It was so cold up here, even with the windows closed, that all the boys were shivering in their warmest wraps.  The air, too, was so rarefied that it was with considerable difficulty that they could breathe, for they had been in it for some time.  Not one flyer in a hundred can live at an altitude of twenty thousand feet, as he bleeds at the nose and mouth; and our aviators were up to within five thousand feet of that height.  It was now time to descend.