When the air of a room is very calm, have you ever noticed that tobacco smoke rises slowly and in a nearly vertical direction? Have you never watched with interest the grayish or bluish streaks of smoke issuing from the smoke of a cigar or pipe? And in seeing the smoke rise in such a capricious fashion, have you ever ascertained that it is due to the calm of the surrounding air? One may get some amusement out of the agitation of the atmosphere. For the materials you only require a square or round cardboard box, in the lid of which cut a round hole of about two inches in diameter. In the interior of the box place two sheets of blotting paper, the one impregnated with muriatic acid, the other with ammoniac, in equal quantities. Immediately a whitish smoke will escape through the hole and rise straight to the ceiling. If with both hands you give a series of simultaneous taps on the sides of the box, you will see the smoke issue in well defined rings, which will disperse rapidly in the air, and succeed each other as long as you continue the pressure. These rings are the result of the concussion of the air which you occasion in the box. The experiments can be made also with ordinary tobacco smoke, but it will last longer in the way we have indicated. [Illustration] The Walking Cork. Stick two knives in a cork, on the same level, and opposite each other, so as to form a balance. In the bottom of the cork, at an equal distance, insert two pins, sufficiently deep not to bend under the weight which they will have to carry. Rest the pins on a flat ruler, slightly inclined, and give them a slight balancing movement. The weight of the apparatus will fall on the pin, A, on which the whole turns the knife placed at the side, B, will knock against the support, and will tend to bring the apparatus again into its original position as the oscillating movement continues, pin, B, will, in its turn, support the whole weight, and pin, A, will shift on to the other point, A, indicated in the figure. The walking cork will thus continue its movement till it has gone over the course assigned to it. This recreation is interesting, for it demonstrates once more that all bodies are attracted by the earth, and that, as soon as they are thrown off their balance they obey the force which constantly draws them down. [Illustration] The Obstinate Cork. Take a glass or metal tube, closed at one end, and cut a stopper in cork or india-rubber to its size so that it closes the tube hermetically, and take care that it glides in the tube without difficulty, and pierce it with a hole. On the top of this hole adapt a small piece of leather, rather larger than the hole, which you must take care to wet before proceeding with the following experiment. In order to be able to withdraw the cork when in the tube, you take the precaution to fasten in it both ends of a piece of string, as indicated in the figure. [Illustration]