p
pilots. Subsequently, the two ballonets fitted inside the envelope were
made to serve for trimming by the extent of their inflation, and this
method of securing inclination proved the best until exterior rudders,
and greater engine power, supplanted it, as in the Zeppelin and, in
fact, all rigid types.

In the kite balloon, the ballonet serves the purpose of a rudder,
filling itself through the opening being kept pointed toward the
wind--there is an ingenious type of air scoop with non-return valve
which assures perfect inflation. In the S.S. type of airship, two
ballonets are provided, the supply of air being taken from the propeller
draught by a slanting aluminium tube to the underside of the envelope,
where it meets a longitudinal fabric hose which connects the two
ballonet air inlets. In this hose the non-return air valves, known
as 'crab-pots,' are fitted, on either side of the junction with the
air-scoop. Two automatic air valves, one for each ballonet, are fitted
in the underside of the envelope, and, as the air pressure tends to
open these instead of keeping them shut, the spring of the valve is set
inside the envelope. Each spring is set to open at a pressure of 25 to
28 mm.

II. THE FIRST DIRIGIBLES

Having got off the earth, the very early balloonists set about the task
of finding a means of navigating the air but, lacking steam or other
accessory power to human muscle, they failed to solve the problem.
Joseph Montgolfier speedily exploded the idea of propelling a balloon
either by means of oars or sails, pointing out that even in a dead
calm a speed of five miles an hour would be the limit achieved. Still,
sailing balloons were constructed, even up to the time of Andree, the
explorer, who proposed to retard the speed of the balloon by ropes
dragging on the ground, and then to spread a sail which should catch
the wind and permit of deviation of the course. It has been proved that
slight divergences from the course of the wind can be obtained by this
means, but no real navigation of the air could be thus accomplished.

Professor Wellner, of Brunn, brought up the idea of a sailing balloon
in more practical fashion in 1883. He observed that surfaces inclined to
the horizontal have a slight lateral motion in rising and falling, and
deduced that by alternate lowering and raising of such surfaces he would
be able to navigate the air, regulating ascent and descent by increasing
or decreasing the tempera