Underfeed Stokers are either horizontal or inclined. The fuel is fed from underneath, either continuously by a screw, or intermittently by plungers. The principle upon which these stokers base their claims for efficiency and smokelessness is that the green fuel is fed under the coked and burning coal, the volatile gases from this fresh fuel being heated and ignited in their passage through the hottest portion of the fire on the top. In the horizontal classes of underfeed stokers, the action of a screw carries the fuel back through a retort from which it passes upward, as the fuel above is consumed, the ash being finally deposited on dead plates on either side of the retort, from which it can be removed. In the inclined class, the refuse is carried downward to the rear of the furnace where there are dumping plates, as in some of the overfeed types. Underfeed stokers are ordinarily operated with a forced blast, this in some cases being operated by the same mechanism as the stoker drive, thus automatically meeting the requirements of various combustion rates. Traveling Grates are of the class best illustrated by chain grate stokers. As implied by the name these consist of endless grates composed of short sections of bars, passing over sprockets at the front and rear of the furnace. Coal is fed by gravity onto the forward end of the grates through suitable hoppers, is ignited under ignition arches and is carried with the grate toward the rear of the furnace as its combustion progresses. When operated properly, the combustion is completed as the fire reaches the end of the grate and the refuse is carried over this rear end by the grate in making the turn over the rear sprocket. In some cases auxiliary dumping grates at the rear of the chain grates are used with success. Chain grate stokers in general produce less smoke than either overfeed or underfeed types, due to the fact that there are no cleaning periods necessary. Such periods occur with the latter types of stokers at intervals depending upon the character of the fuel used and the rate of combustion. With chain grate stokers the cleaning is continuous and automatic, and no periods occur when smoke will necessarily be produced. In the earlier forms, chain grates had an objectionable feature in that the admission of large amounts of excess air at the rear of the furnace through the grates was possible. This objection has been largely overcome in recent models by the use of some such device as the bridge wall water box and suitable dampers. A distinct advantage of chain grates over other types is that they can be withdrawn from the furnace for inspection or repairs without interfering in any way with the boiler setting.