which oscillated from one side
of the sun to the other.

We can easily imagine how the discovery of Mercury was made in the clear
skies over an Eastern desert. The sun has set, the brief twilight has
almost ceased, when lo, near that part of the horizon where the glow of
the setting sun still illuminates the sky, a bright star is seen. The
primaeval astronomer knows that there is no bright star at this place in
the heavens. If the object of his attention be not a star, what, then,
can it be? Eager to examine this question, the heavens are watched next
night, and there again, higher above the horizon, and more brilliant
still, is the object seen the night before. Each successive night the
body gains more and more lustre, until at length it becomes a
conspicuous gem. Perhaps it will rise still higher and higher; perhaps
it will increase till it attains the brilliancy of Venus itself. Such
were the surmises not improbably made by those who first watched this
object; but they were not realised. After a few nights of exceptional
splendour the lustre of this mysterious orb declines. The planet again
draws near the horizon at sunset, until at length it sets so soon after
the sun that it has become invisible. Is it lost for ever? Years may
elapse before another opportunity of observing the object after sunset
may be available; but then again it will be seen to run through the same
series of changes, though, perhaps, under very different circumstances.
The greatest height above the horizon and the greatest brightness both
vary considerably.

Long and careful observations must have been made before the primaeval
astronomer could assure himself that the various appearances might all
be attributed to a single body. In the Eastern deserts the phenomena of
sunrise must have been nearly as familiar as those of sunset, and in the
clear skies, at the point where the sunbeams were commencing to dawn
above the horizon, a bright star-like point might sometimes be
perceived. Each successive day this object rose higher and higher above
the horizon before the moment of sunrise, and its lustre increased with
the distance; then again it would draw in towards the sun, and return
for many months to invisibility. Such were the data which were presented
to the mind of the primitive astronomer. One body was seen after sunset,
another body was seen before sunrise. To us it may seem an obvious
inference from the observed facts that the two bodies we