MAGNETIC VARIATION. 119 



line of no variation, on which the needle turns to the true 

 north, the pole of the axis of the earth was determined 

 by Columbus on the 13th of September, 1492, and it did 

 not escape his notice that the knowledge of the magnetic 

 declination might serve in the determination of geographical 

 longitudes. I have elsewhere shewn, from the Admiral's log, 

 that when he was uncertain of the ship's reckoning, he 

 endeavoured, on his second voyage, April, 1496, to ascertain 

 his position by observations of declination. M The horary 

 changes of variation which were simply recognized as certain 

 facts by Hellibrand and Father Tachard, at Louvo, in Siam, 

 were circumstantially and almost conclusively observed by 

 Graham in 1722. Celsius was the first who made use of 

 these observations to institute simultaneous measurements 

 at two widely remote points. 51 



Passing to the consideration of the phenomena observed 

 in the variation of the magnetic needle, we must first notice 

 its alterations in respect to the different hours of the 

 night and day, the different seasons of the year and the 

 mean annual values ; next, in respect to the influence which 

 the extraordinary, although periodically recurring disturb- 

 ances, and the magnetic position, north or south of the 

 equator, exert on these alterations, and finally in respect to 

 the different lines passing through the terrestrial points at 

 which the variation is equal, or even null. These linear 

 relations are certainly most important in respect to the direci 



50 Humboldt, Examen. Grit, de VHut. de la Geogr. t. iii, pp. 29, 36. 

 38, 44 51. Although Herrera (Dec. i, p. 23) says that Columbus had 

 remarked that the magnetic variation was not the same by day and 

 by night, it does not justify us in ascribing to this great discoverer a 

 knowledge of the horary variation. The actual Journal of the admira 1 

 which has been published by Navarrete, informs us that from the 17tk 

 to the 30th of September, 1492, Columbus had reduced everything to a 

 so-called "unequal movement" of the polar star and the pointers 

 (Guardas), Examen Grit. t. iii, pp. 56 59. 



51 See pages 60, 70. The first printed observations for London are 

 those by Graham, in the Phil. Transact, for 1724 and 1725, vol. xxxijn, 

 pp. 96 107 (An Account of Observations made of the Horizontal Needle 

 at London, 17221723, by Mr. George Graham). The change of the 

 variation depends " neither upon heat nor cold, diy or moist air. 

 The variation is greatest between 12 and 4 in the afternoon, and the 

 least at 6 01 7 in the evening." These however, are not the true turning 

 hours. 



