fferent from those of larger
spaces that we must necessarily be unable to conceive them.

Still, considering mere possibilities, it is not impossible that the
twentieth century may be destined to make known natural forces which
will enable us to fly from continent to continent with a speed far
exceeding that of the bird.

But when we inquire whether aerial flight is possible in the present
state of our knowledge, whether, with such materials as we possess, a
combination of steel, cloth, and wire can be made which, moved by the
power of electricity or steam, shall form a successful flying-machine,
the outlook may be altogether different. To judge it sanely, let us
bear in mind the difficulties which are encountered in any
flying-machine. The basic principle on which any such machine must be
constructed is that of the aeroplane. This, by itself, would be the
simplest of all flyers, and therefore the best if it could be put into
operation. The principle involved may be readily comprehended by the
accompanying figure. A M is the section of a flat plane surface, say a
thin sheet of metal or a cloth supported by wires. It moves through the
air, the latter being represented by the horizontal rows of dots. The
direction of the motion is that of the horizontal line A P. The
aeroplane has a slight inclination measured by the proportion between
the perpendicular M P and the length A P. We may raise the edge M up or
lower it at pleasure. Now the interesting point, and that on which the
hopes of inventors are based, is that if we give the plane any given
inclination, even one so small that the perpendicular M P is only two
or three per cent of the length A M, we can also calculate a certain
speed of motion through the air which, if given to the plane, will
enable it to bear any required weight. A plane ten feet square, for
example, would not need any great inclination, nor would it require a
speed higher than a few hundred feet a second to bear a man. What is of
yet more importance, the higher the speed the less the inclination
required, and, if we leave out of consideration the friction of the air
and the resistance arising from any object which the machine may carry,
the less the horse-power expended in driving the plane.

[Illustration]

Maxim exemplified this by experiment several years ago. He found that,
with a small inclination, he could readily give his aeroplane, when it
slid forward upon ways, such a speed that it woul