ad been to the south, and by December it had
got back to its original position. It had described, in fact, a small
oscillation in the course of the year. The motion affected neighbouring
stars in a similar way, and was called an "aberration," or wandering
from their true place.

For a long time Bradley pondered over this observation, and over others
like them which he also made. He found one group of stars describing
small circles, while others at a distance from them were oscillating in
straight lines, and all the others were describing ellipses. Unless this
state of things were cleared up, accurate astronomy was impossible. The
fixed stars!--they were not fixed a bit. To refined and accurate
observation, such as was now possible, they were all careering about in
little orbits having a reference to the earth's year, besides any proper
motion which they might really have of their own, though no such motion
was at present known. Not till Herschel was that discovered; not till
this extraordinary aberration was allowed for could it be discovered.
The effect observed by Bradley and Molyneux must manifestly be only an
apparent motion: it was absurd to suppose a real stellar motion
regulating itself according to the position of the earth. Parallax could
not do it, for that would displace stars relatively among each other--it
would not move similarly a set of neighbouring stars.

At length, four years after the observation, the explanation struck him,
while in a boat upon the Thames. He noticed the apparent direction of
the wind changed whenever the boat started. The wind veered when the
boat's motion changed. Of course the cause of this was obvious
enough--the speed of the wind and the speed of the boat were compounded,
and gave an apparent direction of the wind other than the true
direction. But this immediately suggested a cause for what he had
observed in the heavens. He had been observing an apparent direction of
the stars other than the true direction, because he was observing from a
moving vehicle. The real direction was doubtless fixed: the apparent
direction veered about with the motion of the earth. It must be that
light did not travel instantaneously, but gradually, as Roemer had
surmised fifty years ago; and that the motion of the light was
compounded with the motion of the earth.

Think of a stream of light or anything else falling on a moving
carriage. The carriage will run athwart the stream, the occupants of