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 if he be alone, be forced to rectify his altitude continually by means of his propeller and shifting weights. He ought to be free to navigate his air-ship; if on pleasure bent, with ease and leisure to enjoy his flight; if on war service, with facility for his observations and hostile manœuvres. Therefore any automatic guarantee of vertical stability is peculiarly welcome to him. You know already what the guide rope is. I have described it in my first experience of spherical ballooning. Overland, where there are level plains or roads or even streets, where there are not too many troublesome trees, buildings, fences, telegraph and trolley poles and wires and like irregularities, the guide rope is as great an aid to the air-ship as to the spherical balloon. Indeed, I have made it more so, for with me it is the central feature of my shifting weights (Figs. 8 and 9, page 148). Over the uninterrupted stretches of the sea my first Monaco flight proved it to be a true stabilisateur. Its very slight dragging resistance through the water is out of all proportion to the considerable weight of its floating extremity. According to its greater or less immersion, therefore, it ballasts or unballasts the air-ship (Fig. 11). The balloon is