The note in NATURE for February 22, 1894 (p. 397) concerning “Sun-spots and Magnetic Disturbances” illustrates most clearly the necessity for the adoption of a proper method in order to arrive at any conclusion respecting the relation between these phenomena. I must continue to insist, as I have done heretofore in the columns of NATURE and elsewhere, that the study of the periodicity of magnetic storms and auroras at intervals of about twenty-seven and one quarter days must precede that of the attendant solar conditions, otherwise no results will be obtained. For example, during the month of August 1893, to which the note above mentioned refers, sun-spots were so numerous that it would be utterly impossible to determine which group, if any, were in a location upon the sun capable of originating terrestrial magnetic effects. The proper way is to begin by disregarding solar conditions entirely, and arrange the magnetic storms or auroras of the period that it is desired to study, in series as they actually occurred at the twenty-seven and one quarter day interval. This being done, it is possible at a glance to determine what particular solar conditions reappear invariably when magnetic perturbations are recorded. In this way, and in this way alone, it becomes evident that whenever these magnetic effects appear, there is always a disturbed portion of the sun at the eastern limb and near the plane of the earth's orbit in that location. If the series of recurrences is sufficiently persistent to last through many solar rotations, it will be found that the disturbed area continues to have its effect in spite of considerable variations in the size of the spots, and that at times these effects may continue even when nothing but groups of faculæ remain, these being however, unusually bright and extensive in such a case. By following the history of such recurrences into the portion of the year in which any given disturbed portion of the sun is at a distance from the plane of the earth's orbit, when at the eastern limb, it is found that outbreaks of violent thunderstorms, which do not produce any disturbance of the magnetic needle, take the place of magnetic storms and auroras in the regular order of recurrence. There have been some phenomenal illustrations of this the past winter. Usually in this part of the United States a thunderstorm in winter is very rare, and, if it occurs, stands forth as a prominent event. Thus the thunderstorms of Christmas-day and night, in which buildings were struck by lightning in this State, were most exceptional, and, falling as they did upon the proper date to form the continuation of the strongest and most persistent series of thunder storms and auroras that has been current the past year, were most striking. The above method of attacking the question is that which the writer has gradually developed for the purpose of systematic study. The relation having once been established by tracing the history faithfully and in detail, in the manner described, it is no longer absolutely essential to enter into the question of periodicity in order to secure evidence bearing upon the question. As soon as it is known what has to be looked for, it will only be necessary, when any very large increase of thunder storms occurs, or any notable magnetic perturbations, as the case may be, to look at the proper part of the sun, and see whether it is the seat of disturbance. In this way it will be found that it is not the size of solar disturbances, but their activity at the critical date when they are in the proper location, that determines the terrestrial effects to which reference has been made. Thus it is a question throughout of the adoption of a proper method of investigation.
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