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.

Assiduous as Ptolemy may have been in the study of the heavenly
bodies, it seems evident that he cannot have devoted much thought to
the phenomena of motion of terrestrial objects. Simple, indeed, are
the experiments which might have convinced a philosopher much less
acute than Ptolemy, that, if the earth did revolve, the air must
necessarily accompany it. If a rider galloping on horseback tosses a
ball into the air, it drops again into his hand, just as it would
have done had he been remaining at rest during the ball's flight; the
ball in fact participates in the horizontal motion, so that though it
really describes a curve as any passer-by would observe, yet it
appears to the rider himself merely to move up and down in a straight
line. This fact, and many others similar to it, demonstrate clearly
that if the earth were endowed with a movement of rotation, the
atmosphere surrounding it must participate in that movement. Ptolemy
did not know this, and consequently he came to the conclusion that
the earth did not rotate, and that, therefore, notwithstanding the
tremendous improbability of so mighty an object as the celestial
sphere spinning round once in every twenty-four hours, there was no
course open except to believe that this very improbable thing did
really happen. Thus it came to pass that Ptolemy adopted as the