Under the article Stove, the different means of heating apartments, both directly by stoves, and also by throwing in a supply of warm air, have been already noticed; and under the article Steam, the means of using it as a source of heat for warming buildings and fluids, and also for drying articles, have been fully described. There is still another method of warming to which we have to advert; we mean by water. In considering the subject of heat, its communication from one object to another, and the mode by which it is conveyed through fluids, have been fully discussed; but before proceeding to describe the process of warming by water, it may be proper to allude to the circumstances accompanying the distribution of heat in this way.

When heat is applied to the end of a bar of iron, it passes from particle to particle, and the whole of the bar would in this way become warm, were it not for the operation of other causes. But the process of heating a fluid is different. When heat is applied to the bottom of a vessel of water, the particles below, as they receive caloric, are expanded, become specifically lighter, and ascend; cold particles must therefore fall to supply their place, which in their turn gain caloric and also rise; and in this way, by the constant ascent of warm and descent of cold particles, the whole of the fluid is heated. The communication of heat in this case is therefore not from particle to particle, as in solids, but by currents.

If, instead of a jar, an apparatus of the form fig. I be used,

![Fig. 1]

the currents will be established in \(ab\) when it is heated, and water will be found to flow from one vessel to another, owing to a difference in the gravity of the water in the different parts of the apparatus. Suppose that the commencement that the water is at 50°, and heat is applied to the bottom of \(ab\). Owing to the currents, the fluid in it becomes warm, and is therefore of less specific gravity than before, while that in \(cd\) continues cold, and is consequently not altered in gravity; the pressure on \(c\), occasioned by the column \(de\), is therefore greater than that on \(b\), occasioned by the column \(ab\). There is therefore a movement towards \(b\) corresponding to the difference in gravity, and consequent difference of pressure, on \(c\) and on \(b\). As the water flows along \(c\) into \(ab\), there must be a flow along \(ad\) into \(cd\), and this current in the circuit \(a\), \(d\), \(b\) will continue as long as there is any difference between the temperature of the water in \(ab\) and \(cd\). Now if \(ab\) have heat constantly applied to it, and \(cd\) be constantly parting with it by exposure to a colder atmosphere, there must be kept up a difference in temperature, and thus the current will be continued; and the heat which the water in \(ab\) receives will be given off during the circulation of the fluid along the other parts of the apparatus. Hence the mode of conveying heat generated by combustion to distant parts, as through the rooms of a house. The method to be followed will obviously depend on the construction of the building and the purposes for which the heat is required. If the whole of the apartments to be warmed are on the same level, the apparatus is very simple. At a convenient part of the building an open boiler is erected, and so situated that a fire can be kindled under it. From the upper part of the side of the boiler a tube, as \(ad\) in fig. I, passes along the floor or near the floor, and is made to traverse on the same level to the distant parts, and again to return on the same level, and, after making a turn downwards at any convenient part, to enter the side of the boiler near the bottom, as at \(b\), fig. I. In this way the heat which the water in the boiler receives from the fire is conveyed by the fluid travelling along the tubes, and in its passage is communicated to the surrounding atmosphere of the apartment. The more rapidly the heat is abstracted from the tubes, the greater will be the difference between the temperature of the water in the boiler and in the other parts of the apparatus; the more rapid therefore will be the current, owing to the difference in gravity, and consequently the more abundant will be the supply of heat to the apartment.

In the method now described, the boiler is an open one, being placed on it to prevent loss of heat by evaporation. The tube conveying the warm water cannot go above the level of the fluid in it, and hence this form of apparatus is restricted in its application. It is well adapted for hot-houses, for churches, and buildings with all the rooms on the same floor. When the apartments are situated above another, such as in common dwelling-houses and manufactories, the boiler must be a closed one, so that the water may be transported to a greater height. Numerous forms of apparatus have been recommended, and are now in use for this purpose. Suppose that the boiler is situated in a room on the lower part of the building, and that not only the apartments on the same level, but also those above, are to be warmed; all that is required is to carry a pipe from the upper part of the boiler through the different rooms, and after traversing these, to make it enter near the lower part of the boiler, as in the apparatus already described.

Thus \(a\), fig. 2, being the boiler, the water in it, when heated, will flow in the direction \(a b c d e f g h\), and again in the boiler at \(i\). If we suppose the pipes to traverse different apartments, then each apartment will receive heat from the water in its passage through it.

From what has been said of the currents in the fluid, it is evident, that as the last process there must be a greater difference between the gravity of the water in different parts of the apparatus, the current must be more rapid, and consequently the heat from the original source must be more quickly carried away. There is evidently a limit to this; for when the height becomes great, the pressure on the pipes is greatly increased, which requires a corresponding strength in the materials, and a solidity in the junctures of the different parts of the apparatus. Not that there is any danger from the formation of high-pressure steam, so as to cause explosion; but should the pipes be faulty at any part, there would be an escape of hot water, which, under the pressure given by the high column, would rush out with great force.

