MAGNETIC DECLINATION AT TREVANDRUM. 743 



when reduced to the same directive force) that the solar action is greater in 

 December than in June (for the whole earth), yet it seems not improbable that 

 the great lunar action observed at Trevandrum during the months of December 

 and January is connected with the greater proximity of the earth and moon to 

 the sun at that time of the year. 



Changes of the Lunar Diurnal Variation with the Moon's Declination. 



24. The next question of importance which presented itself was that relating 

 to the moon's passage from one hemisphere to the other. The first investiga- 

 tions in answer to this question seemed to show, as already mentioned, that the 

 law of lunar diurnal variation not only varied with the moon farthest north and 

 farthest south, but even with the position on the equator according as she was 

 moving towards the north or towards the south. There was nothing contrary 

 to our knowledge in this result, the solar diurnal law for March not resembling 

 in any way that for September or October. This discussion, which had been 

 made for the two groups of months, April to September, and October to March, 

 from the observations in 1854 to 1858, was extended to the eleven years' 

 observations, and to each month of the year. The results obtained were not 

 consistent. The change of law which appeared due to the moon's passage from 

 the southern to the northern hemisphere in January was not the same as that 

 shown for the same movement of the moon in February, and resembled still less 

 that which occurred in March. The discussions (each requiring the combina- 

 tions of nearly 80,000 quantities) made in a similar manner with reference to the 

 moon's phase and distance from the earth threw no light on this discrepancy. 



25. In another discussion with reference to the law of disturbance as related 

 to the moon's hour angle/" it occurred to me that it would be desirable to com- 

 pare the result which might be deduced from the night hours, when the sola 1 * 

 disturbance is least at Trevandrum, with that derived from the day hours, when 

 the solar disturbance is greatest. The conclusions arrived at from this discus- 

 sion, together with a consideration of the curves representing the apparent 

 variation of the law of lunar diurnal variation with the moon's declination, 

 induced me to employ a similar analysis for the lunar diurnal variation. As 

 the conclusions from this investigation are most important, it is necessary to 

 describe the process made use of. The apparent effect of the moon's change of 

 declination will be considered thereafter. 



Change of the Laiv of Lunar Diurnal Variation with the Day and Night. 



26. Having subducted the solar diurnal variation from the hourly observa- 



* See Proc. Eoy. Soc. Lond., 20tli June 1861. The law of lunar disturbance is that relating to 

 the greater or lesser irregularity of the position of the needle in the diurnal variation for the moon on 

 the different meridians. 



VOL. XXVI. PART IV. 9 H 



