6 Wilde, Evolution of the Mental Faculties. 



and others. This differential motion, as is well known, 

 presents an obstacle in the way of determining the exact 

 time in which the sun revolves on his axis. * 



While the merit of suggesting the earth's internal 

 rotation as the cause of the secular variation of the declin- 

 ation is entirely due to the genius of Halley, the particular 

 disposition of his internal spheres, as well as his hypothesis 

 of magnetic poles rotating with them, are not explicable by 

 any known principles of science. Electro-dynamics, 

 electro-magnetism, and the physics of the earth's 

 crust were unexplored regions to seventeenth-century 

 philosophers, without knowledge of which sciences 

 Halley's theory admitted of no further development. 



In a paper which was read before the Royal Society 

 in 1890, I showed that all the principal phenomena of 

 terrestrial magnetism, and the secular changes in its 

 horizontal and vertical components, could be demonstrated 

 on the assumption of an electro-dynamic substance (liquid 

 or gaseous) rotating within the crust of the earth in the 

 plane of the ecliptic, that is to say, at an angle of 23-5 

 degrees. By means of some new electro-mechanism, 

 which I named a "Magnetarium," the slow period of 

 backward rotation of the internal electro-dynamic sphere 

 required for the secular variation of the magnetic elements, 

 on different parts of the terraqueous globe, was found to 

 be 960 years 



The striking analogies between the action of a compass 

 needle and a gyroscope have been pointed out by several 

 writers, as, from the fixity of its plane of rotation, the 



* That a high value was set on Halley's theory of the rotation of the 

 internal parts of the earth, and that it was intended to be handed down to 

 posterity, is evident from his fine portrait, which adorns the apartments of 

 the Royal Society, being embellished with the diagrammatic representation, 

 from the Philosophical Transactions, of the terrestrial globe enclosing an 

 inner sphere having the same common axis of rotation. 



