his masthead showed the wind to be blowing constantly in the same
direction, but that the wind appeared to vary with every change in the
direction of his boat. 'Here,' as Whewell says, 'was the image of his
case. The boat was the earth, moving in its orbit, and the wind was
the light of a star.'

We may ask, in passing, what, without the faculty which formed the
'image,' would Bradley's wind and vane have been to him? A wind and
vane, and nothing more. You will immediately understand the meaning of
Bradley's discovery. Imagine yourself in a motionless railway-train,
with a shower of rain descending vertically downwards. The moment the
train begins to move, the rain-drops begin to slant, and the quicker
the motion of the train the greater is the obliquity. In a precisely
similar manner the rays from a star, vertically overhead, are caused
to slant by the motion of the earth through space. Knowing the speed
of the train, and the obliquity of the falling rain, the velocity of
the drops may be calculated; and knowing the speed of the earth in her
orbit, and the obliquity of the rays due to this cause, we can
calculate just as easily the velocity of light. Bradley did this, and
the 'aberration of light,' as his discovery is called, enabled him to
assign to it a velocity almost identical with that deduced by Roemer
from a totally different method of observation. Subsequently Fizeau,
and quite recently Cornu, employing not planetary or stellar
distances, but simply the breadth of the city of Paris, determined the
velocity of light: while Foucault--a man of the rarest mechanical
genius--solved the problem without quitting his private room. Owing
to an error in the determination of the earth's distance from the sun,
the velocity assigned to light by both Roemer and Bradley is too
great. With a close approximation to accuracy it may be regarded as
186,000 miles a second.