made to my present position.'

This flash of genius was immediately succeeded by another. 'If this
surmise be correct,' Roemer reasoned, 'then as I approach Jupiter
along the other side of the earth's orbit, the retardation ought to
become gradually less, and when I reach the place of my first
observation, there ought to be no retardation at all.' He found this
to be the case, and thus not only proved that light required time to
pass through space, but also determined its rate of propagation.

The velocity of light, as determined by Roemer, is 192,500 miles in a
second.

For a time, however, the observations and reasonings of Roemer failed
to produce conviction. They were doubted by Cassini, Fontenelle, and
Hooke. Subsequently came the unexpected corroboration of Roemer by the
English astronomer, Bradley, who noticed that the fixed stars did not
really appear to be fixed, but that they describe little orbits in the
heavens every year. The result perplexed him, but Bradley had a mind
open to suggestion, and capable of seeing, in the smallest fact, a
picture of the largest. He was one day upon the Thames in a boat, and
noticed that as long as his course remained unchanged, the vane upon
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