MAGNETIC DECLINATION AND HORIZONTAL FORCE. 
37 
25. The thick lines of figs. 48 and 49 have been drawn to represent the two last 
terms alone of our formula, viz., of 
f c . 2 (h) cos 2 +/;- 2 sin 2 (^j. 
Noticing that they deviate but little from the dotted lines, it may be inferred that 
the bulk of the systematic part of the variations is embraced by the restricted 
formula; and accordingly it is to that part of the variation that goes through a cycle 
of change in half a lunation that attention should first be directed in seeking for 
efficient physical causes. 
TUE VAN DRUM. 
26. Having thus found that order and system of a simple kind pervade the manifes¬ 
tations of minute lunar variations of magnetic force at Bombay, we turn with interest to 
records of a similar nature obtained at the near, but more nearly magnetically equatorial, 
station of Trevandrum. In a volume entitled Observations of Magnetic Declination 
made at Trevandrum and Agustia Malley in the Observatories of H. H. the 
Maharajah of Travancore/ and published by H. S. King and Co., of London, in 
1874, the late John Allan Broun discussed the declination observations made at 
Trevandrum during the ten years 1854 to 1864, and—by a method of his own, which 
is, in essential points, similar to that described in the preceding pages—obtained 
values of the lunar diurnal variations for each month of the year and at each of the 
four quarters of the moon. Curves representing these variations appear as Plate 6 
of the volume, and a glance at those for the months December and January, which— 
as at Bombay—have the largest range, is sufficient to show that our formula would 
correctly represent their principal features at each of the four quarters of the moon ; 
indeed, those curves have a close resemblance to the corresponding winter curves for 
Bombay. An inspection of the curves for all the months leads to a suspicion that a 
period of ten years is scarcely sufficient to secure a counterbalancing, in all months, of 
casual irregularities ; but, by combining together the variations of those consecutive 
months, the curves of which are similar in character, greater regularity is obtained, 
and the nature of the change of the typical variations with change of season is more 
distinctly brought out. The grouping of months adopted with these objects in view 
was as follows, viz. :—(l) December and January ; (2) Februai'y and March ; (3) April 
to June; (4) July to September; and (5) October and November. The average lunar 
diurnal variations for these several groups of months were calculated from the varia¬ 
tions for individual months given in Tables XXXVII. to XL. (pages 125 and 126) 
of the book referred to. These average variations, which were expressed in minutes 
of arc, were then converted into force by multiplying by an adopted value of the 
horizontal force at Trevandrum (0‘37141 O.G.S. unit), and by sine L, with results 
that are exhibited in the following - Tables. 
O 
