ring 30 feet in length by 6 feet in width,
carried out in wood, is a striking example of the maximum of strength
with the minimum of weight, being as steady and as solid as a boat's
deck. The propellers are collapsible, although in the latest craft of
this class they are semi-rigid.

The mechanical equipment is also interesting. There are two propellers,
and two motors, each nominally driving one propeller. But should one
motor break down, or motives of economy, such as husbanding of fuel,
render it advisable to run upon one engine, then the two propellers may
be driven by either of the motors.

The inventor has perfected an ingenious, simple, and highly efficient
coupling device to attain this end, but to ensure that the propeller
output is of the maximum efficiency in relation to the engine, the pitch
of the propellers may be altered and even reversed while the engine is
running. When one motor only is being used, the pitch is lowered until
the propellers revolve at the speed which they would attain if both
engines were in operation. This adjustment of the propeller pitch to the
most economical engine revolutions is a distinctive characteristic, and
contributes to the efficiency and reliability of the Parseval dirigible
to a very pronounced degree.

Steering in the vertical plane is also carried out upon distinctive
lines. There are no planes for vertical steering, but movement is
accomplished by tilting the craft and thus driving the gas from one end
of the balloon to the other. This is effected by the manipulation of the
air-ballonets, one of which is placed at the prow and stem of the gas
bag respectively. If it is desired to descend the gas is driven from the
forward to the after end of the envelope, merely by inflating the bow
ballonet with air by means of a pump placed in the car. If ascent is
required, the after-ballonet is inflated, thereby driving the gas to the
forward end of the balloon, the buoyancy of which is thus increased.
The outstanding feature of the "Drachen-Balloon" is incorporated in
the airship. This is the automatic operation of the safety valve on the
gas-bag directly by the air ballonets. If these ballonets empty owing
to the pressure of the gas within the envelope, a rope system disposed
within the balloon and connecting the ballonets and the gas-valve at the
top is stretched taut, thereby opening the gas-valve. In this manner the
gas-pressure becomes reduced until the ballonets are enable