our globe, and that the momentary streak of fire in the
summer sky represents a feeble survival of the glowing hailstorm by
which in old times it was fashioned and warmed. Mr. E. W. Brayley
supported this view of planetary production in 1864,[1157] and it has
recommended itself to Haidinger, Helmholtz, Proctor, and Faye. But the
negative evidence of geological deposits appears fatal to it.

The theory of solar energy now generally regarded as the true one was
enounced by Helmholtz in a popular lecture in 1854. It depends upon the
same principle of the equivalence of heat and motion which had suggested
the meteoric hypothesis. But here the movement surrendered and
transformed belongs to the particles, not of any foreign bodies, but of
the sun itself. Drawn together from a wide ambit by the force of their
own gravity, their fall towards the sun's centre must have engendered a
vast thermal store, of which 453/454 are computed to be already spent.
Presumably, however, this stream of reinforcement is still flowing. In
the very act of parting with heat, the sun develops a fresh stock. His
radiations, in short, are the direct result of shrinkage through
cooling. A diminution of the solar diameter by 380 feet yearly would
just suffice to cover the present rate of emission, and would for ages
remain imperceptible with our means of observation, since, after the
lapse of 6,000 years, the lessening of angular size would scarcely
amount to one second.[1158] But the process, though not terminated, is
strictly a terminable one. In less than five million years, the sun will
have contracted to half its present bulk. In seven million more, it will
be as dense as the earth. It is difficult to believe that it will then
be a luminous body.[1159] Nor can an unlimited past duration be
admitted. Helmholtz considered that radiation might have gone on with
its actual intensity for twenty-two, Langley allows only eighteen
million years. The period can scarcely be stretched, by the most
generous allowances, to double the latter figure. But this is far from
meeting the demands of geologists and biologists.

An attempt was made in 1881 to supply the sun with machinery analogous
to that of a regenerative furnace, enabling it to consume the same fuel
over and over again, and so to prolong indefinitely its beneficent
existence. The inordinate "waste" of energy, which shocks our thrifty
ideas, was simultaneously abolished. The earth stops and turns vari