142. If you remove about six or seven yards from the candle, and place
yourself so that it’s flame may be just about the height of your eye,
and then desire the other person to move the ball slowly round the
candle as before, keeping it as near of an equal height with the flame
as he possibly can, the ball will appear to you not to move in a circle,
but rather to vibrate backward and forward like a pendulum; moving
quickest when it is directly between you and the candle, and when
directly beyond it; and gradually slower as it goes farther to the right
or left side of the flame, until it appears at the greatest distance
from the flame; and then, though it continues to move with the same
velocity, it will seem to stand still for a moment. In every Revolution
it will shew all the above Phases § 141; and if two balls, a smaller and
a greater, be moved in this manner round the candle, the smaller ball
being kept nearest the flame, and carried round almost three times as
often as the greater, you will have a tolerably good representation of
the apparent Motions of Mercury and Venus; especially, if the bigger
ball describes a circle almost twice as large in diameter as the circle
described by the lesser.
[Sidenote: Fig. III.
The elongations or digressions of Mercury from the Sun.