Page:My Airships.djvu/261

 Rather than spend time over illusory calculations on paper I have always preferred to go on materially improving my air-ships. Later, when they come in competition with the rivals which no one awaits more ardently than myself, all speed calculations made on paper and all disputes based on them must of necessity yield to the one sublime test of air-ship racing. Where speed calculations have their real importance is in affording necessary data for the construction of new and more powerful air-ships. Thus the balloon of my racing "No. 7," whose motive power depends on two propellers each 5 metres (16$\tfrac{1}{2}$ feet) in diameter, and worked by a 60 horse-power motor with a water cooler, has its envelope made of two layers of the strongest French silk, four times varnished, capable of standing, under dynamometric test, a traction of 3000 kilogrammes (6600 pounds) for the linear metre (3*3 feet). I will now try to explain why the balloon envelope must be made so very much stronger as the speed of the air-ship is designed to be increased; and in so doing I shall have to reveal the unique and paradoxical danger that besets high-speed dirigibles, threatening them, not with beating their heads in against the outer