482 



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 : 



1st. At the magnetic equator the lunar diurnal law of magnetic 

 declination varies with the moon's declination and with 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 three maxima and three minima, a result wholly false, 

 excepting as an arithmetical operation due to combination of very 

 different laws. 



3rd. The lunar diurnal law varies 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 : 



5th. When the moon is furthest north. 



a. About perihelion. The lunar diurnal law of magnetic declina- 

 tion consists of two maxima* when the moon is near the upper and 

 lower meridians, the maximum for the latter being much the great- 

 est ; of the two minima at intermediate epochs, that for the setting 

 moon is the most marked. 



b. About aphelion. The law consists of two nearly equal minima 

 near the upper and lower transits : of the two intermediate maxima, 

 that near the moonset is the most marked. 



c. Thus the law about the winter solstice is inverted about the 



* The declination is easterly at Trevandrum, and the maxima indicate greater 

 easterly declination. 



