dening; only particular attention must be paid to the equability of the heat, and the gradation of it in different parts of the building. The heat in the flue gradually diminishes as it recedes from the fire-place. It must therefore be fo conducted through the building by frequent returns, that in every part there may be a mixture of warmer and cooler branches of the flue, and the final chimney fhould be close by the fire-place. If it is the flue of a greenhouse, it is beft to allow the flue to pro ceed gradually up the wall in its different returns, by which the loweft part will be the warmeft, and the heated air will afcend among the pots and plants. In the hypocaufta and sudaria of the an cient Greeks and Romans, the flue was conducted chiefly under the floors. See HYPOCAUSTUM. Malt-kilns are a fpecies of ftove which merit attention. Many attempts have been made to improve them on the principle of flue ftoves; but they have been unfuccefsful, because heat is not what is chiefly wanted in malting; it is a copious current of very dry air to carry off the meifture. We refer the examination of this fubject alfo to the article STOVE, and proceed to confider the current of heated air in the chief varieties of furnaces. All that is to be attended to in the different kinds of melting furnaces is, that the current of air be fufficiently rapid, and that it be applied in as extenfive a furface as poffible to the fubftance to be melted. The more rapid the current it is the hotter, because it is confuming more fuel; and therefore its effect increases in a higher proportion than its rapidity. It is doubly effectual if twice as hot; and if it then be twice as rapid, there is twice the quantity of doubly hot air applied to the fubject; it would therefore be four times more powerful. This is procured by raifing the chimney of the furnace to a greater height. The clofe application of it to the fubject can hardly be laid down in general terms, becaufe it depends on the precife circumftances of each cafe. In reverberatory furnaces, fuch as refining furnaces for gold, filver, and copper, the flame is made to play over the furface of the melted metal. This is produced entirely by the form of the furnace, by making the arch as low as the circumftances will allow. (See CHEMISTRY, Index; and FURNACE, 15.) Experience has taught the chief circumstances of their conftru&tion, viz. that the fuel fhould be at one end on a grate, through which the air enters to maintain the fire; and that the metal fhould be placed on a level floor between the fuel and the chimney which produces the current. But there is no kind of furnace more variable in its conftruction and effects. This has occafioned many whimsical varieties in their form. This uncertainty feems to depend much on a circumftance feldom taken notice of by mineralogical writers. It is not heat alone that is wanted in the refining of filver by lead, for inftance. We muft make a continual application to its furface of air which has not contributed to the combuftion of the fuel. Any quantity of the hottest air, already faturated with the fuel, may play on the furface of the metal for ever, and keep it in the ftate of moft perfect fufion, but without refining it. Now, in the ordinary conftruction of a furnace, this is much the cafe. If the whole air has come in by the grate, and paffed through the middle of the fuel, it must be nearly faturated with it; and if air be alfo admitted by the door (which is generally done), the pure air lies above the vi tiated air, and during the paffage along the horizontal part of the furnace and the furface of the metal, it ftill keeps above it. Thus the metal does not come into contact with air fit to act on the bafe metal and calcine it, and the operation of refining goes on flowly. Trifling circumftances in the form of the arch or canal may tend to promote the jumbling of the airs together, and thus render the operation more expeditious; and as these are but ill understood, they are confidered as fo many noftrums of great importance. Changes in the form of the roof fhould therefore be tried, directed to this circumftance. The glafs-houfe furnace exhibits the chief variety in the management of the current of heated air. (See GLASS-MAKING, Sect. IV.) In this it is neceffary that the hole at which the workman dips his pipe into the pot fhall be as hot as any part of the furnace. This could not be the cafe, if the furnace had a chimney above the dipping hole; as in this cafe cold air would immediately rush in. To prevent this, the hole is made the chimney; but as this would be too short, and would produce very little current and very little heat, the whole furnace is fet under a tall dome. Thus the heated air from the real furnace is confined in this dome, and conftitutes a high column of very light