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.
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