In military airships, and, it may be added, aëroplanes also, the colour should be a neutral tint that is as invisible as possible against the sky. Most of the airships have been made a glaring yellow, so that the india-rubber in the envelopes may be better preserved from the action of light. This protection may have to be sacrificed to the overpowering advantages of invisibility in the case of naval and military airships. CHAPTER VI THE ADVENT OF THE AËROPLANE The year of 1908 will be memorable in aëronautical science for its demonstration of the possibility of mechanical flight. Day after day in France and America was then seen the spectacle of men flying in the air, with a grace equal to that of the soaring bird. This was done with a machine not raised by the buoyancy of a gas, but with one that was heavier than the medium in which it travels, and whose sustentation and direction was accomplished by dexterity and skill. The experiments of the brothers Wright were new triumphs of man, new examples of the old truths that a difficulty is a thing to be overcome, and that the impossibility of to-day may be the achievement of to-morrow. This progress in human flight was not the result of any new discovery; it was the sequence of a long series of experiments; nor was it one nation only that forged the links that connected past researches to the successful issues of the present century. It is, however, not without honour to the British nation that one of the fundamental principles of the biplane was proposed and elucidated by a Briton in 1866. I refer to the important principle of superposed surfaces advanced in that year by the late F. H. Wenham. He pointed out that the lifting power of such a surface can be most economically obtained by placing a number of small surfaces above each other. Wenham built flying machines on this principle with appliances for the use of his own muscular power. He obtained valuable results as to the driving power of his superposed surfaces, but he did not accomplish flight. In 1872, H. von Helmholtz emphasised the improbability that man would ever be able to drive a flying machine by his own muscular exertion. After his statements there came a period of stagnation in the attempts to navigate the air by bodies heavier than air.