_Answer._ Yes; when no air is admitted above the fire, large grates are found to produce the best combustion. But while it is true that the same amount of heat will be produced by the union of each equivalent of oxygen and fuel, yet if we can force _more_ air and fuel to unite in the _same place_, a higher temperature is produced in that place, just as a fire in a blacksmith’s forge is hotter because of the forced blast than that in an ordinary stove, or a smelting furnace than a parlor grate. If, then, we can concentrate the draft in the fire of a locomotive, we secure a greater _intensity_ of combustion; and when the air is urged against the solid carbon with considerable force, it comes in contact with every point of its surface, and therefore less dilution of the air is needed, and consequently the products of combustion have a higher temperature; and, as has been explained, a larger proportion of the heat is then transferred to the water than if the temperature is lower and the volume greater. Intensity of combustion also has the effect of maintaining an igniting temperature; whereas, if the same amount of fuel is burned slowly, its heat may not be high enough to ignite the gases as they are produced. It is desirable, however, to have all the space that is possible in the fire-box, so as to give room for the mixing of the gases; but with a large fire-box and large grate a decided improvement and economy will often result by diminishing the effective area of the grate by covering a part of it with dead plates, but at the same time making provision for the admission of air above the fire. QUESTION 397. _What is meant by the “Total Heat of Combustion?”_ _Answer._ It is the number of units of heat given out by the combustion of a given quantity (usually a pound) of fuel. QUESTION 398. _How is this determined?_