156 



COSMOS. 



ation of the position of such magnetic axes of a mountain 

 would be of the greatest interest, if it could be ascertained, 

 after considerable periods of time, that the three variable 

 elements of the total force of terrestrial magnetism caused 

 either an alteration in the direction of the axes, or that such 

 small systems of magnetic forces were at least apparently 

 independent of these influences. 



earth, have led Dr. Zaddach to hazard the conjecture (see s. 74, 80) 

 that the polar property, which always appears to be manifested with 

 the greatest intensity in rocks to which the air has been freely admit- 

 ted, and which are intersected by open fissures, " diffuses itself from 

 without inward, and generally from above downward." Gmelin ex- 

 presses himself as follows in respect to the great magnetic mountain, 

 Ulu-utasse-Tau, in the country of the Baschkiri, near the Jaik : " The 

 sides which are exposed to the open air exhibit the most intense mag- 

 netic force, while those which lie under ground are much weaker" 

 (Reise durch Siberien, 1740-1743, bd. iv., s. 345). My distinguished 

 teacher, Werner, in describing the magnetic iron of Sweden, in his 

 lectures, also spoke of " the influence which contact with the atmos- 

 phere might have, although not by means of an increased oxydation, 

 in rendering the polar and attracting force more intense." It is as- 

 serted by Colonel Gibbs, in reference to the magnetic iron mines at 

 Succassuny, in New Jersev, that " the ore raised from the bottom of 

 the mine has no magnetism at first, but acquires it after it has been 

 some time exposed to the influence of the atmosphere" {On the connec- 

 tion of Magnetism and Light, in Silliman's American Journal of Science, 

 vol. i., 1819, p. 89). Such an assertion as this ought assuredly to stim- 

 ulate observers to make careful and exact investigations ! When I 

 drew attention in the text (see page 154) to the fact that it was not 

 only the quantity of the small particles of iron which were intermixed 

 in the stone, but also their relative distribution (their position), which 

 acted as the resultant upon the intensity of the polar force, I consid- 

 ered the small particles to be so many small magnets. See the new 

 views regarding this subject in a treatise by Melloni, read by that dis- 

 tinguished physicist before the Royal Academy at Naples, in the month 

 of January, 1853 (JEsperienze intorno alMagnetismo delle Rocche, Mem. 

 i., Sulla Polarita). The popular notion which has been so long cur- 

 rent, more especially on the shores of the Mediterranean, that if a 

 magnetic rod be rubbed with an onion, or brought in contact with the 

 emanations of the plant, the directive force will be diminished, while 

 a compass thus treated would mislead the steersman, is mentioned in 

 Prodi Diadochi Paraphrasis Ptolem., libri iv., de Siderum- ajfectionibus, 

 1635, p. 20 (Delambre, Hist, de V Astronomic Ancienne, t. ii., p. 545). 

 It is difficult to conceive what could have given occasion to so singular 

 a popular error. 



