magnetic virtue for sustaining heavy
bodies with which it is in contact. To satisfy all these doubts I have
contrived the following experiment to demonstrate how truly the air does
support these bodies; for I have found, when one of these bodies which
floats when placed lightly on the water is thoroughly bathed and sunk to
the bottom, that by carrying down to it a little air without otherwise
touching it in the least, I am able to raise and carry it back to the
top, where it floats as before. To this effect, I take a ball of wax,
and with a little lead make it just heavy enough to sink very slowly to
the bottom, taking care that its surface be quite smooth and even. This,
if put gently into the water, submerges almost entirely, there remaining
visible only a little of the very top, which, so long as it is joined to
the air, keeps the ball afloat; but if we take away the contact of the
air by wetting this top, the ball sinks to the bottom and remains there.
Now to make it return to the surface by virtue of the air which before
sustained it, thrust into the water a glass with the mouth downward,
which will carry with it the air it contains, and move this down towards
the ball until you see, by the transparency of the glass, that the air
has reached the top of it; then gently draw the glass upward, and you
will see the ball rise, and afterwards stay on the top of the water,
if you carefully part the glass and water without too much disturbing
it."(3)

It will be seen that Galileo, while holding in the main to a correct
thesis, yet mingles with it some false ideas. At the very outset, of
course, it is not true that water has no resistance to penetration; it
is true, however, in the sense in which Galileo uses the term--that
is to say, the resistance of the water to penetration is not the
determining factor ordinarily in deciding whether a body sinks
or floats. Yet in the case of the flat body it is not altogether
inappropriate to say that the water resists penetration and thus
supports the body. The modern physicist explains the phenomenon as due
to surface-tension of the fluid. Of course, Galileo's disquisition
on the mixing of air with the floating body is utterly fanciful. His
experiments were beautifully exact; his theorizing from them was, in
this instance, altogether fallacious. Thus, as already intimated, his
paper is admirably adapted to convey a double lesson to the student of
science.

WILLIAM GILBERT AND THE