Observations on days of unusual magnetic disturbance, made at British Colonial Magnetic Observatories, under the Departments of the Ordnance and Admiralty. Part 1.--1840-1841. / Printed ... under the superintendence of ... E. Sabine.

  • Great Britain. Board of Ordnance
Date:
1843
Catalogue details

Licence: Public Domain Mark

Credit: Observations on days of unusual magnetic disturbance, made at British Colonial Magnetic Observatories, under the Departments of the Ordnance and Admiralty. Part 1.--1840-1841. / Printed ... under the superintendence of ... E. Sabine. Source: Wellcome Collection.

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    of currents of air within the external magnetometer case, may possibly in succeeding years somewhat diminish the value of the mean fluctuations which have been derived from the observations of 1841 ; but the correspondence which has been stated to exist, and which may be pursued in detail in the Observations contained in this volume, in the times of the occurrence of the disturbances (of those of principal magnitude at least), at stations so widely remote from each other as Toronto and Van Diemen Island, and so opposed in season and horary angle, appear fully to justify the persuasion, that the fluctuations, of which we have sought to obtain at least approximate mean values, are actual natural phenomena, and not, as some have imagined, mere instrumental deceptions. Further evidence of the generality of these phenomena may he obtained by comparing the days of principal disturbance at Prague with those at Toronto and Van Diemen Island. At Prague, as at the British Colonial Observatories, observations at short intervals are commenced whenever the attention of the Director is arrested by the occur- rence of a change of more than usual magnitude between one regular observation and the next; and are continued until the fluctuations assume again their ordinary character. The disturbance observations at Prague are printed in detail, and in a form very similar to that which has been adopted in this volume, in a separate section of the “ Magnetische und Meteorologische Beobachtungen zu Prag ” now in course of publication, by ]\1. Kreil, Director of the Observatory. This portion of the work has not yet advanced beyond November, 1840 ; but the volume in which it is comprised contains also a table, in which the relative magnitude of the disturbance at the observation hours on every day from July, 1839, to June, 1841, inclusive, is given both in Declination and Horizontal Intensity ; and M. Kreil has been so obliging as to furnish me with a MS. continuation to the end of December, 1841. In this table the days of disturbance are marked by an asterisk(*), and the day of greatest disturbance in each month is marked thus (* 3)). The days which M. Kreil characterizes as days of disturbance are those in which the sum of the fluctuations from one observation hour to the next throughout the twenty-four hours, including both the irregular and diurnal changes, equals or exceeds the doul>le of the sum of the corresponding fluctuations derived from the mean monthly positions at the same hours. This mode of comparison serves to show, equally with the one which has been adopted in this volume, the days in each month when the irregular fluctuation deduced from the usual observation hours has been greatest, and will answer therefore the present purpose ; though inasmuch as in M. Kreil’s method the unit of comparison varies in different months, an amount of irregular fluctuation which may constitute a day of disturbance in one month might not do so in another. The days on which the extra observations recorded in this volume were made at Toronto and Van Diemen Island are inserted in the following table ; and the flnal column shows the character attached to the same day in M. Kreil’s Table. The com-