XXVIII. On the annual variation of the magnetic declination, at different periods of the day

The interest which papers recently communicated to the Royal Society have excited in regard to the physical explanation of the Annual and Diurnal Variations of Terrestrial Magnetism, makes it extremely desirable that the facts which are to be explained should in the first instance be clearly and fully comprehended; and that for this purpose, the different classes of facts, which undergo much additional complication by being viewed together, should be distinguished apart, and that each class should be presented separately, combining at the same time, as far as circumstances may permit, facts of the same class obtained from different parts of the globe. Under this impression I have deemed that an acceptable service might be rendered, by arranging in a small compass and presenting together the Annual Variations which the Magnetic Declination undergoes at every hour of the day at the four Colonial Observatories established by the British Government at Toronto, Hobarton, the Cape of Good Hope and St. Helena;—stations selected, it may be remembered, with the express view (amongst others) of affording, as far as any four stations of equally convenient access might be expected to do, the means of generalizing the facts of the Annual and Diurnal Variations in different quarters of the globe. I have attempted to accomplish this object by a graphical representation (Plate XXVI.), in which the Annual Variation at every hour is shown by vertical lines varying in length according to the amount of the range of the Annual Variation at each hour; each line having also small cross lines marking the mean positions of the several months in the annual range. The scale is the same for all the stations, being one inch to one minute of declination. The declination is that of the north end of the magnet at all the stations; the upper end of the line is always the eastern extremity, and the lower end the western extremity, of the annual range. The broken horizontal line which crosses all the verticals at each station, marks for each of the observation hours the mean declination in the year at that particular hour, obtained by adding together the daily observations of the declination at that hour, and dividing the sum by the number of days of observation in the year. This line is consequently not a line of uniform declination-value throughout, because the mean declination varies at different hours, by quantities which constitute the mean Diurnal Variation: but it is the line, or curve as it is sometimes called, of mean Diurnal Variation projected as a straight line, for the parpose of viewing the phenomena of the Annual Variation at each hour, irrespective of the Diurnal Variation, or the changes which the mean declination undergoes at different hours. The hours are those of mean solar time at each station, the day commencing at noon, and being reckoned through the twenty-four hours; noon is therefore = 0^h. The fractional minutes are occasioned by the observations having been made at the exact hours of Göttingen time, which differ more or less at each station from exact hours of local time.