1. The _orthopters_, or as they are less commonly called, the
ornithopters. The word "orthopter" means straight wing and the word
"ornithopter" bird wing. This class of machines includes those designed
to fly by the flapping of wings, somewhat in imitation of birds.
2. The _helicopters_. The word "helicopter" means spiral wing. Flying
machines of this class are designed to fly by the rapid horizontal
rotation of two spiral propellers moving in opposite directions but so
shaped that their combined effect is to move the machine upward. They
are like a pair of tractor propellers of the modern aeroplane but
arranged horizontally to lift the machine instead of drawing it forward
in a vertical position.
3. The _gliders_. As the name suggests, these were designed to coast or
glide down the air, to start from a high elevation and by sailing
through the air in an oblique direction reach a lower elevation at some
distance from the starting point. Down to the latter part of the
nineteenth century only the gliders were successfully used in man
flight. In reality they can scarcely be called flying machines for they
could not lift their own weight, though late experiments prove that when
once in air they may rise above their starting point under the influence
of a strong wind. The glider, however, performed a most important part
in the evolution of the aeroplane. In coasting the air from hills, sand
dunes and towers against steady wind currents a number of inventors
through a series of years learned how to guide and control these gliders
in their downward flight--an essential preparation for the application
of motive power to lift the glider against the force of gravity and thus
make it a veritable flying machine or aeroplane.