Alberto Santos-Dumont,
My Airships; The Story of My Life
“ The spherical balloon is always open. When it is taut with gas it is shaped like an [106] 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 ”
