Another plan of fire-box, which was designed and patented by Mr. William Buchanan, Master Mechanic of the Hudson River Railroad, and used extensively on that line, is shown in fig. 218. This consists of a water-table, but it extends completely across the fire-box from the tube sheet to the back-plate, thus dividing the fire-box into two compartments, _M_ and _N_. In order to afford communication from the lower one to the upper one a round hole, _D_, about 24 in. in diameter, is put in the water-table in the position shown. It will thus be seen that all the currents of gas, smoke and air must unite in passing through this opening, and are thus brought into close contact with each other. After they enter the upper chamber and before they enter the tubes, there is room and time for combustion. The position of the lower side of the table, it will be seen, is similar to that of the deflector shown in fig. 216, so that it acts in somewhat the same way, by directing the currents of air, which enter through the furnace door, downward on the fire. [Illustration: Fig. 218. Scale ³⁄₈ in. = 1 foot.] QUESTION 402. _How do these different plans operate?_ _Answer._ They will all burn coal more perfectly, and therefore more economically, if they are carefully and skillfully managed, than is possible in ordinary plain fire-boxes; but it is probable that more economy in the consumption of coal would result from the improvement of the practice and knowledge of firemen than can be expected from the use of any of the appliances described, if they are used without care, or knowledge of the principles of combustion. QUESTION 403. _In what respect does anthracite coal differ from bituminous?_ _Answer._ It differs chiefly in the fact that it contains a much larger proportion of carbon and less of hydrogen, and in the fact that it consequently gives off very little or no coal gas. Its combustion is therefore more simple than that of bituminous coal, as there is very little else than solid carbon to burn. QUESTION 404. _In what kind of a fire-box is anthracite usually burned?_ _Answer._ It is usually burned in a very long grate, and as the heat is very intense, the grate-bars are usually made of iron tubes, through which a current of water circulates, so as to prevent them from melting. QUESTION 405. _Is it important to admit air above an anthracite fire to facilitate combustion?_