OF THE MAGNETIC DECLINATION. 



21 



We likewise obtain : 



and 



Secondary minimum of eastern deflection in winter 9 h - 42 m - P. M. Amount 0'.97 



" maximum of western " " " 1 15 A.M. " 0.26 



Differences: 3 h - 33 m - 0'.71 



Secondary minimum of eastern deflection for the year 10 h - ll m - P. M. Amount 0'.62 



" maximum of western " " " 1 13 A.M. " 0.47 



Differences: 3 h - 02 m - 0'.15 



The effect of the seasons on the critical hours is well marked in the above table. 

 The eastern elongation occurs earliest between the summer solstice and the autumnal 

 equinox, and latest about the winter solstice. The western elongation occurs earliest 

 about the autumnal equinox, and latest about the winter solstice; and the same 

 holds good for the morning epoch of the mean declination. The afternoon epoch, 

 however, occurs earliest, shortly after the autumnal equinox, and latest, shortly 

 after the summer solstice. 



The critical hours which vary least during the year are those of the western 

 elongation and those of the morning mean declination. The extreme difference 

 between the value for any month and the mean annual value is 31 minutes in the 

 former and 28 minutes in the latter. 



The following graphical representation of three variables (Diagram G) will serve 

 to show at a glance the various features of the diurnal variation and its annual 

 inequality: The magnetic surface is formed by contour lines, O.'o apart; the dotted 

 curves are lines of mean position, the curves represented by dashes correspond to 

 eastern, and the full curves to western deflection from the normal position. This 

 diagram, as well as the computed tabular values from which it has been constructed, 

 serve equally to furnish the correction necessary to reduce any single observation 

 taken at any hour of the day and month to its mean value. It also enables us in 

 a measure to dispense with developing the annual variability of the coefficients j5, 



2? 2 A and C r C z C 3 (or rather the equivalents a, ^ a 2 & 2 a s ^ from 



which they are derived) in the general expression A -f- R^ sin (6 + Ci) + etc. In 

 most cases either a tabular or graphical interpolation between the two adjacent 

 monthly values will fully answer the purpose. The diagram also distinctly exhibits 



