You fasten one extremity of the string to your foot, and take the other in your hand so as to produce at will the rigidity of the string. You can then command the apple to go down, or to stop, and it will obey your order immediately. Indeed, when you straighten the string, the part which enters the apple pushes against the angle formed by the two passages, and by the pressure, holds the apple. When on the contrary you let go a little, you take away the rigidity and the apple glides down. [Illustration] You can therefore alternately let the apple go down or stop its course, and we repeat it, persons not in the secret cannot imagine by what means you get this curious result. If, instead of an apple one takes a wooden ball, the experiment will be more interesting and the article will last longer. Economical Letter-Scales. Take a watch or small clock spring, and fix it by the center on a stick. At the other end attach a small brass hook to hold letters, etc., as shown in the figure. At the top of the hook fix horizontally a small band, running over a strip of cardboard, likewise hanging on the stick. Now graduate the cardboard strip with real weights, or their exact equivalents, and after this any small articles may be weighed with sufficient accuracy. The spring, being of steel, always turns to its original position when the scale is empty. [Illustration] Tracing a Spiral. In geometry the process for tracing a spiral by the help of compasses is pretty long and tedious. The following method will enable you to do it far more quickly and as accurately. [Illustration] Take a wooden or cardboard cylinder, with a diameter equal to a fourth part of the distance you require between the spires (or trelices) to be traced. On this cylinder fasten one end of a string, B, and wind it up, and attach to the other end a pencil, C, or a point, according to what you want to do. Now you have only to turn to right or left according to the direction in which the string was wound up, by holding the pencil down and keeping the string tight, and a spiral of perfect regularity will be traced. The above figure clearly shows the process. The cylinder A has a diameter equal to the distance R S divided by 4. The Inclined Plane. Take a piece of paper, roll it up into a tube large enough to hold a marble, and gum it lengthwise. Then introduce a marble and close the extremities with a strip of paper as shown below. [Illustration] When you think that it is well dried you place it upright on the upper end of an inclined board, or flat ruler, leaning on a pile of books for example. You will then see the paper cylinder lie down, get up and so on till it reaches the bottom of its course. The effect is very curious and will be more so if you are somewhat of an artist, and able to draw or paint a figure on the cylinder. To Cut a Bottle With a String.