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XXVIII. On the Annual V aviation of the Magnetic Declination, at different periods of 
the Day, By Lieut. -Col. Edward Sabine, B.A., V.P. and Treas. of the Royal Society. 
Received April 30, — Read May 22, 1851 ; Revised October 1851. 
The interest which papers recently communicated to the Royal Society have ex- 
cited 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 com- 
plication by being viewed together, should be distinguished apart, and that each 
class should be presented separately, combining at the same time, as far as circum- 
stances 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 con- 
venient 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 sta- 
tions ; 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 dif- 
ferent 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 purpose of viewing the phenomena of the Annual Variation at 
