Improved Fire-grates.

545

btained by causing the fire to burn like a candle from above downwards, and by using the poker as a simple lever to lift the fuel as required on a movable bottom. This arrangement is now employed to a considerable extent.

English grates, by slight changes in their construction, can be made to effect several of the ends sought, thus

Ist. The surfaces of the back and sides of the grate should be continued vertically, in fire-brick or clay, to the chimney-throat, so as to contract the open space between the burning fuel and the throat.* This prevents the great waste of heat caused in common grates, by the mixing of much of the pure warmed air of the room with the smoke, which must ascend the chimney to escape. 2nd. A throttle-valve, or a sliding damper, may be placed in the chimney-throat, with its handle projecting in front, so that the size of the passage may be regulated at will, and its state may always be known. This damper can be so far closed as to leave a passage for little more than the true smoke, or hot foul air rising directly from the fire. Thus will the flue be filled with hot air, almost undiluted, and the great heat will cause a chimney-draught much stronger than exists with the common grate, absolutely preventing any return of smoke into the room. Such a draught will also quicken remarkably the ventilation of the room, through the opening made near the ceiling to receive the balanced ventilatingvalve, and it may be made to draw through that opening the hot impure air arising from the burning of candles or lamps, or from other sources. 3rd. On the bottom of the grate can be laid a plate of sheet iron, covering it to within about half an inch of the border all round, so that only a little air may enter there, and a length of plate-iron may be placed to rest on edge, as a lining or shutter between the lower bar and the bottom of the grate-still further preventing the entrance of air there. 4th. To prepare for lighting the fire at the top and making it burn downwards, the grate may be filled with common coal to near the level of the upper bar, and upon that, pieces of twisted paper and firewood will be laid as usual, and over the wood a layer of the cinder or caked coal left from the fire of the preceding day. The combustibles being so arranged, and a match being applied to the paper, the fire, by

* No greater space should be left between the fire and the opening of the flue than is necessary to allow of the passage of the machine used for sweeping.

546

Defects of the Close Stove.

reason of the strong draught, blazes up with singular rapidity. It will burn for many hours, and if the grate be deep, even for a whole day, without being touched. In the evening, when the fuel is nearly consumed, the fire may be extinguished instantly by lifting off one or two of the remaining pieces. At any time when desired, the fire, by a touch of the poker, and sometimes by the application in front of a sheet blower, may be excited to great activity. Fresh air may be admitted to the room through a channel from the outside, which discharges the current under the fender, and that air, tempered by contact with the warmed fender, is then diffused all around.

A grate, constructed to act without re-charging for a long day, as above described, has beneath the firebox a receptacle for coal, which coal, supported on a movable pistonlike bottom, can be easily raised, when there is need, by a hand using the poker as a simple lever. At any time a lump of coal placed on the very hot upper surface of this fire, blazes quickly and lasts long, giving out hardly any smoke. Since the publication of the writer's book, various manufacturers have engaged in making stoves with the simple lever to lift the coal, but wishing to claim some peculiar new merit, have often deviated so as to fail in their object.

Fig. 181.

771. THE CLOSE STOVE.-The old form of this stove, with its narrow flue to carry away the smoke, is here sketched (fig. 181). By such a stove, a close room can be readily warmed to any desired temperature at the cost of a fourth part of the fuel required in an open grate. But this stove has important defects. It scarcely at all promotes the ventilation of the room; hence the air soon becomes irrespirable. It may vitiate the air of the room by becoming overheated; and there is much difficulty in regulating satisfactorily the rate of combustion. Notwithstanding these objections, close stoves are the chief means of warming dwelling houses during the winter, in the northern parts of continental Europe. The writer, when his attention was drawn to the subject, in relation to the preservation of health, saw the possibility of connecting with this kind of stove, such a self-acting current-regulating air-valve as would completely remedy the evils above referred to.

Construction of Air-valve.

547

The fire, when once lighted in this stove, if properly attended to, will burn uninterruptedly night and day for the whole of the 、winter. It will maintain in the room a temperature of about 62° of Fahrenheit, or higher if desired, and if the room-door is left open, the warmed air issues to the staircase, and so pervades the whole house. An air-channel under the floor, direct from the outer atmosphere, having an area of eight inches by four, admits fresh air immediately under the stove to be warmed, and to spread. Experience in many other cases has shown that such a self-regulating fire in the entrance-hall of a house, goes far to secure to the inhabitants, the advantages of the climate of the south of Europe or Madeira. It may also save to many invalids, the pain of banishment from home and its comforts.

