116 COSMOS. 



Declination. 



We have already referred to the historical facts of the 

 earliest recognition of those phenomena which depend upon 

 the third element of terrestrial magnetism, namely, declina- 

 tion. The Chinese, as earlv as the 12th centurv of our era, 

 were not only well acquainted with the fact of the varia- 

 tion of a horizontal magnetic needle (suspended by a cot- 

 ton thread) from the geographical meridian, but they also 

 knew how to determine the amount of this variation. The 

 intercourse which the Chinese carried on with the Malays 

 and Indians, and the latter with Arab and Moorish pilots, 

 led to the extensive use of the mariner's compass among the 

 Genoese, Majorcans, and Catalans, in the basin of the Med- 

 iterranean, on the west coast of Africa, and in hi^h northern 

 latitudes ; while the maps, which were published as early as 

 1436, even give the variation for different parts cf the sea.* 

 The geographical position of a 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 de- 

 termination of geographical longitudes. I have elsewhere 

 shown, from the Admiral's loo;, that when he was uncertain 

 of the ship's reckoning, he endeavored, on his second voyage, 

 April, 1496, to ascertain his position by observations of dec- 

 lination.! The horary changes of variation, which were sim- 

 ply recognized as certain facts by Hellibrand and Father 

 Tachard, at Louvo, in Siam, were circumstantially and al- 

 most 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.J 



* See page 53; Petrus Peregrine informs a friend that he found 

 the variation in Italy was 5° east in 1269. 



f Humboldt, Examen. Crit. de lllist. de la GLogr., t. hi., p. 29, 36, 

 38, 44-51. Although Hen-era {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 admiral, 

 which has been published by Navarre te', informs us that from the 17th 

 to the 30th of September, 1492, Columbus had reduced every thing to 

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

 (Guardas), Examen Crit., t. iii., p. 56-59. 



X See pages 61, 70. The first-printed observations for London are 

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

 xxxiii., p. 96-107 {An Account of Observations made of the Horizontal 



