To prevent smoke, the chimney ought to be so constructed, that a current of air may pass immediately over the fire, so as to be rarefied in its passage, and not to pass entirely through the tire, as many have erroneously imagined. For this purpose, the throat should be so near to the fire. as to prevent the cold air from passing over it, and its horizontal dimension in the thickness of the wall should nut exceed four inches and a half. or five inches at most.
This contraction is to be formed by l'acing up the back, and bevelling the covings, so that no cold air may be admitted by the ends of the fire ; by thus obliging the overplus above the quantity necessary to produce combustion, to pass over the fire, it becomes so heated, as to consume the smoke in part. ;red to drive the remaining portion before it, with cele rity and violence.
The covings are in general placed at an angle of one hundred and thirty-five degrees with the back and breast, and should be made to form an abrupt plane on their top, so as to break the current of a sudden gust of wind.
The greater the quantity of rarefied air that passes up the flue, and in general the higher the chimney, the more celerity and three will it ascend with. The flee ought, therefore, to be carried as high as convenieney will admit.
To prevent the absorption of heat, the back and covings should he constructed of white materials, or, if not, they should be covered with plaster, and whitened as often as they become black, and thus they will reflect a greater quan tity of heat.
Must metals absorb the heat, and are therefore unfavour able for this purpose.
The back and covings are most conveniently put up after the house is built. ']'he introduction and general use of registers." has obviated any difficulty in this respect ; they thrui a great improvement on the old method.
Some of the principles in the construction of chimneys are very well ascertained. others are not easily discovered till tried.
The tops of flues should not have such wide apertures, as to permit a greater quantity of air to rush down the chimney, and counteract the of the ascending rarefied steam.
Smoky chimneys are frequently occasioned by the situation of doors in a room, the grate being placed too low, or the mantle too high. There are many cases in which it is not easy to discover the cause ; but if once known, it may be easily removed.
Flues with circular sections are, with some reason, sup posed to be more favourable for the venting of smoke, than those whose sections are square or rectangular.
There is much difference of opinion as to the origin of chimneys. They do not seem to have been in use among the classics, as they are not found. as Winklemann informs us, amongst the ruins of Herculaneum, although coals have been discovered in some of the rooms, from which he conjectures that the Romans Used charcoal fires.; Arr. Lvsons, however, describes a fire-place, which he found in one of the rooms of the Roman villa at Bignor, in Sussex. There does not seem to be any evidence of the use of chimneys in England before the twelfth century, when we meet with them in the castles of Rochester, Hedingham, &c., also in a Norman house at Winwall in Norfolk, in these cases, however, the flue is carried up only a short distance in the thickness of the wall, and is then turned out at the back, the apertures being small oblong holes. Shortly afterwards we meet with flues carried up the whole height of the wall, as at the castles of Collis borough, Newcastle, Sherhomne, &c., as also at Christ Church, Hants. At this period the shafts were carried up to a considerable height, and are generally circular ; in after times the forms varied considerably, and terminated fre quently with it spire. pinnacle, or gable, with apertures of ornamental forms in the sides underneath for the escape of the smoke. During the tburteenth century the shafts were very short, and of great variety of forms. In the fifteenth, the shafts Were more usually octangular. someliines square, with the aperture at the top ; at the latter end of this cen tury we find clustered shafts, which afterwards beeame so common in Elizabethan buildings. These clustered chimneys are most frequently of brick, variously and elaborately orna mented all the way up the shaft, and indeed form a very prominent and beautiful feature in buildings of this period. Fine specimens of the kind are to be seen at Hampton Court Palace, Eton College, East Basham Norfidk, and all the larger buildings of the Elizabethan style ; examples in stone, though more rare, exist at Bodiam Castle, Sussex, and on houses at South Petherton and Lambrook, Somer setshire.