e
images on what is called a negative--that is, a glass coated with a
silvered collodion (gun-cotton dissolved in alcohol and ether) film.
From this plate another image is taken on silvered paper, which we
call the positive image. There are also other chemicals used instead
of silver.

What is a Camera Obscura?

A small box or dark room into which the light is admitted through a
lens.

What is a Stereoscope?

It is an instrument exhibiting the effects and advantages of seeing
with two eyes. The instrument is so constructed that from a flat
picture we may see the solid body in its reality in nature.

What is a Kaleidoscope?

An instrument invented by Sir David Brewster, consisting of a tube
with slips of reflecting glass so arranged in the interior that small
beads, bits of colored glass, and similar things are, by revolving the
tube, thrown into an endless variety of beautiful shapes.

CHAPTER XXI.

ELECTRICITY, ELECTRIC CURRENTS, ELECTRIC BATTERY, ELECTROTYPING,
STEREOTYPING, TELEGRAPH, OCEAN CABLE, LIGHTNING ROD, THE GULF STREAM,
THE MT. CENIS TUNNEL, THE SUEZ CANAL, SUSPENSION BRIDGES, EMINENT
AMERICANS.

What is the nature of Electricity?

A form of energy into which all other forms can readily be converted.

What is an Electric current?

Electricity manifests itself in a variety of ways, but all may be
arranged under two heads, _viz._, 1, as a charge; 2, as a current. By
means of friction, many bodies become electrified--that is, have
acquired an electrical charge. If this charge is in great quantity we
call it high tension. When a body containing an electrical charge is
brought in contact with other bodies through which electricity is
capable of passing, there ensues a current of electricity. Such bodies
are called conductors.

What are the sources of currents?

There are currents produced by chemical action called voltaic
currents; by the action of heat, or thermo-electric currents; by the
motion of magnets, or magneto-electric currents.

[Illustration: REMOVING THE EARTH FROM THE CANAL BY MEANS OF
DROMEDARIES.]

[Illustration: OPENING THE SUEZ CANAL--PROCESSION OF SHIPS.]

What is positive and what negative electricity?

No difference in electricity in itself. When a body has more than its
natural amount of electricity, it is said to be charged positively;
when it has less than its natural amount it is negatively charged.

What is a Cell; what a Battery?

If a p