air, which therefore rises with great force up the dome, and escapes at the top. The dome is therefore the chimney, and will produce a current proportioned to its height. Some are raised above 100 feet. When all the doors are shut, and no supply given except through the fire, the current and heat become prodigious. But the workmen being in this chimney, they must have refpirable air. Yet notwithstanding this fupply by the house doors, the draught of the real furnace is vaftly increased by the dome, and a heat produced fufficient for the work, which could not be produced without the dome. This has been applied with great ingenuity and effect to a furnace for melting iron from the ore, and an iron finery, both without a blaft. The common blaft iron furnace is well known. It is a tall cone with the apex undermoft. The ore and fluxes are thrown into this cone, mixed intimately with the fuel, till it is full, and the blaft of moft powerful bellows is directed into the bottom of this cone through a hole in the fide. The air is thrown in with fuch force, that it makes its way through the mafs of matter, kindles the fuel in its paffage, and fluxes the materials, which then drop down into a receptacle below the blaft-hole, and thus the paffage for the air is kept unobftructed. It was thought impoffible to produce or maintain this current without bellows; but Mr Cotterel, an ingenious founder, tried the effect of a tall dome placed over the mouth of the furnace; and though it was not half the height of many glasshoufe domes, it had the defired effect. It is extremely difficult to place the holes below, at which the air is to enter, at fuch a precife height as neither to be choked by the melted matter, nor to leave ore and ftones below them unmelted; but but the invention is very ingenious, and promises to be of immense service. Another important ufe made of the currents produced by heating the air is to free mines, fhips, prifoss, &c. from the noxious vapours which often int them. As a drift or work is carried on in the mine, let a trunk of dale boards, about 6 or fisches fquare, be laid along the bottom of the drift,communicating with a trunk carried up in the corner of one of the thafts. Let the top of this laft trunk open into the afh-pit of a fmall furnace, having a tall chimney. Let fire be kindled in the furnace; and when it is well heated, shut the fireplace and afh-pit doors. There being no other fapply for the current produced in the chimney of this furnace, the air will Row into it from the trask, and will bring along with it all the offen five vapours. This is the most effectual method yet found out. In the fame manner may trunks be conducted into the afh-pit of a furnace from the cells of a prifon or the wards of an hofpital. In the account above given, we have mentioned the application of air to the burning fuel as neceffary for its combuftion. This is a general fact. That any inflammable body may be really inflamed, and its combustible matter confumed and ahes produced, it is not enough that the body bemide hot. A piece of charcoal inclofed in a bir of iron may be kept red-hot for ever, without wifing its fubftance in the fmalleft degree. It is farther neceffary, that it be in contact with oxyge. (See OXYGEN.) In contributing to the combuftion of an inflammable body, this air combines with fome of its ingredients, and becomes fixed air. Combustion may therefore be confider ed as a folution of the inflammable body in air. This doctrine was firft promulgated by the celebrated Dr HOOKE in his Micrographia, published in 1660, and afterwards improved in his treatise on Lamps. It is now completely established, not as a mechanical but a chemical phenomenon. See CHEMISTRY, Index, and COMBUSTION. In all fire-places, therefore, the conftruction is directed fo as to produce a clofe application of the air to the fuel. A candle will confume fafter in the low countries than in the elevated regions of Quito and Gondar, because the air is nearly one half denfer below, and will act proportionally fafter in decompofing the candle. A curious phenomenon is obferved in many places, with respect to certain fprings or fountains, which have periods of repletion and fcantinefs, or ebb and flow, at regular intervals; and fone of these periods are of a complicated nature. Thus a well will have feveral returns of high and low water, the difference of which gradually increases to a maximum, and then diminishes, as in the ocean. A very ingenious explanation of this has been given in N° 424. of the Philof. Tranf. by Mr ATWELL, as follows: Let ABCD (fig. 68, Plate CCLXXXIII.) reprefent a cavern, into which water is brought by the fubterraneous paffage OT. Let it have an outlet MNP, of a crooked form, with its higheft part N confiderably raised above the bottom of the cavern, and thence