Latterly the non-rigid
system has been embraced exclusively, the craft being virtually
mechanically driven balloons. They have proved efficient and
reliable so far as they go, but it is the personal element in
this instance also which has contributed so materially to any
successes achieved with them.
But although Great Britain and France apparently lagged behind
the Germans, appreciable enterprise was manifested in another
direction. The airship was not absolutely abandoned: vigilance
was maintained for a superior type of craft. It was an instance
of weighing the advantages against the disadvantages of the
existing types and then evolving for a design which should
possess the former without any of the latter. This end appears
to be achieved with the Astra type of dirigible, the story of the
development of which offers an interesting chapter in the annals
of aeronautics.
In all lighter-than-air machines the resistance to the air
offered by the suspension ropes is considerable, and the
reduction of this resistance has proved one of the most
perplexing problems in the evolution of the dirigible.
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