A Guide to the Scientific Knowledge of Things Familiar. Ebenezer Cobham Brewer
a chimney through which the smoke passes, is called the funnel, or flue.
Q. Why does a chimney smoke, if the funnel be very short?
A. Because the draught of a short flue is too slack to carry the smoke up the chimney.
Q. Why is the draught of a short flue more slack that that of a long one?
A. For many reasons. 1st—The fire is always dull and sluggish if the chimney be too short.
2ndly—The smoke rolls out of the chimney, before it has acquired its full velocity.
3rdly—The wind, rain, and air, have more influence over a short funnel, than over a long one.
Q. Why is the fire always dull and sluggish if the chimney-flue be very short?
A. Because the draught is so bad: and as the rarefied air passes up the chimney very tardily, fresh air flows as tardily towards the fire, to supply it with oxygen.
Q. On what does the intensity of fire depend?
A. The intensity of fire is always in proportion to the quantity of oxygen with which it is supplied.
Q. Why does not smoke acquire its full velocity in a short funnel?
A. Because the higher smoke ascends in a flue, (provided it be clear and hot) the faster it goes; (as a stone falls faster and faster the lower it descends): if, therefore, a funnel be very short, the smoke never acquires its full velocity.
Q. Does the draught of a chimney depend on the speed of the smoke through the flue?
A. Yes. The more quickly hot air flies up the chimney, the more quickly cold air will rush towards the fire to supply the place; and, therefore, the longer the flue, the greater the draught.
Q. Why is the draught of a long flue greater than that of a short one?
A. Because the higher smoke ascends, the faster it goes; (as a stone falls faster and faster, the nearer it approaches to the earth): if, therefore, a funnel be long, the smoke acquires great velocity, and the draught is great.
Q. If a chimney be too short, and cannot be lengthened, what is the best remedy to prevent smoking?
A. To contract the opening of the chimney contiguous to the stove.
Q. Why will a smaller opening against the stove prevent the smoking?
A. As all the air (which enters the chimney) must pass near the fire, it will become greatly heated, and rise rapidly through the funnel; and this increase of heat will compensate for the shortness of the flue.
Q. Why will a room smoke, if there be two fires in it?
A. Because the fiercer fire will exhaust the most air; and draw from the smaller one, to supply its demand.
Q. Why will a chimney smoke if there be a fire in two rooms communicating with each other?
A. Whenever the door between the two rooms is opened, air will rush from the chimney of the inferior fire, to supply the other; and both rooms will be filled with smoke.
Q. What is the remedy in this case?
A. Let a tube be carried from the hearth of each stove, into the external air; and then each fire will be so well supplied, that neither will need to borrow from the other.
Q. Why do vestry chimneys so often smoke?
A. Because the wind (striking against the steeple) is reflected back; and tumbles down the vestry chimney, forcing the smoke into the room.
Q. what winds make vestry chimneys smoke?
A. Those from the north-east or south-east; according to the position of the vestry.
Q. Why will the eastern winds make vestries smoke, more than those from the west?
A. Because they strike against the steeple, and bound back to the vestry chimney: but western winds cannot rebound over the roof of a church.
(N. B. The steeple of a church is always due west, and the other end of the church due east; if, therefore, a western wind rebound, it would rebound to the west, or away from the church, and not towards it.)
Q. Why does a house in a valley very often smoke?
A. Because the wind (striking against the surrounding hills) rebounds back again upon the chimney, and destroys its draught.
Q. What is the common remedy in both these cases?
A. To fix a cowl on the chimney top, to turn like a weather-cock, and present its back to the wind.
Q. Why will not a cowl always prevent a chimney smoking?
A. If the wind be strong, it will keep the opening of the cowl towards the steeple or hill; and then the reflected wind will blow into the cowl, and down the chimney.
Q. As a cowl is such a poor remedy, can any other be devised?
A. If the chimney flue can be carried higher than the steeple or hills, no wind can enter the flue.
Q. Why cannot the wind enter a chimney flue, if it be carried up higher than the steeple or hills?
A. Because the reflected wind would strike against the sides of the chimney-flue, and not pass over the opening at all.
Q. In what other cases will a chimney smoke?
A. If both door and chimney be placed on the same side of a room, the chimney will often smoke.
Q. Why will a chimney smoke, if the door and stove are both on the same side?
A. Because when the door is opened, a current of air will blow into the chimney-place, and drive the smoke into the room.
Q. What remedy can be applied to this evil?
A. The door must be set opposite to the chimney, or nearly so; and then the draught from the door will blow the smoke up the chimney, and not into the room.
Q. Why will a chimney smoke if it needs sweeping?
A. Because the obstruction in the chimney (presented by the loose soot, to the free passage of the smoke) delays its current, and prevents the draught.
Q. Why will a chimney smoke, if out of repair?
A. 1st—Because the loose mortar and bricks obstruct the smoke: and
2ndly—The cold air (oozing through the chinks) chills the air in the chimney, and prevents its ascent.
Q. Why will an arnott’s stove smoke, if the joints of the flue do not fit air-tight?
A. Because the cold air (which gets through the joints) chills the air in the flue, and prevents its ascent.
Q. Why does an old fashioned farm chimney-place so often smoke?
A. Because the opening is so very large, that much of the air which goes up the chimney, has never passed near the fire; and this cold air mixing with the other, so reduces its temperature, that it ascends