COSMOS, VOL. II - World eBook Library

718 COSMOS.
the inflections 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 magnetic
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 character
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. Gilbert
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 (inclinatorium)
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 believed
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 G-assendi 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 aneedle,
compare my Relation hist., t. i. pp. 260-264, and Cosmos, pp. 179
181.