B84 :osM03. 



the consideration of teiTestrial magnetism and atiiiosplieric 

 temperature, as far as these sciences are included in the cen- 

 tury which we have attempted to describe. The able and 

 important work on magnetic and electric forces, the Physio- 

 logia nova de Mag7ief.e, by William Gilbert, to which I have 

 frequently had occasion to allude,* appeared in the year 1600 

 This writer, whose sagacity of mind was so highly admired 

 by Galileo, conjectured many things of which we have now 

 acquired certain knowledge. t Gilbert regarded terrestrial 

 magnetism and electricity as two emanations of a single fun- 

 damental force pervading all matter, and he therefore treated 

 of both at once. Such obscure conjectures, based on analogies 

 of the effect of the Heraclean magnetic stone on iron, and the 

 attractive force exercised on dry straws by amber, when ani- 

 mated, as Pliny expresses it, with a soul by the agency of 

 heat and friction, appertain to all ages and all races, to the 

 Ionic natural philosophy no less than to the science of the 

 Chinese physicists. t According to Gilbert's idea, the earth 

 itself is a magnet, while he considered that the inflections of 

 the lines of equal declination and inclination depend upon the 

 distribution of mass, the configuration of continents, or the 

 farm and extent of the deep, intervening oceanic basins. It 

 is difficult to connect the periodic variations which character- 

 ize the three principal forms of magnetic phenomena (the iso- 

 clinal, isogenic, 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 modi- 

 fied 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 rotation- 

 magnetism by Arago in 1825, has shown practically that ev- 

 ery kind of matter is susceptible of magnetism ; and the most 

 recent investigations of Faraday on dia-magnetic substances 



* Cosmos, vol. i,, p. 177, 179, and vol. ii., p. 278. 



t Lord B.icon, whose comprehensive, and, generally speaking, free 

 and methodical views, were unfortunately accompanied by very limit 

 ed mathematical and physical knowledge, even for the age in which 

 he lived, was veiy unjust to Gilbert. '' Bacon showed his inferior apt- 

 itude for physical research in rejecting the Copernican doctrine which 

 William Gilbert adopted" (Whewell, Philosophy of the Inductive Sci 

 ences, vol. ii.. p. 378"). I Cosmoi;, vol. i., p. 188. 



