114 COSMOS. 



inversion of the poles ; when I found, after very careful in- 

 vestigation, that the neighboring rock, which was composed 

 of gneiss, exerted no action on the magnetic needle. The 

 depth below the surface was 854 feet, and the difference be- 

 tween the inclination of the subterranean parts of the mine 

 and those points which lay immediately above it, and even 

 with the surface, was only 2 /< 06 ; but, considering the care 

 with which my experiments were made, I am inclined to 

 think, from the results given for each needle, as recorded in 

 the accompanying note^" that the inclination is greater in 

 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 Bo- 

 hemia, 3778 feet in depth.j 



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

 November, 1799, I found that the inclination was dimin- 

 ished 0-90, or nearly a whole degree. The circumstances 



* In the Churprinz mine at Freiberg, in the mountains of Saxony, 

 the subterranean point was 133 fathoms deep, and was observed with 

 Freiesleben and Reich at 2 P.M. (temperature of the mine being 

 60-08 F.). The dipping-needle A showed 67 37'-4, the needle B 

 67 32''7, the mean of both needles in the mine was 67 35''05. In 

 the open air, at a point of the surface which lies immediately above 

 the point of subterranean observation, the needle A stood at 11 A.M. 

 at 67 33'-87, and the needle B at 67 32'-12. The mean of both 

 needles in the upper station was 67 32'-99, the temperature of the 

 air being 60'44 F., and the difference between the upper and lower 

 result 2'-06. The needle A, which, as the stronger of the two, in- 

 spired me with most confidence, gave even 3'-53, while the influence 

 of the depth remained almost inappreciable when the needle B only 

 was used (Humboldt, in Poggend., AnnaL, bd. xv., s. 326). I have 

 already described 'in detail, and elucidated by examples, in Asic 

 Centr., t. iii., p. 465-467, the uniform method which I have always 

 employed in reading the azimuth circle in order to find the magnetic 

 meridian by corresponding inclinations, or by the perpendicular posi- 

 tion 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 read- 

 ings at both points, before and after the poles had been reversed. The 

 position of the two needles has, in each case, been read off sixteen 

 times, in order to obtain a mean result. TV here so small an amount 

 has to be determined, it is necessary to enter fully into the individual 

 details of the observation. 



t Cosmos, vol. i., p. 157. 



