ight of coal. We have stated that by falling
through a space of 16 feet our lead bullet would be heated
three-fifths of a degree; but a body falling from an infinite distance
has already used up 1,299,999 parts out of 1,300,000 of the earth's
pulling power, when it has arrived within 16 feet of the surface; on
this space only 1/1,300,000 of the whole force is exerted.

Let us now turn our thoughts for a moment from the earth to the sun.
The researches of Sir John Herschel and M. Pouillet have informed us
of the annual expenditure of the sun as regards heat; and by an easy
calculation we ascertain the precise amount of the expenditure which
falls to the share of our planet. Out of 2300 million parts of light
and heat the earth receives one. The whole heat emitted by the sun in
a minute would be competent to boil 12,000 millions of cubic miles of
ice-cold water. How is this enormous loss made good--whence is the
sun's heat derived, and by what means is it maintained? No
combustion--no chemical affinity with which we are acquainted, would
be competent to produce the temperature of the sun's surface.
Besides, were the sun a burning body merely, its light and heat would
speedily come to an end. Supposing it to be a solid globe of coal,
its combustion would only cover 4600 years of expenditure. In this
short time it would burn itself out. What agency then can produce the
temperature and maintain the outlay? We have already regarded the
case of a body falling from a great distance towards the earth, and
found that the heat generated by its collision would be twice that
produced by the combustion of an equal weight of coal. How much
greater must be the heat developed by a body falling against the sun!
The maximum velocity with which a body can strike the earth is about 7
miles in a second; the maximum velocity with which it can strike the
sun is 390 miles in a second. And as the heat developed by the
collision is proportional to the square of the velocity destroyed, an
asteroid falling into the sun with the above velocity would generate
about 10,000 times the quantity of heat produced by the combustion of
an asteroid of coal of the same weight.

Have we any reason to believe that such bodies exist in space, and
that they may be raining down upon the sun? The meteorites flashing
through the air are small planetary bodies, drawn by the earth's
attraction. They enter our atmosphere with planetary velocity, and by
fr