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XXIV. — On the Lunar Diurnal Variation of Magnetic Declination at Trevan- 

 drum, near the Magnetic Equator, deduced from Observations made in the 

 Observatory of His Highness the Maharajah of Travancore, G. C.S.I. 

 By J. A. Broun, F.R.S. (Plates XXVI-XXVIII.) 



(Read 6th May 1872.) 



1. The lunar diurnal variation of magnetic declination as first discovered by 

 Kreil, depended on too few observations to be free from the errors introduced 

 by irregular disturbing causes. The independent discovery of the lunar action 

 on the magnetic needle made afterwards by myself, was liable to the same 

 criticism ; but the agreement of the results obtained, both for the magnetic 

 declination and the horizontal force, was sufficiently great to give a consider- 

 able value to the conclusion, that the magnetic needle obeys a diurnal law, 

 depending on the moon's hour angle, both as to its direction and the force with 

 which it is directed. This conclusion was farther confirmed in the discussion 

 first made by myself, for the lunar diurnal variation of the vertical magnetic 

 force, which gave, within an hour, the same epochs of maxima and minima as 

 those obtained previously by me for the horizontal component.* 



The results obtained afterwards from longer series of observations^ while less 

 affected by the irregularities due to disturbances, still showed variations of so 

 small a range, that the fact of the existence of a lunar diurnal variation was 

 not accepted by all men of science without reserve. Whatever doubt may 

 have existed before has been dissipated, I have every reason to believe, by the 

 results communicated in a paper to the Royal Society of Edinburgh in 1867, J 

 where variations were shown due to the lunar action, which equalled those 

 produced by the sun. 



2. This action of the moon is not constant ; it not only varies with the 

 period of the year, but it varies also for the same month in different years ; so 

 that, in some seasons, the variation is still so small, as to require the combina- 

 tion of large masses of observations to eliminate the effects of irregular causes, 

 in the determinations relating to the laws of variation under different circum- 

 stances. 



3. Though observations made in different parts of the world, both in the 

 northern and southern hemispheres, seemed to prove the existence of a law of 



* Trans. Eoyal Soc. Edin., vol. xvi. p. 143, § 19. 



"f* The most important of these have been discussed by General Sir E. Sabine. 



| Trans., vol. xxiv. p. 669. 



VOL. XXVI. PART. IV. 1) !<' 



