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Classification of the Sci
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is that the balloon does not cease to fall until it is again far too light. Of rigid, light balloons, only one has ever been made and that one, instead of being made of magnalium as it should have been, was made of aluminium. It seems to have been a perfect success; but a piece of Prussian stupidity destroyed it after a life of ten minutes; so that we have little experience on the subject. It cost about fifty thousand dollars. Had it been made of magnalium it would have been vastly more expensive. Such a balloon will have a certain barometric height at which it will be in equilibrium; and this can be accurately known beforehand. It should carry up so much extra ballast as to be without upward velocity when it reaches that point, and the aeronaut with his eye on the barometer should instantly discharge the whole of the extra ballast when that point is reached. The balloon will then float at that height and if it be stanch the balloonist can remain there as long as he likes, till he reaches the north pole or wherever else he wishes to go. The effect of leakage would simply be that the level of equilibrium