At
last, however, the explanation of these remarkable phenomena dawned
upon him, and his great discovery was made.
One day when Bradley was out sailing he happened to remark that every
time the boat was laid on a different tack the vane at the top of the
boat's mast shifted a little, as if there had been a slight change in
the direction of the wind. After he had noticed this three or four
times he made a remark to the sailors to the effect that it was very
strange the wind should always happen to change just at the moment
when the boat was going about. The sailors, however, said there had
been no change in the wind, but that the alteration in the vane was
due to the fact that the boat's course had been altered. In fact,
the position of the vane was determined both by the course of the
boat and the direction of the wind, and if either of these were
altered there would be a corresponding change in the direction of the
vane. This meant, of course, that the observer in the boat which was
moving along would feel the wind coming from a point different from
that in which the wind appeared to be blowing when the boat was at
rest, or when it was sailing in some different direction. Bradley's
sagacity saw in this observation the clue to the Difficulty which had
so long troubled him.
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