the wind, and by
which storms, at a distance, can be predicted.

What is a Chronometer?

A time-piece of delicate and exact construction, chiefly employed by
astronomers and navigators. It differs only from an ordinary watch in
its delicate springs, in not being so much influenced by heat and
cold, and consequently in its accuracy in giving the time.

CHAPTER XX.

LIGHT, LIME LIGHT, MAGNESIUM LIGHT, ELECTRIC LIGHT, RAINBOW,
PRISM, SPECTRUM, COLORS, PHOTOGRAPHY, CAMERA OBSCURA, STEREOSCOPE,
KALEIDOSCOPE.

Do you know something about the nature of Light?

Light is a mere form of vibration like sound, and like sound it
requires some source to set this vibration going, and some medium to
carry this vibration as air carries sound.

Is not the air this medium?

No, it is supposed that there is an elastic fluid called "ether" which
pervades all space and matter, and if the molecules of a body are in
motion they have the power of setting this ether in motion. The
movement thus produced will appear either as heat or light according
to its velocity.

What sources of light do you know?

We are told that the principal source of light on earth is the sun,
either directly with its own beams or indirectly by supplying us with
combustibles to produce light; for oil, gas, candles, and most of the
substances used for producing light and heat when burning are but
sending forth in another form the rays of the sun which were stored up
in nature's economy.

Another source of light is the result of chemical action, such as the
lime, magnesium, and electric light. A third source of light is
phosphorescence, as we see it in the glow-worm and fireflies.

What is the Drummond or Lime Light?

It is one of the most brilliant of artificial lights. When a stream of
oxygen and one of hydrogen under pressure are brought together and
mixed within a few inches of the end of a blowpipe, the mixture on
lighting burns with a colorless flame possessing intense heat. If this
flame be made to play upon a ball of carbonate of lime, the lime on
becoming white hot gives off a powerful incandescence.

_Incandescence_, the glowing whiteness of a body caused by
intense heat.

What is a Blowpipe?

A tube, usually bent near the end, terminated with a finely-pointed
nozzle, for blowing through the flame of a lamp or gas-jet, producing
thereby a small conical flame possessing intense heat. It is used in
soldering silve