there are certain
special questions regarding which we are able to interrogate the sun
because of his proximity, and which have, furthermore, a peculiar
interest for the residents of our little globe because of our dependence
upon this particular star. One of the most far-reaching of these is
as to where the sun gets the heat that he gives off in such
liberal quantities. We have already seen that Dr. Mayer, of
conservation-of-energy fame, was the first to ask this question. As
soon as the doctrine of the persistence and convertibility of energy was
grasped, about the middle of the century, it became clear that this
was one of the most puzzling of questions. It did not at all suffice to
answer that the sun is a ball of fire, for computation showed that, at
the present rate of heat-giving, if the sun were a solid mass of coal,
he would be totally consumed in about five thousand years. As no such
decrease in size as this implies had taken place within historic times,
it was clear that some other explanation must be sought.

Dr. Mayer himself hit upon what seemed a tenable solution at the very
outset. Starting from the observed fact that myriads of tiny meteorites
are hurled into the earth's atmosphere daily, he argued that the sun
must receive these visitants in really enormous quantities--sufficient,
probably, to maintain his temperature at the observed limits. There was
nothing at all unreasonable about this assumption, for the amount of
energy in a swiftly moving body capable of being transformed into heat
if the body be arrested is relatively enormous. Thus it is calculated
that a pound of coal dropped into the sun from the mathematician's
favorite starting-point, infinity, would produce some six thousand times
the heat it could engender if merely burned at the sun's surface. In
other words, if a little over two pounds of material from infinity
were to fall into each square yard of the sun's surface each hour, his
observed heat would be accounted for; whereas almost seven tons per
square yard of stationary fuel would be required each hour to produce
the same effect.

In view of the pelting which our little earth receives, it seemed not
an excessive requisition upon the meteoric supply to suppose that the
requisite amount of matter may fall into the sun, and for a time this
explanation of his incandescence was pretty generally accepted. But soon
astronomers began to make calculations as to the amount of matter which