throw
it, you will see the clam shell curve downward.

That is the principle on which airships mount upward and descend while
in motion. In a biplane there is either a forward or rear deflecting
rudder, as well as one for steering from side to side. The latter works
an the same principle as does the rudder of a boat in the water. If
this rudder is bent to the right, the craft goes to the right, because
of the pressure of air or water on the rudder twisted in that
direction. And if the rudder is deflected to the left, the head of the
craft takes that direction.

Just as the curve of a clam shell helps it to mount upward, so the
curve of the elevating or depressing rudder on an airship helps it to
go up or down. If the rudder is inclined upward the aeroplane shoots
toward the clouds. When the rudder is parallel to the plane of the
earth's surface, the airship flies in a straight line. When the rudder
is tilted downward, down goes the craft.

I hope I have not wearied you with this description, but it was,
perhaps, needful, to enable those who have never seen an aeroplane to
understand the working principle. One point more. A gasolene motor,
very powerful, is used to whirl the wooden propeller blades that shove
the airship through the air, as the propeller of a motor-boat shoves
that craft through the water.

Faster and faster across the grassy ground went the biplane containing
Dick Hamilton and the army officers. It was necessary to get this
"running start" to acquire enough momentum so that the craft would
rise, just as a heavy bird has sometimes to run along the ground a few
steps before its wings will take it up.

"Here we go!" suddenly exclaimed the captain, and as he raised the
elevating rudder the big craft slowly mounted on a slant.

Dick caught his breath sharply as he felt himself leaving the earth. He
had once gone up in a captive balloon at a fair, but then the earth
seemed sinking away beneath him. This time it seemed that he was
leaving the earth behind.

Higher and higher they went, and Dick could feel the strong wind in his
face. His eyes were protected by goggles, made of celluloid to avoid
accidents from broken glass in case of a fall, and on his head he wore
a heavy leather helmet, not unlike those used by football players. He
was strapped to his seat, as were the others, in case the machine
should turn turtle. The straps would then prevent them from falling
out, and give them a cha