=247. Scissors and Shears.= In popular language, there is no distinction made between scissors and shears. Technically defined, scissors are less than six inches in length. Any similar cutting device of greater length is called shears. Both are devices used for cutting cloth, paper, pruning trees, and many other purposes. They consist of two knives riveted together at some point between the handle and the point of the blade. The two blades are so adjusted that as the open scissors are closed, they touch lightly as they pass each other until the tip is reached. When the scissors are closed, the blades should touch only at rivet and tip. Scissors not so adjusted will not cut well, even the the blades are very sharp. Dropping scissors often bends the blades. Blades may be straightened as well as sharpened, and thus make good metal scissors like new. =248. Principles Upon Which Incubator Works.= A device for hatching chickens is called an incubator. In order to hatch chickens, the incubator must keep an average temperature of 102-1/2 degrees Fahrenheit. The thermometer should be placed in the center of the tray and on a level with the top of the eggs. The temperature of 102-1/2 degrees Fahrenheit must not vary greatly during the incubation of eggs. The incubator must also permit of suitable ventilation and control of the moisture in the eggs. There are incubators heated with hot water and others with hot air. The air or water in those commonly used in homes is heated with a kerosene lamp. The device consists of a heating unit, a regulator or thermostat which, acting upon a valve or damper, regulates the admission of heat into the insulated box containing the trays of eggs, ventilators and a thermometer (Fig. 140). =249. The Body of the Incubator.= The box-like body of a good incubator is set on strong legs which raise it to a convenient height. The trays slide into the box on cleats about two or three inches from the bottom of the body. They fit so that a slit about two inches wide is left between for the chickens to drop down under the tray as they hatch. Usually this is near the door. If the door is furnished with a glass to admit light, the chickens are attracted toward light and fall thru the slit. [Illustration: FIG. 140. Incubator.] The walls of the incubator are usually double so that air can be let in without making a draft. Dampers in the side of the machine regulate the admission of air. Ventilation both regulates the amount of air circulating in the incubator and the amount of moisture. Air from a damp room keeps the eggs moist. Air from a dry room dries them. =250. Incubators Heated by a Lamp.= Choose a lamp which holds enough oil to last for twenty-four hours. Good lamps are usually made of metal and as plain as possible (Fig. 141). [Illustration: FIG. 141. Incubator lamp.]