MAGNETIC DECLINATION AT TREVANDRUM. 



741 



TABLE I. — Epochs of Maxima and Minima of easterly Declination in the Lunar Diurnal 

 Variation for each Month, as derived from the calculated Variations and by estimation 

 from the Projection of the observed Means. 

















Month. 



Diurnal term. 





Semi-diurnal term. 









Calculated. 



Estimated from observations. 



Max. 



Min. 



Max. 



Min. 



Max. 



Min. 



Max. 



Min. 



Max. 



Min. 



Max. 



Min. 



Max. 



Min. 





h. m. 



h. m. 



h. 



m. 



h. m. 



h. m. 



h. 



m. 



h. 



m. 



h. m. 



h. m. 



h. m. 



h. m. 



li. m. 



h. m. 



h. m. 



Jan. 



14 6 



2 6 



11 



38 



17 38 



23 38 



5 



38 



11 



42 



17 44 



23 34 



5 33 



11 40 



17 44 



23 55 



5 8 



Feb. 



14 15 



2 15 



11 



16 



17 16 



23 16 



5 



16 



11 



17 



17 21 



23 11 



5 12 



11 14 



17 35 



23 5 



5 8 



Mar. 



16 22 



4 22 



10 



21 



16 21 



22 21 



4 



21 



11 







16 21 



21 43 



4 21 



10 55 



16 50 



21 45 



4 



April 



18 49 



6 49 



9 



46 



15 46 



21 46 



3 



46 



10 







15 41 



21 41 



3 52 



10 5 



15 22 



21 50 



3 50 



May 



22 48 



10 48 



7 



25 



13 25 



19 25 



1 



25 



6 







12 48 



20 26 



2 58 



5 15? 



11 15? 



20 40 



2 



June 



9 8 



21 8 



6 



23 



12 23 



18 23 







23 



6 



30 



12 



18 11 



30 



6 15 



11 20 



18 35 



30 



July 



6 52 



18 52 



5 



8 



11 18 



17 18 



23 



18 



5 



25 



11 33 



17 



22 56 



4 50 



12 15 



16 20 







Aug. 



18 



6 



4 



52 



10 52 



16 52 



22 



52 



4 



50 



10 45 



16 55 



23 



4 20 



12 30 



16 40 



23 



Sept. 



5 18 



17 18 



4 



10 



10 10 



16 10 



22 



10 



4 



23 



11 7 



15 50 



21 23 



4 50 



11 30 



16 10 



20 55 



Oct. 



14 40 



2 40 







51 



6 51 



12 51 



18 



51 







29 



6 8 



13 17 



19 30 



23 40 



6 40 



13 30 



18 15 



Nov. 



19 6 



7 6 



23 



59 



5 59 



11 59 



17 



59 



23 



22 



6 8 



12 40 



17 47 



23 30 



5 30 



13 25 



18 30 



Dec. 



21 11 



9 11 







20 



6 20 



12 20 



18 



20 







9 



6 30 



12 32 



18 9 



23 45 



6 25 



12 25 



18 35 



17. No conclusion of any value can be drawn from the epochs for the 

 diurnal term; the irregularity shown in the hours for the different months for 

 this term, probably depends on causes which will be noticed hereafter (35). 

 The semi-diurnal term which gives the largest oscillation, excepting in May and 

 October, shows a rather regular change of the epochs from one month to the 

 other, the most marked exception being from September to October, when the 

 change is 3 h. 19 m. It is, however, the whole variations that we have at present 

 especially to consider, and for these the calculated epochs are the most certain. 



18. If we commence with the maximum, which occurs in January near the 

 moon's upper passage (23 h. 34 m.), and follow the change of hour for this 

 maximum till June, when it happens six hours before the upper passage, and 

 thence to December, when it is near the lower passage (thus going through 

 twelve hours in the twelve months), we find that the greatest leaps occur from 

 May to June (2| h.), and from September to October (2| h.) In the months 

 of March and April this maximum occurs about 2 h. 20 m. before the upper 

 passage, and in September 3 h. 50 m. after the lower passage. 



On the other hand, the maximum which happens near the inferior passage 

 (11 h. 42 m.) in January, changes suddenly four hours from April to May, and 

 from September to October. 



The two minima change hour with considerable regularity, with one marked 

 exception, which occurs again from September to October. 



On the whole, we can conclude that though the maxima occur near the 

 upper and lower passages in December, and at six hours after these epochs in 

 June, that they happen nearly midway between the two in April and September. 



where a is the correction to an estimated epoch of the maximum or minimum. See footnote to a 

 paper on the " Horizontal Form of the Earth's Magnetism," Trans. Roy. Soc. Edin., vol. xxii. p. 529. 



