One of the many unphilosophical terms still retained in modern technology is the word "draft," as applied to the motion of air and gases through a chimney. There is no case whore tliis term can be applied in its true meaning. It was like many other terms still retained, employed at first to ox-press an utterly false idea of the nature of the thing for which a name was sought. The misfortune of retaining such terms in scientific language, is, that although the scientific man is not often misled by them, the unscientific are sure to be misled. A chimney has no dra*'t, if by draft is meant the power to draw anything. The time was, before the fact that air had weight was demonstrated, when it was thought that flame and smoke went up a chimney because the chimney had a mysterious power which drew them up. A similar error prevailed as to the way in which water rises in pumps; hence the term auction pump, long since demonstrated to be a misnomer. It is now generally known that water rises in atmospheric pumps by virtue of the pressure of the atmosphere upon the surface of the water exterior to the pum.p, the office of the plunger being principally to remove the pressure of the atmosphere from the interior of the pump. Tlie principle of the chimney is that a body, fluid or solid, plunged into a fluid of greater density will if unrestrained rise to the top. If as in the case of air the fluid into which the body is plunged is more dense at the bottom than at the top, the body immersed will rise until it reaches a stratum of fluid of equal specific gravity, and then cease rising. Tliis is a natural result of the laws which govern the pressure of fluids. Air when heated has its density decreased, hence a volume of heated air, surrounded by air not heated, will immediately rise, if unrestrained, until it reaches a stratum of air of the same density as itself. A balloon filled with heated air will rise, but no one ever thought of speaking about the draft of a balloon. What then is the use of a chimney, it the heated air will rise without it ? Air rising through a tube and issuing through the top, the air which rushes in to fill the void is forced to enter at the bottom. Rising nnconfined it would expand, and diffuse, mixing with the surrounding air, rapidly losing its heat and ceasing to rise much sooner than would be the case, when confined within the walls of a tube. Besides, the air rushing in to fill the void wou.ld bo distributed more or less along the irregular sides of the heated and ascending current, instead of passing as desired through the fuel to maintain combustion. The utility of high chimneys will now be manifest. The column of heated air rising as one body is impelled by the pressure of an eqira,! volume or cord irentering at the bottom, and the higher the chimney the greater the volume, and of course die greater the force with which it will rush into the bottom through the fuel. It must be obvious, also, if the principles we have stated are clearly comprehended, that anything which tends to check the ascent of the heated air in a chimney will more or less obstruct the flow of air into the bottom. Those correspondents therefore, who have written us with reference to the effect of horizontal and descending flues, arguing that a certain proportion of such flues would facilitate what they call, and is generally called, the draft of chimneys, are radically wrong in their views. All the draft (if we must use the word for want of a better) a chimney has, it has by virtue of the ascending flues, all others are subtracters from the efficiency of the structure. In conclusion, we suggest that the term circulation would be a good substitute for the term draft as applied to chimneys in general, ridding the language of one of its most inappropriate terms.
This article was originally published with the title "Draft in Chimneys" in Scientific American 21, 4, 57 (July 1869)