced in 1904 by E. W. Maunder, ” whowas wholly unaware of Broun's work. Maunder concluded that the period was 27-28 days, coinciding with the sun's rotation period relative to an observer on the earth. Taking magnetic storms at Greenwich from 1882 to 1903, he found the interval between the commencement of successive storms to approach closely to the above period in a considerably larger number of instances than one would have expected from mere chance. He found several successions of three or four storms, and in one instance of as many as six storms, showing his interval. In a later paper Maunder reached similar results for magnetic storms at Greenwich from 1848 to 1881. Somewhat earlier than Maunder, Arthur Harvey” deduced a period of 27~246 days from a consideration of magnetic disturbances at Toronto. A. Schuster, ” examining Maunder's data mathematically, concluded that they afforded rather strong evidence of a period of about é (27-28) or I3-6 days. Maunder regarded his results as demonstrating that magnetic disturbances originate in the sun. He regarded the solar action as arising from active areas of limited extent on the sun's surface, and as propagated along narrow, well defined streams. The active areas he believed to be also the seats of the formation of sun-spots, but believed that their activity might precede and outlive the visible existence of the sun-spot. Maunder did not discuss the physical nature of the phenomenon, but his views are at least analogous to those propounded somewhat earlier by Svante Arrhenius, “ who suggested that small negatively charged particles are driven from the sun by the repulsion of light and reach the earth's atmosphere, setting up electrical currents, manifest in aurora and magnetic disturbances. Arrhenius's calculations, for the size of particle which he regarded as most probable, make the time of transmission to the earth slightly under two days. Amongst other theories which ascribe magnetic storms to direct solar action may be mentioned that of Kr. Birkeland, ” who believes the vehicle to be cathode rays. Ch. Nordmann°° similarly has suggested Rontgen rays. Supposing the sun the ultimate source, it would be easier to discriminate between the theories if the exact time of the originating occurrence could be fixed. For instance, a disturbance that is propagated with the velocity of light may be due to Rontgen rays, but not to Arrhenius's particles. In support of his theory, Nordmann mentions several cases when conspicuous visual phenomena on the sun have synchronized with magnetic movements on the earth-the best known instance being the apparent coincidence in time of a magnetic disturbance at Kew on the 1st of September 1859 with a remarkable solar outburst seen by R. C. Carrington. Presumably any electrical phenomenon on the sun will set up waves in the aether, so transmission of electric and magnetic disturbances from the sun to the earth with the velocity of light is a certainty rather than a hypothesis; but it by no means follows that the energy thus transmitted can give rise to sensible magnetic disturbances. Also, when considering Nordmann's coincidences, it must be remembered that magnetic movements are so numerous that it would be singular if no apparent coincidences had been noticed. Another consideration is that the movements shown by ordinary magneto graphs are seldom very rapid. During some storms, especially those accompanied by unusually bright and rapidly varying auroral displays, large to and fro movements follow one another in closing succession, the changes being sometimes too quick to be registered distinctly on the photographic paper. This, however, is exceptional, even in polar regions where disturbances are largest and most numerous. As a rule, even when the change in the direction of movement in the declination needle seems quite sudden, the movement in one direction usually lasts for several minutes, often for IO, 15 or 30 minutes. Thus the cause to which magnetic disturbances are due seems in many cases to be persistent in one direction for a considerable time. § 38. Attempts have been made to discriminate between the theories as to magnetic storms by a critical examination of the phenomena. A general Connexion between sun-spot frequency and the amplitude of magnetic movements, regular and irregular, is generally admitted. If it is a case of cause and effect, and the interval between the solar and terrestrial phenomena does not exceed a few hours, then there should be a sensible Connexion between corresponding daily values of the sun-spot frequency and the magnetic range. Even if only some sun-s ots are effective, we should expect when we select from a series of) years two groups of days, the one containing the days of most sun-spots, the other the days of least, that a prominent difference will exist between the mean values of the absolute daily magnetic ranges for the two groups. Conversely, if we take out the days of small and the days of large magnetic range, or the days that are conspicuously quiet and those that are highly disturbed, we should expect a prominent difference between the corresponding mean sun-spot areas. An application of this principle was made by Chree” to the five quiet days a month selected by the astronomer royal between 1890 and 1900. These days are very quiet relative to the average day and possess a much smaller absolute range. One would thus have expected on Birkeland's or Nordmann's theory the mean sun-spot frequency derived from Wolfer's provisional values for these days to be much below his mean value, 41-22, for the eleven years. It proved, however, to be 41-28. This practical identity was as visible in 1892 to 1895, the years of sun-spot maximum, as it was in the years of sun-spot minimum. Use was next made of the Greenwich projected sun-spot areas, which are the result of exact measurement. The days of each month were divided into three groups, the first and third-each normally of ten days-containing respectively the days of largest and the days of least sun-spot area. The mean sun-spot area from group 1 was on the average about five times that for group 3. It was then investigated how the astronomer royal's quiet days from ISQO to 1900, and how the most disturbed days of the period selected from the Kew" magnetic records, distributed themselves among the three groups of days. Nineteen months were excluded, as containing more than ten days with no sun-spots. The remaining 113 months contained 565 quiet and 191 highly disturbed days, whose distribution was as follows: Group 1. Group 2. Group 3. K

Quiet days 179 195 191

Disturbed days 68 65 58

The group of days of largest sun-spot area thus contained slightly under their share.of quiet days and slightly over their share of disturbed days. The differences, however, are not large, and in three ears, viz. 1895, 1897 and 1899, the largest number of disturbedydays actually occurred in group 3, while in 1895, 1896 and 1899 there were fewer quiet days in group 3 than in group I. Taking the same distribution of days, the mean value of the absolute daily range of declination at Kew was calculated for the group I and the group 3 days of ea.ch month. The mean range from the group I days was the larger in 57% of the individual months as against 43% in which it was the smaller. When the days of each month were divided into groups according to the absolute declination range at Kew, the mean sun-spot area for the group 1 days (those of largest range) exceeded that for the group 3 days (those of least range) in 55% of the individual months, as against 45% of cases in which it was the smaller.

Taking next the five days of largest and the five days of least range in each month, sun-spot areas were got out not merely for these days themselves, but also for the next subsequent day and the four immediately preceding days in each case. On Arrhenius's theory we should expect the magnetic range to vary with the sun-spot area, not on the actual day but two days previously . The following figures give the percentage excess or deficiency of the mean sun-spot area for the respective groups of days, relative to the average value for the Whole epoch dealt with. rt denotes the day to which the magnetic range belongs, n-I-1 the day after, n-1 the day before, and