604. COTTAGE, FARM, AND VILLA ARCHITECTURE. J 163 Ft. 40 Li- on which they rest may be three inches broad, and five inches deep, also cast hollow. The thickness of the metal, in either case, need not exceed a quarter of an inch ; as the hollow in the centre, by admitting a current of air, keeps the bars cool, and prevents their decay. It is to be observed, that the opening behind the fuel-chamber, for the admission of an extra quantity of air, inust have a grating where it enters the fuel-cham- ber, to prevent its being choked up by the lime while burning, or being drawn. 1289. Remarks. This Design for a limekiln we consider decidedly the best we have ever seen or heard of. We examined it, and several others of dif- ferent kinds, at Closeburn, in 1831 ; we saw Heathorn's kiln in 1828, and we liave seen, we believe, every variety of limekiln in use in Britain, having had occasion to build one for ourselves some years ago. We therefore con- sider that we are entitled to pronounce an opinion on Mr. Menteath's im- provements. The same form of kiln is admirably adapted for burning clay, either to use as a manure, or to form walks or roads ; and we shall presently show how it may be used for burning bricks. Heathorn's limekiln has for its object the preparation of coke as well as of quicklime. For this purpose, the kiln may either be a narrow vertical cylinder, with coking ovens placed around, and opening into it ; or, it may be a long narrow kiln, like Menteath's, with a row of coke ovens on each side. The kiln is filled with broken limestone, and the process of burning is effected entirely by the combustion of the bituminous matter of the coal to be coked. A patent was taken out for this kiln some years ago; and one was erected at Maidstone, by the patentee, who has recently informed us that it continues to answer his expectations. Only one other kiln of the same kind, however, has been erected ; which we conclude to be owing to the expense of building, and the greater time and care required to burn the lime in these kilns. Where lime is to be burned in small quantities, and where there is a great demand for coke, this description of kiln may perhaps be recommended ; but the first cost, and the slowness of the operation of burning, are great drawbacks. This kiln will be found figured and described in our Encyc. of Agric. § 3868. It is evidently founded on the model of a perpetual limekiln, described by Count Rumford, in his sixth Essay, p. 181. The objects proposed to be attained by the perpetual kiln were, first, to cause the fuel to burn in such a manner as to consume the smoke, by obliging the latter to pass through the ignited fuel ; and secondly, to cause the flame from the fire to come in contact with a large quantity of tlie broken limestone, so as to prevent the heat, as much as possible, from escaping into the atmosi)here. The latter object was effected by making the kiln very high in proportion to its width ; by fill- ing it quite to the top with limestone ; and by having the fireplace near the bottom of the kiln. The third object in view, was to render the process of burning lime perpetual, in order to prevent the waste of heat, which unavoidably attends the cooling of the kiln, in emptying and filling it. A fourth object was, so to arrange the process, as that the burnt limestone, while cooling, might give off its heat in such a manner as to assist in heat- ing the fresh quantity of cold limestone with which the kiln was replenished, as often as a portion of burnt lime was taken out of it. To show how these purposes might be effected, Count Rumford had a model erected in the court-yard of the Dublin Society, and another in the farm of the English garden at Munich ; and he has given an engraving and description of the latter in the Essay referred to. Whoever will take the trouble of examining it, will find that Heathorn's kiln only differs from that of Count Rumford in saving the coke ; this object not being proposed by the Count. The fuel in both kilns is not mixed with the limestone, but is burned in a closed fireplace, which opens into the side of the kiln, at some distance from the bottom. For large kilns built on these principles, Rumford observes, there may be several fireplaces, all opening into the same cone ; that shape of kiln being employed both by Count Rumford and Mr. Heathorn. At the bottom of Count Rnmfi)rd's kiln there is a door, which is occasionally opened to take out the lime; and at the bottom of Heathorn's there are movable bars, which can be taken out, for tiic same
