_The Great Law of Conservation of Force._

But how speaks the law of conservation, that law most enduring, and most
inexorable? According to the decrees of that law, whatever is received
by the earth from the sun, an equivalent for the same must again be
returned from the earth to the sun, to the uttermost fraction.[2] Such
being the conditions, how may this retro-acting process that all analogy
and the profoundest scientific axiom prove to be in constant
operation--how, I ask, may this retro-acting process be explained? What
equivalent may the earth give back as compensation for such enormous
benefits, for such stupendous powers? The laws of conservation may not
be violated: _the earth will respond_.

_How are the Spheres constructed?_

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.