floping downwards into lower ground, and terminating in an open well at P. Let the dimenfions of this canal be fuch, that it will dif charge much more water than is fupplied by TO. The effect will be a remitting fpring at P: for when the cavern is filled higher than the point N, the canal MNP will act as a fyphon; and, by the conditions affumed, it will discharge the water fafter than TO fupplies it; it will therefore run it dry, and then the fpring at P will cease to furnish water. After fome time the cavern will again be filled up to the height N, and the flow at P will recommence. If befides this' fupply, the well P alfo receive water from a conftant fource, we shall have a reciprocating spring. The fituation and dimenfions of this fyphon canal and the fupply of the feeder may be fuch, that the efflux at P will be conftant. If the fupply increafe in a certain degree, a reciprocation will be produced at P with very short intervals; if the supply diminishes confiderably, we shall have another kind of reciprocation with great intervals and great differences of water. If the cavern has another fimple outlet R; new varieties will be produced in the fpring P, and R will afford a curions fpring. Let the mouth of R, by which the water enters it from the cavern, be lower than N, and let the fupply of the feeding fpring be no greater than R can difcharge, we fhall have a conftant fpring from R, and P will give no water. But fuppofe that the main feeder increafes in winter or in rainy feafons, but not fo much as will fupply both P and R, the cavern will fill till the water gets over N, and R will be running all the while; but foon after P has begun to flow, and the water in the cavern finks below R, the ftream from R will ftop. The cavern will be emptied by the fyphon canal MNP, and then P will ftop. The cavern will then begin to fill, and when near full R will give a little water, and foon after P will run and R ftop as before, &c. M. DESAGULIERS fhows, Vol. II. p. 177, &c. in what manner a prodigious variety of periodical ebbs and flows may be produced by under ground canals, which are extremely fimple and probable. SECT. XII. Of SEVERAL PNEUMATICAL EN GINES. WE cannot conclude this fcience, without defcribing fome pneumatical engines, which have not been particularly noticed under their námes in the preceding part of this work. BELLOWS are of most extenfive and important ufe; and it will be of fervice to describe fuch as are of uncommon construction and great power, fit for the great operations in metallurgy. It is not the impulfive force of the blaft that is wanted in moft cafes, but merely the copious fupply of air, to produce the rapid combuftion of inflammable matter; and the fervice would be better performed in general if this could be done with moderate velocities, and an extended furface, What are called air-furnaces, where a confiderble furface of inflammable matter is acted on at once by the current which the mere heat of the expended air has produced, are found more operative in proportion to the air expended than blaft furnaces animated by bellows; and we doubt not but that the method propofed by Mr COTTEREL above mentioned, of increafing this current in a melting melting furnace by means of a dome, will in time The operation is precifely fimilar to that fuperfile the bli furnaces. There is indeed a blowing with chamber bellows. When the bour great impulive force required in fome cafes; as is lifted up the air eaters by the valves V, V, (fg for blowing of the fourie, from the furface of fil. 72. Pl. 283.) and is expelled at the pipe OQ ver or copher in refairg furnaces, or for keeping depreffing the boards. There is therefore no far a car paftare for the air in the great iron furnace. ther defcription neceffary. Thefe bellows an In general, however, we cannot procure this made of a very great fize, AD being 16 feet, A abultant fupply of air any other way than by five feet, and the circular end AE alfo five feet giving it a great velocity by means of a great the rife, however, is but about 3 or 34 feet. They preffure, f, that the general conftruction of bel- expel at each stroke about 90 cubic feet of air lows is pretty much the fame in all kinds. The and they make about 8 ftrokes per minute. air is admitted into a very large cavity, and then expelled from it through a fmail hole. The furnaces at the mines having been greatly enlarged, it was neceffary to enlarge the bellows alio; and the leathern bellows becoming exceedingly expenfive, wooden ones were fubftituted in Germany about the beginning of the 17th century, and from them became general through Europe. They confift of a wooden box ABCKFE (fig. 69, Pl. 282. A), which has its top and two fides flat or ftraight, and the end BAE formed into an arched or cylindrical surface, of which the line FP at the