670 MR J. A. BROUN ON DIURNAL VARIATIONS OF THE 



In his examination of observations which seem to have been made with much 

 zeal and care, by M. de Tessan, in 1837-38, in different latitudes, at different 

 periods of the year, M. Arago says, with reference to the question , whether the 

 hours of maxima and minima of declination are identical over all the earth : 

 " Nous pouvons affirmer qu'il n'en est pas ainsi l'aiguille horizontale atteint les 

 limites de ses excursions diurnes, a des heures differentes suivant les climats."* 

 With this affirmation M. Arago destroyed the base of his hypothesis. M. de 

 Tessan himself pointed out, " Que la transition de l'un de ces etats (the move- 

 ment in one hemisphere), a l'etat oppose (the movement in the other hemisphere), 

 a mesure qu'on passe d'un hemisphere a Fautre, peut se faire aussi par un 

 deplacement graduel des heures critiques auxquelles la direction du mouve- 

 ment change. Et la realite de ce dernier mode de transition qui a priori, est 

 aussi possible que Fautre me parait indiquee avec beaucoup de vraisemblance par 

 la succession reguliere des courbes de notre tableau graphique."f 



Yet we find in August 1840, after a study of all the observations made in the 

 different voyages terminating with that of the " Venus," that M. Arago thought 

 we should still seek the line of no diurnal variation, " Une courbe le long de la 

 quelle Taiguille par exception conservera de jour et de nuit absolument la raeme 

 direction." — " Une courbe qui deviendra aussi l'objet de bien des recherches de 

 bien des voyages." | It will be seen in his posthumous work on " Terrestrial 

 Magnetism," that he preserved this idea to the last. 



A consideration of the observations examined by M. Arago, of the shortness 

 of their duration at each station, and of the difficulties of obtaining accurate 

 observations with an imperfect installation of instruments (signalised by M. de 

 Tessan), will explain his varying conclusions, and show that the materials in his 

 possession were insufficient to answer the questions as to the mode in which the 

 diurnal variation of the magnetic needle changed during the year at any station 

 within the tropics, or from one latitude to another, at the same period. 



The first series of observations of sufficiently long duration within or near the 

 tropics, and capable of showing what laws the magnetic needle really followed 

 there, were made in the Observatories of Bombay, Madras, Trevandrum, Singa- 

 pore, and St Helena. The first, second, and fourth of these observatories were 

 established at the expense of the East India Company, the last by the British 

 Government, and the third by His Highness the Rajah of Travancore. 



The observations made at Trevandrum, in June to December 1841, under the 

 direction of the late Mr Caldecott, F.R.S., were in the possession of the Royal 

 Society of London early in 1842, with the projected diurnal curves of magnetic 



* Instructions, Rapports, etc., p. 283. Voyage de la Venus. Report, dated 1840. 

 f Voyage autour du monde sur la Fregate la Venus. Physique, vol. v. par M. de Tessan, 1844, 

 p. 417. See also p. 461. 



\ (Euvres, Rapports, etc., p. 288. 



