Ptolemy had, therefore, to resort for guidance to indirect lines of reasoning.  One of these suppositions must be true, and yet it appeared that the adoption of either was accompanied by a great difficulty.  It is one of his chief merits to have demonstrated that the celestial sphere was so stupendous that the earth itself was absolutely insignificant in comparison therewith.  If, then, this stupendous sphere rotated once in twenty-four hours, the speed with which the movement of some of the stars must be executed would be so portentous as to seem well-nigh impossible.  It would, therefore, seem much simpler on this ground to adopt the other alternative, and to suppose the diurnal movements were due to the rotation of the earth.  Here Ptolemy saw, or at all events fancied he saw, objections of the weightiest description.  The evidence of the senses appeared directly to controvert the supposition that this earth is anything but stationary.  Ptolemy might, perhaps, have dismissed this objection on the ground that the testimony of the senses on such a matter should be entirely subordinated to the interpretation which our intelligence would place upon the facts to which the senses deposed.  Another objection, however, appeared to him to possess the gravest moment.  It was argued that if the earth were rotating, there is nothing to make the air participate in this motion, mankind would therefore be swept from the earth by the furious blasts which would arise from the movement of the earth through an atmosphere at rest.  Even if we could imagine that the air were carried round with the earth, the same would not apply, so thought Ptolemy, to any object suspended in the air.  So long as a bird was perched on a tree, he might very well be carried onward by the moving earth, but the moment he took wing, the ground would slip from under him at a frightful pace, so that when he dropped down again he would find himself at a distance perhaps ten times as great as that which a carrier-pigeon or a swallow could have traversed in the same time.  Some vague delusion of this description seems even still to crop up occasionally.  I remember hearing of a proposition for balloon travelling of a very remarkable kind.  The voyager who wanted to reach any other place in the same latitude was simply to ascend in a balloon, and wait there till the rotation of the earth conveyed the locality which happened to be his destination directly beneath him, whereupon he was to let out the gas and drop down!  Ptolemy knew quite enough natural philosophy to be aware that such a proposal for locomotion would be an utter absurdity; he knew that there was no such relative shift between the air and the earth as this motion would imply.  It appeared to him to be necessary that the air should lag behind, if the earth had been animated by a movement of rotation.  In this he was, as we know, entirely wrong.  There were, however, in his days no accurate notions on the subject of the laws of motion.