other end is the axis. This box is open below, and receives within it the fhallow box KHGNML (fig. 71, Pl. 282. B), which exactly fills it. The line FP of the one coincides with FP of the other, and along this line is a set of hinges on which the upper box turns as it rifes and finks. The lower box is made faft to a frame fixed in the ground. A pipe OQ proceeds from the end of it, and terminates at the furnace, where it ends in a fmall pipe called the tower or TUYERE. This lower box is open above, and has in its bottom two large valves V, V, opening inwards: Fig. 72. C Pl. 283. The conducting pipe is fometimes furnished with a valve opening outwards, to prevent burning coals from being fucked into the bellows when the upper box is drawn up. The joint along PF is made tight by thin leather nailed along it. The fides and ends of the fixed box are made to fit the fides and curved end of the upper box, fo that this laft can be raised and lowered round the joint FP without fenfible friction, and yet without fuffering much air to escape: but as this would not be fufficient ly air-tight by reafon of the fhrinking and warping of the wood, a farther contrivance is adopted. A flender lath of wood, divided into feveral joints, and covered on the outer edge with very foft lea ther, is laid along the upper edges of the fides and ends of the lower box. This lath is fo broad, that when its inner edge is even with the infide of the box, its outer edge projects about an inch. It is kept in this pofition by a number of fteel wires, which are driven into the bottom of the box, and stand up touching the fides, as reprefented in fig. 73, Pl. 280, where abc are the wires, and e the lath, projecting over the outside of the box. By this contrivance the laths are preffed clofe to the fides and curved end of the moveable box, and the fpring wires yield to all their ine. qualities. A bar of wood RS (fig. 69, Pl. 282.) is fixed to the upper board, by which it is either raifed by machinery, to fink again by its own weight, having an additional load laid on it, or it is forced downward by a crank or wiper of the machinery, and afterwards raifed. Such are the bellows in general ufe on the cam tinent. We have adopted a different form in thi kingdom, which feems much preferable. W ufe an iron or wooden cylinder, with a pifton fliding along it. This may be made with much greater accuracy than the wooden boxes, at lefs expence, if of wood, because it may be of coopers work, held together by hoops; but the great advantage of this form is its being more eafily made air-tight. The pifton is furrounded with a broad ftrap of thick and foft leather, and it has around its edge a deep groove, in which is lodged a quantity of wool. This is called the packing or ftuffing, and keeps the leather very closely applied to the inner furface of the cylinder. Iron cylinders may be neatly bored and smoothed, so that the pifton, even when very tight, will flide along it very fmoothly. To promote this a quantity of black lead is ground very fine with water, and a little of this is fmeared on the infide of the cylinder from time to time. The cylinder has a large valve, or fometimes two, in the bottom, by which the atmospheric air enters when the pifton is drawn up. When the pifton is thruft down, this air is expelled along a pipe of great diameter, which terminates in the furnace with a small cr fice. This is the fimpleft form of bellows which can be conceived. It differs in nothing but fize from the bellows ufed by the rudeft nations. The Chinese fmiths have a bellows very similar, being a square pipe of wood ABCDE, (fig. 70, Pl. 281.) with a fquare board G which exactly fits it, moved by the handle FG. At the farther end is the biaft pipe HK, and on each side of it a valve in the end of the fquare pipe opening inwards. The piften is fufficiently tight for their purposes without ary leathering. The pifton of this cylinder bellows is moved by machinery. In fome blaft engines the pifton 3 fimply raised by the machine, and then let go, and it defcends by its own weight, and compreffes the air below it to fuch a degree, that the velocity of efflux becomes conftant, and the piffen defcends uniformly: for this purpose it must be loaded with a proper weight. This produces a very uniform blaft, except at the beginning, while the pifton falls fuddenly and comprefies the air: but in moft engines the piston rod is forced down the cylinder with a determined motion, by means of a beam, crank, or other contrivance. This gives a more unequal blaft, becaufe the motion of the pifton is neceffarily flow in the beginning and end of the ftroke, and quicker in the middle. Put in all it is plain that the blaft must be defultory. It ceafes while the pifton is rifing; for this realon it is ufual to have two cylinders, as it was former |