If a card, such as an ordinary visiting card, is turned down about a quarter of an inch at each end, at right angles to the rest of the card, and then placed on a table so as to rest on the turned edges, you may safely challenge most persons to blow it so as to make it turn over on the other side. It would naturally seem easier to overturn a card so prepared, than one whose shape remained unchanged; but whoever tries it will find that the facts are otherwise.
The card can be overthrown, however, by blowing on the table, toward the card, as the stream of air is then reflected against its under side.
This may be constructed by means of three table-knives, in the manner illustrated in the figure. Three goblets or tumblers will serve as the piers; these are to be arranged in a triangle, a little farther from each other than the length of the knives. Lay two of the knives on the table, with the blades crossing each other. Then pass the blade of the third knife over the uppermost blade of the other two, and under the undermost; then take them up and place them with the ends of the handles on the rims of the glasses. The bridge now sustains itself, and if a moderate weight be placed upon it, it will be all the firmer.
Inertia is defined as the tendency of a body to persevere in its state either of rest or motion. It is generally used in the sense of persisting in a state of rest. Among the many illustrations of this property of matter, is one which figured in the text-books of thirty or forty years ago, and which the boys of that time adapted to their amusement by constructing the apparatus here illustrated.
It consists of three parts: the board which forms the base, a post about six inches high, and a strip of stout whalebone, or dry, elastic wood.
The board should be as much as seven-eighths of an inch in thickness, and the elastic strip or spring should be firmly inserted in an inclined slit cut through the board. The places of the spring and post should be so adjusted to each other, that when the latter is secured solidly by a good-sized screw passing up through the board, the former will press with its upper end against the top of the post (as shown by the dotted line) with some degree of force.