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 difficulties of this kind. The sluggishness and unreliability of the compass in our submarines have been met by the adoption of the gyroscope. Special fuses have been designed for anti-airship use which will detonate even when they meet the flimsy covering of an airship. We have learnt how to select our airmen by tests which reveal their powers and their weaknesses to a degree quite unknown to themselves and how to supply them with the oxygen they need at their highest altitudes. When acting as scouts, aeroplanes have had at their command the accuracy that photography gives and their surveys—thanks to the work of Sir William Pope—could be continued when the light was too poor for observation by the naked eye. Powerful but slow-acting incendiary mixtures have been made to burst into full operation with the speed of an explosive. Sounds beyond our power of hearing have been made to convey signals. Alloys have been made to give strength with unheard of lightness. An airman sitting behind a propeller revolving at say 25 times a second has been 42