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, to great heights. If I have not done so it is because it would have served no useful purpose during a period of pleasure navigation, while it would but have added danger to experiments from which I have sought to eliminate all danger. Dangers like these are to be accepted only when a good cause justifies them.

The experiments above named are, of course, of a nature interesting warfare by land. I cannot abandon this topic, however, without referring to one unique maritime advantage of the air-ship. This is its navigator's ability to perceive bodies moving beneath the surface of the waterCruising at the end of its guide rope, the air-ship will carry its navigator here and there at will at the right height above the waves. Any submarine boat, stealthily pursuing its course underneath them, will be beautifully visible to him, while from a warship's deck it would be quite invisible. This is a well-observed fact, and depends on certain optical laws. Thus, very curiously, the twentieth century air-ship must become from the beginning the great enemy of that other twentieth century marvel—the submarine boat—and not only its enemy but its master. For, while the submarine boat can do no harm to the air-ship, the latter,