18 COSMOS 



the -nflectioiis of the lines of equal declination and inclina- 

 tion depend upon the distribution of mass, the configuration 

 of continents, or the form and extent of the deep intervening 

 oceanic basins. It is difficult to connect the periodic varia- 

 tions which characterise the three principal forms of mag- 

 netic phenomena (the isoclinal, isogonic, and isodynamic 

 lines), with this rigid system of the distribution of force and 

 mass, unless we represent to ourselves the attractive force of 

 the material particles modified by similar periodic changes of 

 temperature in the interior of the terrestrial planet. 



In Gilbert's theory, as in gravitation, the quantity of the 

 material particles is merely estimated, without regard to the 

 specific heterogeneity of substances. This circumstance 

 gave his work, at the time of Galileo and Kepler, a charac- 

 ter of cosmical greatness. The unexpected discovery of rota- 

 tion-magnetism, by Arago in 1825, has shown practically, 

 that every kind of matter is susceptible of magnetism ; and 

 the most recent investigations of Faraday on dia-magnetic 

 substances have, under especial conditions of meridian or 

 equatorial direction, and of solid, fluid, or gaseous inactive 

 conditions of the bodies, confirmed this important result. Gil- 

 bert had so clear an idea of the force imparted by telluric 

 magnetism, that he ascribed the magnetic condition of iron 

 rods on crosses of old church towers to this action of the 

 earth.* 



The increased enterprise and activity of navigation to the 

 higher latitudes, and the improvement of magnetic instruments 

 to which had been added since 15 76, the dipping needle (incli- 

 natorium) constructed by Robert Norman of Ratcliff, were 

 the means, during the course of the seventeenth century, of 

 extending the general knowledge of the periodical advance 

 of a portion of the magnetic curves or lines of no vari- 

 ation. The position of the magnetic equator which was be- 

 lieved to be identical with the geographical equator, remained 



* The first observation of the kind was made (1590,) on the tower of 

 the church of the Augustines at Mantua. Grimaldi and Gassendi were 

 acquainted with similar instances, all occurring in geographical latitudes 

 where the inclination of the magnetic needle is very considerable. On 

 the first measurements of magnetic intensity by the oscillation of a needle, 

 compare my Relation hisL, i. i. pp. 260-264, and Cosmos, pp. 179 

 -181. 



