MAGNETIC DECLINATION AT TREVANDRUM. 747 



moon's lower passage, to be much greater than the other near the upper pas- 

 sage ; while for the moon farthest south in January it is just the reverse. 

 Now this difference is connected also with the fact, that in each of these cases 

 the greatest maximum happened in the day hours — that is to say, when the 

 moon is farthest north in January the lower passage occurs in the day-time ; 

 and when farthest south in January, the upper passage occurred in the day- 

 time. 



A similar difference is shown for the weeks in January, when the moon was 

 near the equator, moving north and moving south ; when moving north, the 

 most marked minimum occurred about six hours before the upper passage, and 

 when moving south, the marked minimum happened six hours after the upper 

 passage ; both of these epochs corresponded to the day hours. 



31. Had, however, the change in the moon's declination been a cause of 

 these differences in the values of the extreme elongations, we should expect 

 the same cause to produce similar effects in other months. If in January, the 

 moon being farthest north caused the maximum at the lower passage to be the 

 greatest, a similar result might be expected in February and March, for which 

 months the law of the mean diurnal variation is nearly the same. This is not 

 the case. In February and March the greatest maximum occurs near the 

 lower passage, not when the moon is farthest north, but when on the equator, 

 moving south/"" 



In June, when the moon was farthest north,t the variation may be said to 

 have been zero near the lower passage, instead of, as in January, being the 

 most important for this position of the moon. Also, when the moon was 

 farthest south in June, the movement near the upper passage is very small, J 

 which is just the reverse of the fact for the same position of the moon in 

 January. 



32. An examination of the results for each month of the year, in the same 

 way, leads to the conclusion that the variable magnitude of the maxima and 

 minima depends chiefly, if not wholly, on the hours of the day corresponding 

 to the hour angles of the moon for which there is a maximum or minimum of 

 easterly declination. This conclusion will be evident on examining the projected 



the times of the moon's phases, render unnecessary ; for the conclusions here drawn the curves for the 

 phases may be consulted, approximately, as follows : — 



In January, for moon farthest north, see curve for full moon. 

 „ ,, south, „ new moon. 



„ „ on equator moving north, see curve for 1 st quarter. 



„ „ „ south, , „ 3d quarter. 



In February, on equator moving south, see 3d quarter. 

 In March, „ „ see full moon. 



* See curves for third quarter in February, and for full moon in March, 

 "f" See curve for new moon in June. 

 % See curve for full moon in June, Plate XXVII. 



VOL. XXVI. PART IV. 9 I 



