Cut from a card two discs or circular pieces, about two inches in
diameter; in the centre of one of them make a hole, into which put the
tube of a common quill, one end being even with the surface of the
card. Make the other piece of card a little convex, and lay its centre
over the end of the quill, with the concave side of the card downward;
the centre of the upper card being from one-eighth to one-fourth of an
inch above the end of the quill. Attempt to blow off the upper card by
blowing through the quill, and _it will be found impossible_.
If, however, the edges of the two pieces of card be made to fit each
other very accurately, the upper card will be moved, and sometimes it
will be thrown off; but when the edges of the card are on two sides
sufficiently far apart to permit the air to escape, the loose card will
retain its position, even when the current of air sent against it be
strong. The experiment will succeed equally well, whether the current
of air be made from the mouth or from a pair of bellows. When the quill
fits the card rather loosely, a comparatively light puff of air will
throw both cards three or four feet in height. When, from the humidity
of the breath, the upper surface of the perforated card has a little
expanded, and the two opposite sides are somewhat depressed, these
depressed sides may be distinctly seen to rise and approach the upper
card, directly in proportion to the force of the current of air.
Another fact to be shown with this simple apparatus, appears equally
inexplicable with the former. Lay the loose card upon the hand with the
concave side up; blow forcibly through the tube, and, at the same time,
bring the two cards towards each other, when, within three-eighths of
an inch, if the current of air be strong, the loose card will suddenly
rise and adhere to the perforated card. If the card through which
the tube passes has several holes made in it, the loose card may be
instantly thrown off by a slight puff of air.
For the explanation of the above phenomenon, a gold medal and one
hundred guineas were offered, some years since, by the Royal Society.
Such explanation has been given by Dr. Robert Hare, of Philadelphia,
and is as follows:
Supposing the diameter of the discs of card to be to that of the hole
as 8 to 1, the area of the former to the latter, must be as 64 to 1.
Hence, if the discs were to be separated, (their surfaces remaining
parallel,) with a velocity as great as that of the air blast, a column
of air must meanwhile be interposed, sixty-four times greater than that
which would escape from the tube during the interim; consequently, if
all the air necessary to preserve the balance be supplied from the
tube, the discs must be separated with a velocity as much less than
that of the blast, as the column required between them is greater than
that yielded by the tube, and yet the air cannot be supplied from any
other source, unless a deficit of pressure be created between the
discs, unfavourable to their separation.
It follows, then, that, under the circumstances in question, the
discs cannot be made to move asunder with a velocity greater than
one-sixty-fourth of that of the blast. Of course, all the force of the
current of air through the tube will be expended on the moveable disc,
and the thin ring of air which exists around the orifice between the
discs: and, since the moveable disc can only move with one-sixty-fourth
of the velocity of the blast, the ring of air in the interstice
must experience nearly all the force of the jet, and must be driven
outwards, the blast following it in various currents, radiating from
the common centre of the tube and discs.