=55. Steam Cookers.= There are several steam cookers in use in homes. The simplest of these is a covered pan which has a perforated bottom, which is set over another pan (_A_, Fig. 25), in which water is placed for forming steam. One of the difficulties of this cooker is that the water in the lower pan cannot be watched and may boil dry. On the more improved cookers a whistling device (_B_, Fig. 25) is attached to the pan, and when the water becomes low and steam ceases to flow through it, air begins to come in, and the device makes a whistling noise. QUESTIONS FOR PART I 1. What is smoke? Under what conditions is the greatest amount of heat for cooking or other household purposes produced from fuel? 2. How is an oven made to heat evenly? 3. Explain the purpose of each draft and damper on a stove. 4. Observe the amount of fuel used in a coal stove from day to day. Make the same kind of observation for a gas or electric stove. How was the stove managed when the least fuel was used? 5. Describe the construction of a gas stove. Find the vent thru which the gas enters the burner. Is this large or small? 6. Where is the air regulator? For what is it used? 7. What has happened when the gas in a burner "burns back"? 8. How should a kerosene stove be regulated? How should it be cared for? 9. What precautions should you take against fire from kerosene and gasoline stoves? 10. Describe the heating unit of an electric stove. 11. How may electric current be saved in the operation of an electric stove? 12. How does a fireless cooker cook food? 13. How may one determine when it is economical to use a fireless cooker? PART II HEATING DEVICES CHAPTER VII WARM-AIR FURNACES =56. Principle Upon Which a Furnace Works.= The success of warm-air heating depends on a natural circulation of air thruout all the rooms which are to be heated. The air is the vehicle of transmission of the heat from the fire to the rooms to be warmed. A warm-air furnace is simply a large stove encased in a sheet-metal jacket (Figs. 26 and 27). The jacket is usually insulated with asbestos, since the stove is set in the basement where radiation of heat is not desired. The air entering the casing is warmed by the stove. As the air is warmed, it expands and becomes lighter, so rises to the top of the furnace; from here it is conducted to the rooms above. The warm air which has passed upward must be replaced by cooler air entering at the bottom of the jacket. In the rooms above, there must be outlets for the cold air, already in them, so that it may be replaced by the incoming warm air. Cold-air shafts from the floor leading downward serve as outlets. Sometimes they return the cooled air to the base of the furnace jacket. [Illustration: FIG. 26. Warm-air furnace.]