MY AIRSHIPS
apple; when it has lost part of its gas it takes the shape of a pear; but in each case there is a great hole in the bottom of the spherical balloon where the stem of the apple or the pear would be, and it is through this hole that the gas has opportunity to ease itself in the constant alternations of condensation and dilatation. Having such a free vent, the spherical balloon runs no risk of bursting in the air; but the price paid for this immunity is great loss of gas and, consequently, a fatal shortening of the spherical balloon's stay in the air. Some day a spherical balloonist will close up that hole; indeed, they already talk of doing it.
I was obliged to do it in my air-ship balloon, whose cylindrical form must be preserved at all cost. For me there must be no transformations as from apple to pear. Interior pressure only could guarantee me this. The valves to which I refer have since my first experiments been of all kinds—some very ingeniously interacting, others of extreme simplicity. But their object in each case has always been the same: to hold the gas tight in the balloon up to a certain pressure and then let only enough out to relieve dangerous interior pressure. It is easy to realise, therefore, that
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