will not be necessary to lose gas during flight, provided that an
ascent is not made over 1,000 feet.

Valves are provided to prevent the pressure in the envelope from
exceeding a certain determined maximum and are fitted both to
ballonets and the gaschamber. They are automatic in action, and,
as we 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