The current-regulating air-valve for close stoves, on which their important qualities depend, is here represented by a sectional diagram (fig. 182).

rent passing through. The line, E, marks the movable internal part which modifies the current. It is a lever-frame perfectly balanced and turning on the upper edge of the cross partition, C B, as its axis. One half or arm, C E, of the lever, is a breadth of wire-gauze through which the current of air forces itself, causing a downward pressure and motion. The other arm, C G F, carries at its end a bent plate, G F (here seen edgeways), which, on being raised as the lever moves, becomes a shutter narrowing the air-entrance, H D.

The self-weakening action of the current would soon extinguish a fire dependent on it, if there were not a small weight, w, placed near the extremity of the arm at F, which counteracts, and by pressing down the shutter, F G, admits just air enough for its purpose, and the current then continues quite uniform until changed by lessening or increasing the weight, w, or altering its power by moving it nearer to or further from the centre of motion, C. It will be observed that the current blowing perpendicularly on the bent plate, F G, has no effect either to raise or lower it. This kind of valve may be

548

The self-acting Close Stove.

put on any close stove with much advantage, and it will be found a useful addition to all channels serving for ventilation.

772. The subjoined woodcut (fig. 183) exhibits in section the complete self-regulating, self-feeding close stove above referred to. The letters, A B C D, mark the external case, which prevents the intense heat of the inner stove,

K

e

e

A

Fig. 183.

16

B

a b c d, from damaging the air of the room. It is not allowed to

be hotter than a tea-urn containing boiling water.

F is the regulating valve, c and d, the fire-brick lining of the fuel-box.

H, the coal-reservoir or hopper containing coal enough to last for twenty-four hours, which falls down as the coal below is consumed. It is charged with coal through the lids, k and K, both of which are rendered air-tight by having their turned-down

rims or edges dipping into grooves filled with sand, at e e.

The burned air from the fire, M, rises up in the space between the hopper and the inner stove case, to pass away by the internal flue, x, into the other flue, X, of the outer case.

L is the ash-pit, G the ash-pit door,-ground close.

M is the fuel intensely ignited below, where the fresh air is entering to maintain combustion.

The fuel must be good stone-coal (anthracite) or coke, or a mixture in the proportion of one part of coke to two of anthracite. Common bituminous coal must not be used, since this produces gases which may cause an explosion by admixture with air.

The Mechanical Equivalent of Heat.

773. Within a recent period, certain means have been devised for the purpose of measuring accurately the expansive force of heat as a mechanical agent, and for determining many important relations existing between heat and the other forms of force operating throughout nature.

We may recall that to measure anything is to adopt some convenient amount or quantity of that as a unit of reference, and then

Mechanical Equivalent of Heat.

549 to find how often that unit is contained in any new quantity. Thus, if a convenient handful of some substance be called a poundweight, any other quantity of ponderable matter is accurately measured when the number of such pounds contained in it is ascertained. So if the average length of a human foot be called a footmeasure, any greater length is determined by finding the number of such feet contained in it. It follows that a foot square is a convenient unit for surfaces, and a foot cube for bulks. A measure for temperature is the length of the mercurial column of a thermometer, standing between the fixed points of freezing and boiling water, when divided into a certain number of equal parts, to be called unit degrees of temperature. Then for quantity of heat, as distinguished from temperature, a convenient unit is that quantity which just suffices to raise the temperature of one pound of water one degree of the standard thermometer. Lastly, in relation to the present subject, a convenient unit of force is that which can lift one-pound weight through one foot of height, -an amount now called a foot-pound. By referring to such standard units once chosen and generally agreed upon, all questions respecting the mechanical equivalents of heat and other forms of force can be satisfactorily answered. An example follows.

Fig. 184.

Let the outline, a b c d, indicate a vessel like the barrel of a pump or steam-engine, of a foot square in transverse area, having in it a movable close-fitting piston, gef. That piston, when one foot from the bottom, would shut up below it in the space, ef c d, just one cubic foot of air. The surface of the piston being an area of a square foot, or 144 square inches, will be bearing on its upper surface the atmospheric pressure of 15 pounds on every inch, in all 2160 pounds, or nearly twenty hundred-weight, which in a state of rest will be exactly balanced by the resisting elasticity of the confined air below the piston. Now experiment shows that by giving heat to produce 490 additional degrees of temperature in the air beneath the piston, the bulk or volume of that air will be just doubled, and it will gradually push up the piston to the line, a b, two feet from the bottom. Thus rising, the piston lifts a weight of 2160 pounds through one foot of height. If instead of the atmosphere pressing down the piston, there were a mass of