332 EARTH MAGNETIC BY ROTATION. SECT. XXXV. 



by their relative electrical conditions, but that the direc- 

 tion of the metallic veins must have been influenced by 

 the direction of the magnetic meridians ; and in fact 

 almost all the metallic deposits in the world tend from 

 east to west, or from northeast to southwest. Though 

 it is impossible to say in the present state of our knowl- 

 edge, how far the sun may be concerned in the phe- 

 nomena of terrestrial magnetism, it is probable that the 

 secular and periodic disturbances in the magnetic force 

 are occasioned by a variety of other combining circum- 

 stances. Among these M. Biot mentions the vicinity of 

 mountain chains to the place of observation, and still 

 more the action of extensive volcanic fires, which change 

 the chemical state of the terrestrial surface, they them- 

 selves varying from age to age, some becoming extinct, 

 while others burst into activity. Should the ethereal 

 medium which fills space be the same with the electric 

 fluid, as M. Mossotti. supposes, may not the heat of the 

 sun rarefy it at the earth's equator, and thus by the in- 

 equality of its distribution, and its superior density at 

 the poles, occasion some of the magnetic phenomena of 

 the globe ? and may not the sun's motion in decimation 

 cause temporary variations of density in the fluid, and 

 produce periodic changes in the magnetic equator and 

 intensity ? Were this the case, all the planets would 

 be magnets like the earth, being precisely in similar cir- 

 cumstances. 



It is moreover probable, that terrestrial magnetism 

 may be owing, in a certain extent, to the earth's rota- 

 tion. Dr. Faraday has proved that all the phenomena 

 of revolving plates may be produced by the inductive 

 action of the earth's magnetism alone. If a copper plate 

 be connected with a galvanometer by two copper wires, 

 one from the center and another from the circumference, 

 in order to collect and convey the electricity, it is found 

 that when the plate revolves in a plane passing through 

 the line of the dip, the galvanometer is not affected. 

 But as soon as the plate is inclined to that plane, elec- 

 tricity begins to be developed by its rotation ; it becomes 

 more powerful as the inclination increases, and arrives 

 at a maximum when the plate revolves at right angles to 

 the line of the dip. When the revolution is in the samo 



