have
said, the gas valve is set to blow off at a pressure in excess of that
for the air valve.

In rigid airships ballonets are not provided for the gasbags, and as a
consequence a long flight results in a considerable expenditure of gas.
If great heights are required to be reached, it is obvious that the
wastage of gas would be enormous, and it is understood that the Germans
on starting for a raid on England, where the highest altitudes were
necessary, commenced the flight with the gasbags only about 60 per cent
full.

To stabilize the ship in flight, fins or planes are fitted to the after
end of the envelope or hull. Without the horizontal planes the ship
will continually pitch up and down, and without the vertical planes it
will be found impossible to keep the ship on a straight course. The
planes are composed of a framework covered with fabric and are attached
to the envelope by means of stay wires fixed to suitable points, in the
case of non-rigid ships skids being employed to prevent the edge of the
plane forcing its way through the surface of the fabric. The rudder
and elevator flaps in modern practice are hinged to the after edges of
the planes.

The airship car contains all instruments and controls required for
navigating the ship and also provides a housing for the engines. In
the early days swivelling propellers were considered a great adjunct,
as with their upward and downward thrust they proved of great value in
landing. Nowadays, owing to greater experience, landing does not
possess the same difficulty as in the past, and swivelling propellers
have been abandoned except in rigid airships, and even in the later
types of these they have been dispensed with.

Owing to the great range of an airship a thoroughly reliable engine is
a paramount necessity. The main requirements are--firstly, that it
must be capable of running for long periods without a breakdown;
secondly, that it must be so arranged that minor repairs can be
effected in the air; and thirdly, that economy of oil and fuel is of
far greater importance to an airship than the initial weight of the
engine itself.

HANDLING AND FLYING OF AIRSHIPS

The arrangements made for handling airships on the ground and while
landing, and also for moving them in the open, provide scope for great
ingenuity. An airship when about to land is brought over the aerodrome
and is "ballasted up" so that she becomes considerably lighter than the
air which s