A _screw_ is another form of the inclined plane. Where is the inclined plane on a screw? It begins at the bottom of the screw, and winds around and around the screw until it reaches the top, as is shown in Fig. 60. [Illustration: Fig. 60.] What is this inclined plane called? It is called the thread of the screw. How do the threads affect the screw? When the threads are far apart, the inclined plane is steep; but it becomes less steep the nearer the threads are together. When will the screw exert the most power? The screw will exert the most power, when its threads are nearest together. Of what use are screws, such as is shown in Fig. 60? Screws are used to fasten hinges and bolts on doors and shutters, and to fasten pieces of wood together in the construction of houses, boats, wagons, and other things. For what other purposes is the screw used? The screw is used where much power is needed, as in the vise; where pressure is desired, as in copying letters, and in coining money; and, also, for squeezing the juice from apples, grapes, and sugar-cane. [Illustration: Fig. 61.] In what does a screw like that in Fig. 61 work? It works in a nut which has threads cut on the inside, so that the threads of the screw move in them. Where is the power applied? The power is applied to the lever. Can heavy bodies be raised by means of this screw? Yes; loaded cars, and even heavy buildings, may be raised by means of a screw of this kind. [Illustration: Sewing machine.] [Illustration: Decoration.] CHAPTER XI. Electricity. [Illustration: Illustrated Capital W] When a cat’s back is rubbed in the dark, what does it give off? It gives off small sparks. What are these small sparks called? They are called electric sparks, or _electricity_. If a horse be rubbed in the winter when it is dark, what will be given off from him? Electricity will be given off from the horse. When sealing-wax is rubbed with a silk handkerchief, or a piece of flannel, what is produced? Electricity is produced in the sealing-wax. What will this electricity in the sealing-wax do? It will attract bits of paper, straw, feathers, and hairs, when it is held near them. What other substance will act the same as the sealing-wax? A dry glass tube will act in the same manner when rubbed. Where is electricity found? Electricity is found in every body, and in every thing. How is electricity developed? Electricity is developed by motion, by friction, and by almost every change that takes place. Is electricity found in the air? Yes; the air is almost constantly electrified. With what is electricity often accompanied? Electricity is often accompanied by heat and light. How many kinds of electricity are there? There are two kinds of electricity. What are they called? They are called _positive_ and _negative_ electricity.