signers do not yet understand all the "factors" that enter into the
"why" of the case.

The makers have, however, succeeded in standardizing their machines to a
degree. The story of how the aeroplane flies is a highly technical and
scientific one, but the basic principle is the reaction of air and an
inclined surface in motion. It might be likened to a stone skipping
across the surface of a pond, if the imagination can conceive of the
water as being air. It is simplicity itself to drive an inclined plane
against the air with such force that the impact will produce a lifting
power. In raising an ordinary kite, for instance, the boy runs into the
teeth of the wind. His kite is so attached to a string as to stand at an
angle, and as he runs the pressure against the air drives the kite
upward. In the aeroplane the propellers drive the machine into the air
with such force that the planes, standing at an angle, guide the machine
upward.

There are innumerable problems to be solved--those of buoyancy, delicacy
of balance and many others--but the designers themselves have not been
able to determine upon a precise formula for their solution. It is
sufficient that the aeroplane has reached a degree of practicability in
construction and use which insures its permanent existence, and has
given the military and the naval forces one of the greatest agencies in
the world for protecting themselves and watching their enemies.

CHAPTER X.

WAR'S STRANGE DEVICES.

CHEMISTRY A DEMON OF DESTRUCTION--POISON GAS BOMBS--GAS MASKS--HAND
GRENADES--MORTARS--"TANKS"--FEUDAL "BATTERING RAMS"--STEEL
HELMETS--STRANGE BULLETS--MOTOR PLOWS--REAL DOGS OF WAR.

Things new and passing strange--thousands of them--have been brought
into being by the great world war. Human minds have developed things
undreamed of by science or fiction--things that a few years ago would
have been considered too strange and fantastic for even the professional
romancer to weave into the tissues of his stories.

Every known science has been called upon to produce its quota of new
things which might be used for the destruction or the protection of men
at war. The wonders of chemistry have always lent descriptive
inspiration to the pen of writers, but mankind to get a vivid conception
of the horrors of chemistry has had to wait for the great world war.

The conflict which has involved the entire world might almost be termed
a warfare of chemists. Without their dia