MAGNETIC OBSERVATIONS. 117 



the Churprinz mine than on the surface of the mountain. 

 It would be very desirable if opportunities were to present 

 themselves in cases, where there is evidence that the rock 

 has not exerted any local influence on the magnet, for care- 

 fully repeating my experiments in mines, in which, like those 

 of Valenciana near Guanaxuato in Mexico, the vertical 

 depth is 1686 feet ; or in English coal mines nearly 1900 

 feet deep, or in the now closed shaft at Kuttenberg in 

 Bohemia, 3778 feet in depth. 4 * 



After a violent earthquake at Cumana on the 4th of 

 November, 1799, 1 found that the inclination was diminished 

 0.90, or nearly a whole degree. The circumstances under 

 which I obtained this result, and which I have elsewhere 

 fully described, 47 afford no sufficient ground for the suspi- 

 cion of an error in the observation. Shortly after my arrival 

 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 harbour 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 

 alterations of inclination were greater than those of varia- 

 tion. I found between the 1st and 2nd of November that 

 the inclination exhibited very steadily the mean value of 

 43. 65. The instrument remained untouched and properly 

 levelled 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 re- 

 mained stationary. In September, 1800, in an expedition of 



inclinations, or by the perpendicular position of the needle ; as also to 

 find the inclination itself on the vertical circle by reversing the bearings 

 of the needle and by taking the readings at both points, before and after 

 the poles had been reversed. The position of the two needles has, in 

 each case, been read off 16 times, in order to obtain a mean result. 

 Where so small an amount has to be determined, it is necessary to enter 

 fully into the individual details of the observation. 



46 Cosmos, vol. i, p. 148. 



47 Humboldt, Voy. aux Regions Equinox, t. i, pp. 515 517. 



