MAGNETIC OBSERVATIONS. 115 



under which I obtained this result, and which I have else- 

 where fully described,* afford no sufficient ground for the sus- 

 picion of an error in the observation. Shortly after my ar- 

 rival at Cumana I found that the inclination was 43°*53. A 

 few days before the earthquake I was induced to begin a 

 long series of carefully-conducted observations in the harbor 

 of Cumana, in consequence of having accidentally noticed a 

 statement in an otherwise valuable Spanish work, Mendoza's 

 Tratado de Navegacion, t. ii., p. 72, according to which it 

 was erroneously asserted that the hourly and monthly alter- 

 ations of inclination were greater than those of variation. I 

 found, between the 1st and 2d of November, that the inclina- 

 tion exhibited very steadily the mean value of 43°*G5. The 

 instrument remained untouched and properly leveled on the 

 same spot, and on the 7th of November, and therefore three 

 days after the great earthquake, and when the instrument 

 had again been adjusted, it yielded 42°*75. The intensity 

 of the force, measured by vertical oscillations, was not 

 changed. I expected that the inclination would, perhaps, 

 gradually return to its former position, but it remained sta- 

 tionary. In September, 1800, in an expedition of more than 

 2000 geographical miles on the waters and along the shores 

 of the Orinoco and the Rio Negro, the same instrument, 

 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 disturbances and elec- 

 trical shocks excite polarity in soft iron by altering its mo- 

 lecular condition, we might suspect a connection between the 

 influences of the direction of magnetic currents and the di- 

 rection of earthquakes ; but carefully as I observed this phe- 

 nomenon, 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, however different were the di- 

 rections in which the undulations of the strata were propa- 

 gated. A very accurate and experienced observer, Erman, 

 likewise found that after an earthquake at Lake Baikal, on 

 the 8th of March, 1828, there Avas no disturbance in the 

 declination"!' and its periodic changes. 



* Humboldt, Voy. aux Regions Equinox., t. i., p. 515-517. 

 t Erman, Reise urn die JErde, bd. ii., s. 180. 



