The draft of furnaces is a question of great importance, and yet it is one to which too little attention has been devoted both by practical and scientific men. At present we can merely throw out a few useful hints on the subject. The utility of every furnace depends on its draft. A fire burns because the oxygen of the atmosphere is brought into contact with fuel at a high heat. A chemical union, called combustion, between the oxygen and the fuel is effected, and heat is p.eveloped. The combustion of fuel converts most of its solid Qonstituents into carbonic acid gas and water ; the$e products of the fire possess the property of extinguishing it, hence the necessity for carrying them off as fast as they are generated in. the furnace, To carry on combustion the fuel has to be continually furnished with fresh air. The supplying of this air to a furnace, and the removal of the gases produced by combustioll, eonstitute " the draft." The efficiency of a boiler fnrnace, or any other kind, depends on the rapidity with which coal can be perfectly bnrned in it. By simply bringing the oxygen of the air into contact with fuel, rapid combustion cannot be effected. A fire built in an open space under a boiler burns very slowly, because the heated gases, in expanding, meet with much resistance owing to the great expanse of the superincumbent atmospheric column ; such a fire is said to have no draft. But when the fire is enclosed in a furnace, and the heated gases of combustion are conducted away in a contracted unbroken column through a chimney, the fire burns rapidlyit has a good draft. The reason of this is the heated ascending gases in this case meet with but a limited resistance in comparison with those of the open fire, becanse they have a much less superincumbent weight of air to displace. The deductions of science show that the draft of a furnace is increased according as a chimney is elevated, because the pressure of_ the atmosphere is less at its top than base. Many mistakes have been committed in building short wide chimneys, thinking thereby to obtain a rapid draft, but few mistakes have been committed in building high chimneys. On this pointthe hight of chimneysthere is no difference of opinion among those who are versed on the subject, but there is in regard to the proper area of chimneys. Some contend that a chimney of Et limited hight does not require to be wider than the aggregate section of all the flues or tubes of the boiler. Others contend that the area of a chimney should be twice that of the flues, and Mr. John Curry, an engineer, of Louisville, Ky., who has constructed the engines aud boilers of many western steamboats, considers that the area of steamboat chimneys should be two and a half times that of the flues, independent of the area of fire grate. Mr. Curry's testimony on the draft of the furnaces of western steamboats, given in the great trial against the Wheeling Bridge Company in 1850, amounts to this, that with these proportions of chimney area to that of the flues, a good natural draft can be obtained in chimneys from 30 to 50 feet high. Mr. Scott Russell, on the other hand, gives the rule that for each square foot of fire grate the flues should be one-fifth, and the areas of chimneys only one-tenth, and of equal diameter throughout. " If the chimney," he says, "is f01ty feet high, and has a cross section of one-tenth of the area of the fire crate, it will give an abundant draft. Some very satisfactory boilers have been made by allowing a proportion of 0.6 of a square foot of fire grate peF nominal horse power, and making the sectional area of the flue at the largest part one-seventh of the area of grate, and at the smaiIest part (back end) one-eleventh area, and chimneys the same." m So also says Bourne, allowing in snch cases sixteen square feet of flue or tube heating sur- C face for each. horse power. A better draft has been obtained with boilers twenty-eight IS feet long on steamboats than with those of greater length. Short wide boilers with vertical tubing afford the best draft, and are the most efficient in evaporation. The higher the heat in a furnace, of course, the more rapid is the draft. Furnaces well lined with fire brick are more efficient than those which are surrounded with iron sides, even when such sides have water spaces behind them. It is common in to allow one-third of the area of the grate bars for open spaces, to admit the air to pass into the furnace, but in some instances equal areas of air spaces and fire bars are used. As no more air can pass into a furnace to supply combustion than that which passes between the grate bars, it would be reasonable to suppose that at least an equal area of space should be allowed for the flues and chimney. Some contend that such are the best proportions, but this is a subject regarding which practical and scientific men have no generally settled opinions. It would be well if an extensive set of judicious experiments were made to ascertain the best proportions of the draft spaces in furnaces of all kinds. It has been found that a chimney of fifty feet in hight gives a good draft for small boilers from up to 20 horse power. A chimney one hundred feet in hight is sufficient for the draft of engine boilers from 50 up to 400 horsc power. A rapid draft can be obtained in any chimney, however short, by a blower, the steam blast exhausting into the chimney. At present, we conclude these articles, but at some future time may recur to the subject.