118 COSMOS. 



more than 2000 geographical miles on the waters and along 

 the shores of the Orinoco and the Rio Negro, the same in- 

 strument, which was one of Borda's, which I had constantly- 

 carried with me, yielded 42. 80, showing, therefore, the 

 same dip as before my journey. As mechanical disturb- 

 ances and electrical shocks excite polarity in soft iron by 

 altering its molecular condition, we might suspect a connec- 

 tion between the influences of the direction of magnetic 

 currents and the direction of earthquakes ; but carefully as 

 I observed this phenomenon, of whose objective reality I 

 did not entertain a doubt in 1799, I have never on any 

 other occasion, in the many earthquakes which I experienced 

 in the course of three years at a subsequent period in South 

 America, noticed any sudden change of the inclination, 

 which I could ascribe to these terrestrial convulsions, how- 

 ever different were the directions, in which the undulations 

 of the strata were propagated. A very accurate and ex- 

 perienced observer, Erman, likewise found that after an 

 earthquake at Lake Baikal, on the 8th of March, 1828, there 

 was no disturbance in the declination 48 and its periodic 

 changes. 



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 early as the 12th century of our era, 

 were not only well acquainted with the fact of the variation 

 of a horizontal magnetic needle (suspended by a cotton 

 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 amongst the Genoese, Majorcans and Catalans, in 

 the basin of the Mediterranean, on the west coast of Africa, 

 and in high northern latitudes ; while the maps, which were 

 published as early as 1436, even give the variation for dif- 

 ferent parts of the sea. 49 The geographical position of a 



48 Erman, Reise um die Erde, Bd. ii, s. 180. 



49 See page 52 ; Petrus Peregrine informs a friend that he found the 

 variation in Italy was 5 east in 1269. 



