"Shut yourself with some friend in the main cabin below decks on
some large ship, and have with you there some flies, butterflies, and
other small flying animals. Have a large bowl of water with some fish in
it; hang up a bottle that empties drop by drop into a wide vessel
beneath it. With the ship standing still, observe carefully how the
little animals fly with equal speed to all sides of the cabin. The fish
swim indifferently in all directions; the drops fall into the vessel
beneath; and, in throwing something to your friend, you need throw it no
more strongly in one direction than another, the distances being equal;
jumping with your feet together, you pass equal spaces in every
direction. When you have observed all these things carefully (though
doubtless when the ship is standing still everything must happen in this
way), have the ship proceed with any speed you like, so long as the
motion is uniform and not fluctuating this way and that. You will
discover not the least change in all the effects named, nor could you
tell from any of them whether the ship was moving or standing still. In
jumping, you will pass on the floor the same spaces as before, nor will
you make larger jumps toward the stern than toward the prow
even though the ship is moving quite rapidly, despite the fact that
during the time that you are in the air the floor under you will be
going in a direction opposite to your jump. In throwing something to
your companion, you will need no more force to get it to him whether he
is in the direction of the bow
or the stern, with yourself situated opposite. The droplets will fall
as before into the vessel beneath without dropping toward the stern,
although while the drops are in the air the ship runs many spans. The
fish in their water will swim toward the front of their bowl with no
more effort than toward the back, and will go with equal ease to bait
placed anywhere around the edges of the bowl. Finally the butterflies
and flies will continue their flights indifferently toward every side,
nor will it ever happen that they are concentrated toward the stern, as
if tired out from keeping up with the course of the ship, from which
they will have been separated during long intervals by keeping
themselves in the air. And if smoke is made by burning some incense, it
will be seen going up in the form of a little cloud, remaining still and
moving no more toward one side than the other. The cause of all these
correspondences of effects is the fact that the ship's motion is common
to all the things contained in it, and to the air also. That is why I
said you should be below decks; for if this took place above in the open
air, which would not follow the course of the ship, more or less
noticeable differences would be seen in some of the effects noted."