A train travelling at the rate of sixty miles per hour would take rather
more than seventeen and a quarter days to go round the earth at the
equator, i.e. a distance of 25,000 miles. Light, which travels at the
rate of 186,000 miles per second, would take between one-seventh and
one-eighth of a second to go the same distance.]
[Illustration: ROTATING DISC OF SIR ISAAC NEWTON FOR MIXING COLOURS
The Spectroscope sorts out the above seven colours from sunlight (which
is compounded of these seven colours). If painted in proper proportions
on a wheel, as shown in the coloured illustration, and the wheel turned
rapidly on a pivot through its centre, only a dull white will be
perceived. If one colour be omitted, the result will be one colour--the
result of the union of the remaining six.]
It is not possible to enter deeply into this subject here. It is
sufficient if we briefly outline its salient aspects. Energy is
recognised in two forms, kinetic and potential. The form of energy which
is most apparent to us is the _energy of motion_; for example, a rolling
stone, running water, a falling body, and so on. We call the energy of
motion _kinetic energy_. Potential energy is the energy a body has in
virtue of its position--it is its capacity, in other words, to acquire
kinetic energy, as in the case of a stone resting on the edge of a
cliff.
Energy may assume different forms; one kind of energy may be converted
directly or indirectly into some other form. The energy of burning coal,
for example, is converted into heat, and from heat energy we have
mechanical energy, such as that manifested by the steam-engine. In this
way we can transfer energy from one body to another. There is the energy
of the great waterfalls of Niagara, for instance, which are used to
supply the energy of huge electric power stations.