366 
COLONEL SABINE ON PERIODICAL LAWS DISCOVERABLE 
of the relative proportion which the aggregate values in the different months of the 
easterly disturbances bear to the aggregate values in the same months of the westerly 
disturbances, and the aggregate values of the disturbances which decrease the force, 
bear to the aggregate values of those which increase it, — we find that indications pre- 
sent themselves of an annual variation of a different kind from that which has been 
hitherto discussed, namely an annual variation in the character of the disturbances of 
two at least of the elements which have been observed ; and although a greater length 
of time and a greater amount or continuance of observation may be required for the 
satisfactory establishment of such a periodical variation, its present indication ought 
not to be overlooked, since the range of the variation is of considerable magni- 
tude, and its systematic character as distinctly marked as could well be expected 
in an annual variation derived from not more than five years. The elements 
in which these phenomena are most distinctly noticeable, are the Declination and 
the Vertical Force, and the correspondence between the indications of these two 
elements is in many respects very remarkable. In both elements, when the relative 
proportions are taken, — in the Declination of the aggregate values in the different 
months of easterly and westerly disturbances, and in the Vertical Force of disturbances 
which decrease and disturbances which increase the force, — we find that in both cases 
the proportions vary from a minimum at the southern solstice to a maximum at the 
northern solstice, the equinoxes being intermediate. At the northern solstice easterly 
disturbances are in considerable excess, as are disturbances which decrease the 
Vertical force ; at the southern solstice, the excess of both is on the other side ; 
westerly disturbances then predominate, as do the disturbances which increase the 
Vertical force. The relative proportion of the aggregate values of easterly to westerly 
disturbances of the Declination, and of disturbances which decrease the Vertical Force 
to those which increase it, varies from the one solstice to the other roughly as about 
3 to 1 ; and in both elements nearly alike. 
In the Ilorizontal Force, the disproportion between the values of the disturbances 
which increase the force and those which decrease it is so great (decreasing disturb- 
ances greatly preponderating at all periods of the year), that a variation correspond- 
ing to that of the two other elements is not so simply arrived at ; but it may be 
stated generally that the proportion of decreasing disturbances is greater at the epoch 
of the southern solstice than at that of the northern solstice. 
Diurnal Variation. — Before we proceed to examine the diurnal variation of the 
Declination, Inclination and Total force which it is the average effect of the larger 
disturbances to produce, it may be desirable to show the proportions in which the 
disturbances of the three observed elements occur at the different hours. This is 
expressed in the following Table by the proportion which the actual aggregate values 
in the five years of the disturbances at each particular hour bear to the mean or 
average disturbance at all the hours taken as unity. 
