“Matter is inert.” That is what you read in every treatise on physics--what does it mean? Here is a very simple experiment that will prove this truth to anyone. Pile up ten draughtsmen, as shown in Fig. 1. Before this pile place another piece on edge, and pressing its circumference with the forefinger, let it glide from underneath so that it strikes the pile with considerable force. The piece so thrown must, you will think, upset the whole pile of draughts; but no: the piece thus sharply sent forward will strike only one piece of the pile, and this alone will be dislodged without putting the others out of their equilibrium, and the whole column above will settle down together on the bottom piece. [Illustration: Fig. 1.] In effect, the force of the impulse, making itself felt on the piece that is touched, the latter leaves the pile without transmitting its movement to the other pieces, which, following another physical law, that of gravity, descend vertically to fill the place left vacant. The experiment may be varied by using a knife and striking with it a sharp horizontal blow on one of the pieces. The piece struck will fall out of the pile without disturbing the symmetry of the others. The Decanter, Card, and Coin. This law of “Inertia” will provide us with a few more experiments as curious as they are conclusive. Place a playing or an ordinary visiting card on a decanter; upon the card and just in the center, over the aperture of the decanter, put a small coin (a dime). Now, if with a sharp fillip, given horizontally on the edge of the card, you succeed in whisking it off (which is very easy), the coin will fall to the bottom of the decanter. The following phenomenon has taken place: the movement was too rapid to be transmitted to the coin, and the card alone was whisked off. [Illustration] The coin being no longer sustained by the card falls, of course, vertically, without having in the least come out of position. A sharp horizontal knock given with a penholder or small stick on the edge of the card, will produce the same result, but the fillip is more effective. A Clever Blow. Take a thin stick about a yard long, and thrust a pin firmly in each of its extremities. This done, place the stick on the bowls of two pipes, which a couple of persons hold by the stems, in such a manner that the pins only rest on the pipes. A third person then strikes the stick sharply in the middle, and it will break without injuring the pipes. Ordinary clay pipes will do very well, as the more brittle the pipes are, the more striking is the experiment. How is this explained? [Illustration] The mechanical effect of the shock has not time to reach the bowls of the pipes (inertia), and is only manifested at the very point on which the blow falls, hence the stick unable to resist the force of the blow at the one point breaks in two pieces. The Obedient Coin.