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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    are also wanting for the Vertical Force Magnetometers of the Antarctic Expedition and of the Van Diemen Island Observatory ; for the present, therefore, the observations with the latter instruments will show only the direction and comparative amount of any changes which may have taken place in the vertical force, leaving the ratio of the changes to the force itself undetermined. The Observatories were completed, and the instruments moved into them at the following dates: at St. Helena in August, 1840; at Toronto in September, 1840; at Van Diemen Island in October, 1840; and at the Cape of Good Hope in April, 1841. At St. Helena and Toronto, however, the instruments were set up, previous to those dates, in rooms in which temporary accommodation was obtained. Whilst observations at many stations yet remain to be published, it would he obviously premature to attempt to trace any single disturbance through the various modifications with which it may have manifested itself in different parts of the globe; but it may not be uninstructive to examine in some small degree the general character of these remarkable phenomena, as they have presented* themselves at the stations, and during the periods at present under notice ; particularly at Toronto and Van Diemen Island,—as the comparison of stations situated in different magnetic hemispheres, and nearly at the oj>posite extremities of a diameter of the globe, would seem to offer especial interest. It has been justly remarked, that in enquiries concerning natural phenomena, which besides their principal cause, have a number of subordinate accidental causes, the course of investigation should be, to separate from the observed march of the phenomena, all that is accidental, for the purpose of drawing forth that which ranges itself under laws, and may thereby be connected with an efficient cause. If such be the course to be pursued when the regular portion of the phenomena forms the subject of investigation, we should adopt the converse proceeding when our attention is designed to be directed to the accidental or irregular portion; we should then endeavour to separate from the general march of the phenomena, all that appears to be subject to laws, leaving the residual to consist entirely of the irregular portion, which we desire to examine. In our present enquiry, therefore, it is proper, in the first instance, to ascertain and to eliminate the regular diurnal movements of the magnetic elements, or those movements which depend upon the hour of the day, and the season of the year. For this purpose, the mean monthly positions of the magnets, at the regular hours of observation, have been taken as representing the mean diurnal march in the respective months; omitting in the means, the ])Ositions on days when disturbances from irregular causes prevailed to any considerable degree. It is probable that the number of days of observation in each month, may not be sufficient to give the diurnal movement with the precision with which it will hereafter be ol)tained by the combinations of more than a single year; but the general harmony of the different months, and the correspondence which is shown by the monthly means at Toronto and Van Diemen Island, are an evidence that the approxi-