On the Magnetic Declination at the Magnetic Equator. 889 
that this interpolation should constitute a second part of the discus- 
sion, the first including all the observations*. 
These considerations may appear somewhat elementary, but it is 
essential that results which present such anomalies as the lunar 
diurnal variation of magnetic declination should be obtained in a 
manner the most free from objection, even though the objections 
should touch on quantities of a second order compared with those 
obtained.: 
The discussion of which I now proceed to note the results, 
includes all the hourly observations without exception, made in the 
Trevandrum Observatory (within a degree and a half of the magnetic 
equator) during the five years 1854 to 1858; the second part of the 
discussion, in which days of great magnetic irregularity have been 
wholly rejected, not being completed, I ‘shall reserve the details for a 
more formal communication to the Royal Society. The results 
obtained are as follows :— 
Ist. At the magnetic equator the lunar diurnal law of magnetic 
declination varies with the moon’s declination and — the sun’s 
declination. 
2nd. This variation is so considerable that the attempt to combine 
all the observations to form the mean law for the year gives results 
that are not true for any period. Hence evidently the impossibility 
of relating the laws at different places. The so-called mean law for 
the year at Trevandrum obtained for the moon furthest north, on the 
equator going south, furthest south, and on the equator going north, 
consists of ¢hree maxima and three minima,—a result wholly false, 
excepting as an arithmetical operation due to combination of very 
different laws. 
3rd. The lunar diurnal lawvaries chiefly with the position of the sun, 
the variation being comparatively small with the position of the moon. 
4th. At the magnetic equator the range of the variations is mark- 
edly greatest in the months of January, February, November and 
December, or about perihelion. 
The following results are derived after grouping the means for differ- 
ent positions of the moon in periods of six months, October to March, 
and April to September ; they are therefore, for the reason given in 
the 2nd conclusion, not quite accurate ; but the change of the law from 
month to month will be followed when the details are presented to the 
Society. The following will give a general idea of the changes :— 
Sth. When the moon is fur thest north. 
a. About perihelion. The lunar diurnal law of magnetic declina- 
tion consists of two maxzimat when the moon is near the upper and 
lower meridians, the maximum for the latter being much the great- 
* T should note here my belief that a peculiarity noticed by General Sabine in 
his discussions as requiring explanation, namely, that the excursions of the decli- 
nation needle east and west in the lunar diurnal variation have very different 
magnitudes, is due to the rejection of observations, while the means by which the 
differences were obtained included the rejected quantities. 
+ The declination is easterly at Trevandrum, and the maxima indicate greater 
easterly declination. 
a 
