TERRESTRIAL MAGNETISM. 123 



with the greatest care of which I am capable, and always 

 reversing the poles of the needles, in a mine in which the 

 strictest examination could detect no influence of the rock 

 (gneiss) upon a magnetic needle. The depth below the 

 surface was 802 Trench feet. The difference of the subterra- 

 nean inclination from that observed at a point immediately 

 above it is, indeed, only 2'- 06, but from the precautions 

 taken in the whole proceeding, the results of the several 

 needles which are stated in a note,( 145 ) induce me to 

 believe that the inclination is really greater below than at the 

 surface. It is very desirable to repeat similar trials with 

 care, in favourable localities and where it can be ascertained 

 that the strata are free from all local influence, in mines of 

 greater depth, as that of Yalenciana, near Guanaxuato in 

 Mexico, 3582 French feet, in English coal mines more 

 than 1800 feet deep, or in the now ruined Eselschacht,( 146 ) 

 at Kuttenberg in Bohemia, 3545 French feet in vertical 



After a violent earthquake at Cumana, on the 4th of 

 November, 1799, I found the inclination was diminished 

 90 centesimal minutes, or nearly a whole degree. The 

 circumstances under which I obtained this result, of 

 which I have given elsewhere an exact relation, ( li7 ) offer no 

 satisfactory reason for supposing an error. A short time 

 after landing at Cumana, I had found the inclination 

 43*53 (Centesimal). The accident of having seen, a few 

 days before the earthquake, in an otherwise estimable 

 Spanish work, Mendoza's Tratado de Navegacion, T. ii. 

 p. 72, the erroneous opinion, that the diurnal and annual 

 variations of the inclination are greater than those of the 



