vistas of time past, and to trace with
plausibility, if not with certainty, certain early phases in the history
of our system. The sun and the moon, the planets and the comets, the
stars and the nebulae, all alike are subject to this universal law, which
is now to engage our attention.

What is more familiar than the fact that when a stone is dropped it
will fall to the ground? No one at first thinks the matter even worthy
of remark. People are often surprised at seeing a piece of iron drawn to
a magnet. Yet the fall of a stone to the ground is the manifestation of
a force quite as interesting as the force of magnetism. It is the earth
which draws the stone, just as the magnet draws the iron. In each case
the force is one of attraction; but while the magnetic attraction is
confined to a few substances, and is of comparatively limited
importance, the attraction of gravitation is significant throughout the
universe.

Let us commence with a few very simple experiments upon the force of
gravitation. Hold in the hand a small piece of lead, and then allow it
to drop upon a cushion. The lead requires a certain time to move from
the fingers to the cushion, but that time is always the same when the
height is the same. Take now a larger piece of lead, and hold one piece
in each hand at the same height. If both are released at the same
moment, they will both reach the cushion simultaneously. It might have
been thought that the heavy body would fall more quickly than the light
body; but when the experiment is tried, it is seen that this is not the
case. Repeat the experiment with various other substances. An ordinary
marble will be found to fall in the same time as the piece of lead. With
a piece of cork we again try the experiment, and again obtain the same
result. At first it seems to fail when we compare a feather with the
piece of lead; but that is solely on account of the air, which resists
the feather more than it resists the lead. If, however, the feather be
placed upon the top of a penny, and the penny be horizontal when
dropped, it will clear the air out of the way of the feather in its
descent, and then the feather will fall as quickly as the penny, as
quickly as the marble, or as quickly as the lead.

If the observer were in a gallery when trying these experiments, and if
the cushion were sixteen feet below his hands, then the time the marble
would take to fall through the sixteen feet would be one second. The
time o