inclined at small angles as
they are at present; nor will it be unreasonable for us at the same
time to bring into parallelism all the axes of rotation, and to
arrange that their common directions shall be perpendicular to the
plane of their common orbits. For the purpose of our present research
this ideal system may pass for the real system.

In its original state, whatever that state may have been, a
magnificent endowment was conferred upon the system. Perhaps I may,
without derogation from the dignity of my subject, speak of the
endowment as partly personal and partly entailed. The system had of
course different powers with regard to the disposal of the two
portions; the personal estate could be squandered. It consisted
entirely of what we call energy; and considering how frequently we use
the expression conservation of energy, it may seem strange to say now
that this portion of the endowment has been found capable of
alienation, nay, further, that our system has been squandering it
persistently from the first moment until now. Although the doctrine of
the conservation of energy is, we have every reason to believe, a
fundamental law affecting the whole universe, yet it would be wholly
inaccurate to say that any particular system such as our solar system
shall invariably preserve precisely the same quantity of energy
without alteration. The circumstance that heat is a form of energy
indeed negatives this supposition. For our system possesses energy of
all the different kinds: there is energy due to the motions both of
rotation and of revolution; there is energy due to the fact that the
mutually attracting bodies of our system are separated by distances of
enormous magnitude; and there is also energy in the form of heat; and
the laws of heat permit that this form of energy shall be radiated off
into space, and thus disappear entirely, in so far as our system is
concerned. On the other hand, there may no doubt be some small amount
of energy accruing to our system from the other systems in space,
which like ours are radiating forth energy. Any gain from this source,
however, is necessarily so very small in comparison to the loss to
which we have referred, that it is quite impossible that the one
should balance the other. Though it is undoubtedly true that the
total quantity of energy in the universe is constant, yet the share of
that energy belonging to any particular system such as ours declines
steadily from age to ag