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XLV. — On the Diurnal Variation of the Magnetic Declination at Trevandrum, 

 near the Magnetic Equator, and in both Hemispheres. By John Allan Broun, 

 Esq., F.R.S., late Director of the Observatory of His Highness the Maharajah 

 of Travancore, G.C.S.I., at Trevandrum. (Plates XL. to XLIV.) 



(Read 29th April 1867). 



Preceding Observations and Conclusions. 



The first observations of the diurnal variation of magnetic declination, made 

 near the equator, seem to have been those of Mr Macdonald, who observed in 

 1794-95 at Fort Marlborough, Sumatra, 3° 46' S., and at St Helena. Two conclu- 

 sions seem to have been deduced from these observations — 1st, That near the 

 equator the range of the diurnal variation was much smaller than in Europe ; 

 2d, That the needle moved in opposite directions south of the equator and in 

 Europe.* This latter conclusion was made use of by M. Arago, in his report 

 made in 1821, on the " Voyage de l'Uranie," as the base of a hypothesis that 

 there must be a line betwixt the two hemispheres on which the magnetic needle 

 moves neither east nor west — that is, remains stationary. M. de Freycinet's 

 observations showed that this line was not the terrestrial equator, and M. Arago 

 supposed it must be the magnetic equator, f 



In 1825 M. Arago again alluded to his hypothesis, in his report on M. Du- 

 perrey's observations. These seemed to show— 1st, That the diurnal variation 

 was not extinguished on either equator ; 2d, That in south latitudes, but with 

 north magnetic inclination, the needle moved as in Europe. M. Arago then 

 suggested, " Peutetre les changements de declinaison du soleil, qui en Europe oc- 

 casionnent de si grandes variations dans Famplitude des oscillations diurnes, 

 amenent ils suivant les saisons sous les tropiques des mouvements de l'aiguille 

 diriges en sens inverse."]; 



M. Arago recurred to the subject in 1835, in his " Instructions pour la Bonite," 

 and he suggests some additional line, some curve of equal magnetic intensity, as 

 the curve of no diurnal variation. § 



* Phil. Trans, abridged, vol. xviii. pp. 29 and 355. The first of the conclusions is given by Mr 

 Macdonald. He appears to have found, that the north end of the needle moved east from 7 a.w. 

 till 5 p.m. The observations having been made from June 1794 till March 1795, the result from 

 the observations for the first four months (June to September) should have been nearly the inverse 

 of that from the observations for the remaining months. There can be little doubt that the instru- 

 ment employed was incapable of showing the variations with much accuracy. 



t (Euvres de F. Arago. Instructions, Rapports, etc., p. 152. Voyage de l'Uranie. 



J Ibid. p. 196. Voyage de la Coquille. § Ibid. pp. 25, 26. 



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