The constitution of these two retro-acting spheres, and consequently of all the others of the heavenly host,[3] at this point demands our attention. How are the spheres made up? How speaks the earth? The earth with which we are familiar--our sample--is formed of a slight crust, a core, to a greater or less extent and degree incandescent, and measuring 250,000 millions of cubic miles in dimensions, also an envelope which we call the atmosphere. Now, from the presence of the vast mass of incandescent material within the enclosure of each sphere-crust, it may reasonably be inferred, nay the very nature of human reason _compels_ the decision, that _they are placed there for some specific purpose_, and that _their operations are commensurate with their immensity_. We may not neglect to make account of so vast an element, and so vital and preponderating, in all globes.[A] We are thus compelled to answer the question, What part in the economy of nature is this great central core particularly fitted to perform? What its function among the great forces? The great problem of the age, which scientists are intently engaged in solving, is the correlation of the leading forces already adverted to. Thus far light, heat, electricity, magnetism, chemical action, vital action, cohesion, etc., have been proved to be parts of one great whole. Now, since the especial characteristic of the great earth-core is heat, it comes directly into relationship with the forces mentioned. How then are its forces expended? Through what channels do they manifest their presence? The philosophical mind would most naturally associate with it the idea of stupendous magnetic power. We may well suppose such a power extending its influence through and beyond the earth-crust, reaching out towards the moon, and retro-acting with that body in preserving their mutual relations. Does not this mighty influence reach out toward the sun also, and act conjointly with that great central orb in producing results, which to us, have ever been great mysteries.[4] _The Grand Magnetic Circuit._ In the retro-acting influence in operation between these great bodies, may be found _A Grand Magnetic Circuit_. In this grand magnetic circuit is found the _key_ to the whole subject of the correlation and identity of all the forces. And now, as preparatory to using this _key_ that we may enter in and consider the intimate nature of the physical forces, we would be impressed with the clear and full idea of this mighty _current_, which bears upon its tide, _as one_, all manner of forces with which we have to do. It remains for us to tell what this great current _is_, and what it _does_. To the child, to the savage, and to the civilized man alike, it comes first and pre-eminently as light. FOOTNOTES: [2] Appendix, p. 98. [3] Appendix, p. 99. [4] Appendix, p. 99. [A] The earth's core constitutes nearly 98/100 of its entire mass. CHAPTER IV.