Page:The American Cyclopædia (1879) Volume IV.djvu/446

438 CHIMNEY ancient customs, are described by Pietro della Velle as making their fires in a round or square hole in the earth floor of their apartments, in which is an iron vessel containing the fuel. Over it is placed a low table, covered with a thick quilted cloth, reaching to the floor. The heat is described as being exceedingly agreeable, and not causing uneasiness to the head, though no provision is made for conveying away the gase- ous products of combustion. A pipe for blow- ing the fire with the mouth is made to project up from the floor. To such expedients were the most refined nations of antiquity probably reduced in their ignorance of the simple chim- ney flue ; a fact which would seem quite irre- concilable with the progress they had made in many of the arts dependent more or less upon the use of fires, were it not that these were less essential to them than to the in- habitants of more northern latitudes. The tendency of heated air to ascend must have been almost as familiar to them as that of water to flow down inclined planes ; and to us it would seem to require not much more inge- nuity to devise pipes for conveying the former away, than to contrive aqueducts which to this day are regarded as extraordinary works of art. It was the every-day need of supplies of pure water, and not of artificial warmth, which alone made the difference. This tendency of warm air to form ascending currents comes from its greater proportional lightness as it is expanded by heat and made to occupy a great- er space. Deep mines in cold countries well exemplify the working of this principle. In the winter season the deep shafts are perfect chimneys of ventilation. The air enters the mines through the lowest openings into them, and mixing with the smoke of the candles and of the gunpowder blasts, acquires from these, and from the warmer underground temperature, a greater heat than that of the air above. Ex- panding in bulk, it floats upward, establishing an ascending current, which is renewed by succeeding portions of air, and which rapidly sweeps off the gaseous impurities of the mine. But as the weather becomes warm, and the air within and without attains the same tempera- ture, no such current is formed, and the smoke, settling in a cloud below, slowly finds its way out by those passages through which at other times the fresh air is wont to flow in. Artifi- cial means of ventilation then become neces- sary. Fires are built in the mine or at the mouth of the shaft, and the foul air is also forced out by blowing machines. The ascen- sional power of a column of heated air is the difference between its weight and that of an outside column of the same height. It is hence increased by adding to the height of the chim- ney, as well as by increasing the temperature. But the chimney may be so high, especially when exposed in cold situations, that the heat taken up below may be dispersed before the air reaches the top. In this case increased height adds nothing to the draught, but di- minishes it. Short chimneys may be made more effectual by increasing the temperature, and in locomotives this is done by the intro- duction of a jet of hot steam. Calculations have been made to estimate the ascending force of air in chimneys at any given increase of temperature over the external air ; but these are affected by many sources of error, which render the results only approximate. The flow of air over rough surfaces is retarded by in- creased friction, and in small flues it expe- riences this resistance more than in large ones of the same construction, in the inverse pro- portion of the diameters of the flues. In or- der to apply these calculations, it is necessary to know the mean temperature of the column of air in a chimney, and this cannot be deter- mined with accuracy. It may be approxima- ted by finding the degree of heat a little above the lower entrance into the flue, and that at the top, and taking half of their sum. Mont- golfier, the inventor of balloons, first gave at- tention to this subject, and proposed the fol- lowing method of determining the force of the draught, or the velocity of the current, which is still considered as simple and accurate as the case admits of. It is the same velocity which a falling body would acquire in passing the distance equal to the difference in height be- tween two equal columns of air at the different temperatures. As the velocity of a heavy fall- ing body is ascertained at any period of its de- scent by multiplying the square root of the number of feet it has fallen by 8, which gives its rate per second of time, we have the data for applying to the calculations for determining the velocity of the current. Suppose the height of the chimney to be 70 ft., and the difference of temperature of the two columns to be 30% we have the expansion of the heated column= 70 x -0661 (this decimal representing one sixth of the expansion due to a difference of 180 of temperature). The difference in the height of the two columns is then 4'627 ft., and the square root of this (2'15), being multiplied by 8, gives 17'2 ft. per second as the velocity of the draught. For the further consideration of chimneys especially intended to feed fires with a draught of air, see FURNACE. Smoky chim- neys have a variety of causes, such as imperfec- tions in the flue, too contracted dimensions, too rough an inner surface, openings which admit cold air and chill it, and, the most com- mon of all, too large an opening at the fire- place or throat. Count Rumford paid much attention to the cure of smoking chimneys. He generally found the cause to be too large a throat, and his usual remedy was to diminish it by building a bench of brick in the back of the fireplace, reaching up to the throat, and to lower the fireplace somewhat. Sometimes the aperture at the top is too large, particularly if it is below the level of some neighboring house, hill, or high trees, from which the wind may be reflected down into the chimney, or over which it may fall, and thus beat down the