, the sound from only one of whose octaves is audible to
us.

The brightest light we know on the earth is dull compared with the light
of the sun. It would, indeed, look quite dark if held up against it.

It is extremely difficult to arrive at a precise notion of the
temperature of the body of the sun. However, it is far in excess of any
temperature which we can obtain here, even in the most powerful electric
furnace.

A rough idea of the solar heat may be gathered from the calculation that
if the sun's surface were coated all over with a layer of ice 4000 feet
thick, it would melt through this completely in one hour.

The sun cannot be a hot body merely cooling; for the rate at which it is
at present giving off heat could not in such circumstances be kept up,
according to Professor Moulton, for more than 3000 years. Further, it is
not a mere burning mass, like a coal fire, for instance; as in that case
about a thousand years would show a certain drop in temperature. No
perceptible diminution of solar heat having taken place within historic
experience, so far as can be ascertained, we are driven to seek some
more abstruse explanation.

The theory which seems to have received most acceptance is that put
forward by Helmholtz in 1854. His idea was that gravitation produces
continual contraction, or falling in of the outer parts of the sun; and
that this falling in, in its turn, generates enough heat to compensate
for what is being given off. The calculations of Helmholtz showed that a
contraction of about 100 feet a year from the surface towards the centre
would suffice for the purpose. In recent years, however, this estimate
has been extended to about 180 feet. Nevertheless, even with this
increased figure, the shrinkage required is so slight in comparison with
the immense girth of the sun, that it would take a continual
contraction at this rate for about 6000 years, to show even in our
finest telescopes that any change in the size of that body was taking
place at all. Upon this assumption of continuous contraction, a time
should, however, eventually be reached when the sun will have shrunk to
such a degree of solidity, that it will not be able to shrink any
further. Then, the loss of heat not being made up for any longer, the
body of the sun should begin to grow cold. But we need not be distressed
on this account; for it will take some 10,000,000 years, according to
the above theory, before the solar orb becomes too