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blaft the ftrength of fire depends folely on thedegree of mechanical power employed in im-pelling the blaft, and the chimney is altogetherl’uperfluous except to carry off the fmoke.Thefe kind of furnaces therefore are muchfimpler than the draught furnaces, as no moreis required than a Ample hearth or a fhallowpot, with a fmall hole at the bottom to condudtthe air from the bellows. Large quantities ofmaterials, as in the fmelting of iron, can hardlybe heated to the utmoft intenfity without theartificial blaft, and the afiiftance of a fteamengine or fome other very ftrong mechanicalforce is neceffary to keep up the conftant fupplyof air. At leaft, much time and fuel are favedby the blaft, for if the heat of the moft powerfuldraught-furnaces, fuch as the porcelain kilns,may be brought to equal that of the iron-fmelt-ing furnace, it requires, however, a very largebody of fuel and a great many hours to bring itto the utmoft intenfity. But in the fmall way,for experimental purpofes, the higheft tempe-rature of a draught-furnace, when well builtand properly attended to, appears to be no wayinferior to the beft blaft, to judge by the com-parative effefts; and the greater confumptionof fuel and time is often fully counterbalancedby the faving of manual labour.

Some chemifts have fuppofed that the heatof a furnace might be refledted from the fides,and concentrated into one focus, by giving it acircular or elliptical form. This however isfound by experience to be perfectly nugatory,for no fenfible difference is obferved between acylindrical, globular, or elliptical fire-place, pro-vided the dimenfions of the chimney and afti-pitate the fame. There is however a method ofconcentrating the heat of a blaft-furnace, bydividing the blaft into a number of fmaller jetsof air, and mechanically driving them towards acommon center, as will be explained hereafter.

The materials of which furnaces are con-ftrutfed is always a refra&ory clay, either inform of bricks, or, where the pieces of the fur-nace are fmall, of entire pottery. The largerkinds of black-lead crucibles make extremelyufeful portable furnaces for a variety of pur-pofes, being very infufible, bearing pretty fuddenheating without fplitting, and having the addi-tional advantage of being fo foft as to bereadily drilled by a gimblet, and cut by a fawor hacked knife, whereby openings may bemade and ftoppers fitted without difficulty.Large moveable furnaces are often made of thiskind of pottery, but much thicker than thecommon crucibles, and (Lengthened on the

outfide by iron hoops. A very ufeful whitecoarfe pottery is employed in France for port-able furnaces, which bears the fire extremelywell, but when baked is too hard to be cut likethe black lead ware. Moveable furnaces areoften made of a wrought-iron cafe lined on theinfide with fmall fire-bricks, and the part im-mediately in contadb with the fuel is furthercovered with a thick coating of Windfor loam,laid on when moift and plaftic, and beat fre-quently with a wooden inftrument when drying,to give it more compadtnefs, and to fill up thefmall cracks which the flirinkage occafions.The fixed furnaces are always built of fire-bricks(a very bard infufible brick, made for this exprefspurpofe) cemented with a very refraftory mor-tar; and the larger openings are either irondoors fet upon hinges, or fometimes a thickpiece of free-ftone, or very large brick properlyfafhioned. There is a very foft red i'andybrick fold in London , under the name of Wind-for brick , which may be cut or fcraped withgreat eafe, and is extremely ufeful for ftoppers,crucible ftands, and many other fmaller purpofes,though it is too foft to bear any confiderablepreffure.

A proper feleftion of the fuel is of firft im-portance. Wood and charcoal were formerlythe only materials employed for this purpofe inthis country, and are ftill fo in moft other partsof the world; the firft, where a large volumeof flame is required, and the latter, for a ftrongheat without flame. Thus glafs-houfe and re-verberatory furnaces were fupplied entirely bydry faggots, and iron' forges by charcoal.

Wood faggots give a ftrong clear flame un-mixed with fulphur, and with but little fmoke,and burn to a clean afh, which alfo is valuablefor the alkali it contains. From its freedomfrom fulphur it muft be in many inftancesbetter than coal, and it feems to have no otherdifadvantage than the rapidity with which itburns out in a ftrong draught, being fo muchmore bulky than coal that only a comparativelyfmall quantity at a time can be thrown into afurnace. Hence in the large foreign glafs-houfes that are fed by faggots, it is the work ofone man to be inceffantly adding fuel. But thefcarcity of wood now forbids its ufe in moftparts of this country, and the abundance of coalon the whole very well fupplies its lofs.

Charcoal is a moft valuable fuel for furnaces.It kindles readily, burns with a very ftrong clearbeat, and requires a much lefs draught of airfor combuftion than coak or charred coal; itcontains no fulphur. nor any earthy or metallic