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.