Ever since the earliest days of the great conquest of the air, first by the dirigible balloon and then by the aeroplane, their use in time of war has been a fruitful theme for discussion. But their arrival was of too recent a date, their many utilities too unexplored to provide anything other than theories, many obviously untenable, others avowedly problematical.

Yet the part airships have played in the Greatest War has come as a surprise even to their most convinced advocates. For every expectation shattered, they have shown a more than compensating possibility of usefulness.

In this volume an endeavour has been made to record their achievements, under the stern test of trial, as an axiom of war, and to explain, in untechnical language, the many services to which they have been and may be applied.

In the preparation of the work I have received assistance from many sources - British, French, Russian and German - from official reports and from men who have played a part in the War in the Air. The information concerning German military aircraft has been obtained from Government documents, most of which were placed at my disposal before the outbreak of war.

The use of aircraft has changed the whole art and science of warfare. With its disabilities well in hand, with its strength but half revealed, the aerial service has revolutionised strategy and shorn the unexpected attack of half its terrors. The Fourth Arm is now an invaluable part of the complex military machine.