The burner furnished with them is an ordinary lamp burner carrying a straight, flat wick. Metal chimneys are used, there being enough mica in one side to permit the flame to be seen. The chimney extends into a metal chamber containing the hot-water pipes, or into a chamber thru which air is taken and heated by the chimney. The fumes from the burning oil pass out into the room and not into the incubator. The heated air passes thru ducts into the incubator. These are often constructed of wood. =251. The Wick.= The wick most generally found practical is the cotton wick, such as is used in ordinary lamps. It should be kept clean and renewed often. The lamp should be kept filled regularly. The wick must always be kept trimmed even, to prevent smoking. Incubators heated by electricity have the heating unit placed either above or below the trays of eggs. The current is controlled by a thermostat. =252. Thermostat.= The thermostat also raises the damper over the top of the lamp and air heater (Fig. 142), when the incubator reaches the temperature for which it is set, and lowers it when the temperature falls. When the damper is lifted, the heated air passes out into the room and not into the incubator. As soon as the incubator cools below this temperature, the thermostat contracts, letting the damper drop in place to retain the heat and direct it into the incubator. The thermostat works the same when a gas flame is used instead of a lamp. In electrical machines, the thermostat operates the switch, admitting much, little or no current, as is needed to maintain 102½ degrees Fahrenheit. [Illustration: FIG. 142. Thermostat for incubator.] =253. The Thermometer.= A thermometer is placed in the incubator to guide the operator in regulating the temperature. It guides him in adjusting the thermostat and the heating device; that is, it shows him when to turn the wick of the lamp up or down. Lamps should never be turned high enough to smoke. Smoke and gas in the room are likely to get into the incubator and harm the growing chicks. =254. Operation of Incubator.= Set the incubator level; it is constructed to work on the level. Heated air rises--if the incubator is not level, the highest point will get most of the heat. It should be set in a dry room or dry cellar, which is well ventilated and well lighted. There should be no artificial heat in the room which is not regular. An uneven temperature gives difficulty in managing the heating of the incubator. The room should be free from dust. Adjust the incubator and run it for two or three days to see that it is operating at a constant temperature before putting in the eggs. Use only the best grade of oil, and use the same kind of oil all thru one hatch. Change in oil may necessitate a change in regulators which is not safe while the eggs are in the incubators.