er
the air they are moving at an average speed of about twenty miles a
second, and that the energy which this motion represents is by the
resistance which the body encounters converted into heat. This fact
will help us to understand how, as the original star dust fell in
toward the centre of attraction, it was able to convert what we have
termed the energy of position into temperature. We see clearly that
every such particle of dust or larger bit of matter which falls upon
the earth brings about the development of heat, even though it does
not actually strike upon the solid mass of our sphere. The conception
of what took place in the consolidation of the originally disseminated
materials of the sun and planets can be somewhat helped by a simple
experiment. If we fit a piston closely into a cylinder, and then
suddenly drive it down with a heavy blow, the compressed air is so
heated that it may be made to communicate fire. If the piston should
be slowly moved, the same amount of heat would be generated, or, as we
may better say, liberated by the compression, though the effect would
not be so striking. A host of experiments show that when a given mass
of matter is brought to occupy a less space the effect is in
practically all cases to increase the temperature. The energy which
kept the particles apart is, when they are driven together, converted
into heat. These two classes of actions are somewhat different in
their nature; in the case of the meteors, or the equivalent star dust,
the coming together of the particles is due to gravitation. In the
experiment with the cylinder above described, the compression is due
to mechanical energy, a force of another nature.

There is reason for believing that all our planets, as well as the sun
itself, and also the myriad other orbs of space, have all passed
through the stages of a transition in which a continually
concentrating vapour, drawn together by gravitation, became
progressively hotter and more dense until it assumed the condition of
a fluid. This fluid gradually parted with its heat to the cold spaces
of the heavens, and became more and more concentrated and of a lower
temperature until in the end, as in the case of our earth and of other
planets, it ceased to glow on the outside, though it remained
intensely heated in the inner parts. It is easy to see that the rate
of this cooling would be in some proportion to the size of the sphere.
Thus the earth, which is relatively s