It is essential in the burning of this class of fuel that a large combustion space be supplied, and on account of the usually high moisture content there should be much heated brickwork to radiate heat to the fuel bed and thus evaporate the moisture. Extension furnaces of the proper size are usually essential for good results and when this fuel is used alone, grates dropped to the floor line with an ashpit below give additional volume for combustion and space for maintaining a thick fuel bed. A thick fuel bed is necessary in order to avoid excessive quantities of air passing through the boiler. Where the fuel consists of hogged wood and sawdust alone, it is best to feed it automatically into the furnace through chutes on the top of the extension. The best results are secured when the fuel is allowed to pile up in the furnace to a height of 3 or 4 feet in the form of a cone under each chute. The fuel burns best when not disturbed in the furnace. Each fuel chute, when a proper distance from the grates and with the piles maintained at their proper height, will supply about 30 or 35 square feet of grate surface. While large quantities of air are required for burning this fuel, excess air is as harmful as with coal, and care must be taken that such an excess is not admitted through fire doors or fuel chutes. A strong natural draft usually is preferable to a blast with this fuel. The action of blast is to make the regulation of the furnace conditions more difficult and to blow over unconsumed fuel on the heating surfaces and into the stack. This unconsumed fuel settling in portions of the setting out of the direct path of the gases will have a tendency to ignite provided any air reaches it, with results harmful to the setting and breeching connection. This action is particularly objectionable if these particles are carried over into the base of a stack, where they will settle below the point at which the flue enters and if ignited may cause the stack to become overheated and buckle. Whether natural draft or blast is used, much of the fuel is carried onto the heating surfaces and these should be cleaned regularly to maintain a good efficiency. Collecting chambers in various portions of the setting should be provided for this unconsumed fuel, and these should be kept clean. With proper draft conditions, 150 pounds of this fuel containing about 30 to 40 per cent of moisture can be burned per square foot of grate surface per hour, and in a properly designed furnace one square foot of grate surface can develop from 5 to 6 boiler horse power. Where the wood contains 50 per cent of moisture or over, it is not usually safe to figure on obtaining more than 3 to 4 horse power per square foot of grate surface.