By using the shut boiler there is another advantage gained, namely, that of being able to carry the water downwards, and again upwards, at any particular part where it may be necessary, and, what is of still greater consequence, to convey it to a level above the boiler. In constructing an apparatus for this purpose, it must be borne in mind, that when the fluid has to rise after having descended, there is always a tendency to a retrograde motion, owing to the difference in pressure at different parts. Thus in a system of boiler and tubes, as in fig. 3, the ascent of the water from \(a\) to \(b\), and its descent from \(g\) to \(h\), and its return to the boiler, go on as in the other forms. At the same time, however, when the fluid flows from \(c\) to \(d\), as that in \(f e\) is colder, and consequently of greater gravity, than that in \(c d\), there is a tendency to cause a motion back again in the direction \(f c d e\), and this will actually occur unless the descending force in \(g h\) be greater than that in \(f e\). This is obtained by taking care to have the ascending pipe of sufficient height, and thus to cause a sufficient difference in the temperature, and consequent gravity, of the water, in the pipe passing from the boiler, and that in the descending tube \(g h\). Much must depend on the size of the pipes, and the heat to be given off by the water in its passage along them; for, as already stated, the greater the difference between the temperature of the fluid in the ascending and descending tubes, the greater the difference in gravity, and consequently the more rapid the current. It has been found, that when the pipes are not to ascend after having descended, a sufficient current can in general be obtained when the exit from the boiler, as in fig. 1, is distant from the entrance to it about sixteen inches. When the pipe ascends after dipping, then the height to which the pipe ascending from the boiler must be carried should be in proportion to the descent and corresponding rise. Thus suppose, in fig. 3, the ascent from \(e\) to \(f\) is twelve inches, then the perpendicular height from \(i\), where the water returns to the boiler, to \(b\), ought to be twenty-eight; that is, sixteen inches greater than \(e f\), or thereabouts; because, as already stated, the other circumstances affecting the current will cause a difference in the requisite height of the tubes.

By carrying the ascending tube to a considerable height, it thus affords the means of constructing an apparatus in which the ascent, after the pipe has dipped, may be much greater; and thus it is also that the pipe may be carried below the level of the boiler itself. In causing water to flow in an apparatus similar to that of fig. 4, a great deal of heat must be lost by the coil, owing to the great surface exposed; and the water will therefore become in it proportionally of great gravity compared to that in \(a b\). At the same time, however, there is a tendency to a back-flow from \(e\) to \(d\), and also in the lower part of the pipe \(i h\), owing to the difference in temperature and gravity of the water in it and in \(k l\). But if, owing to the great loss of heat from the coil, the gravity of the whole fluid in \(e f g i\), be greater than that from \(k\) to \(b\), then the flow will be established and kept up as long as the abstraction of heat from the coil is continued. It is evident, that by varying the form of an apparatus of this kind, water may be conveyed to the different parts of a building, though some of them are below the level of the boiler.

Different forms of boilers are used when the water is confined in a closed apparatus. Much must depend on the situation and on the particular purpose to which the system of heating is to be applied; and the same remarks are also applicable to the construction of the furnaces. When the boiler is a shut one, it is necessary to have a cistern to supply it with water, which must be placed above the highest part of the tubes. The most convenient place... Warming for the entrance of the pipe from it, is immediately before the water returns to the boiler; it is therefore generally connected with the return pipe just before it pierces the boiler.

It is well known that the expansion of iron by increase of temperature is considerable. The pipes must therefore be allowed free movement, so as to admit of expansion, otherwise they will give way. It is also necessary to have some contrivance for securing the escape of the air from the pipes, because, if left in, it impedes, may sometimes prevent altogether, the current of the water; and hence the necessity of having an opening in the highest part of the tube, by which not only the air in them, but also that disengaged from the water when heated, will be expelled. The part at which this aperture must be placed will depend entirely on the form of the apparatus. In some, one opening will be sufficient, but in others two or more will be required; for though the air has a tendency to pass to the highest points, yet, from the difference in the levels, it may accumulate at different places. Thus, in fig. 4, it will be found at b and e, and hence at these points small tubes of a few inches in length, open at top, ought to be placed.

In addition to the forms of apparatus now described, there is still another now much in use, called the High Pressure apparatus. It consists of a coil of tubes, fig. 5, a b, about an inch external, and half an inch internal diameter, which is the part of the apparatus that is surrounded by the fire, and of course acts instead of the boiler. From the upper part of this the ascending tube b e proceeds, and from it the others to the different parts of the building, and is then conveyed to the bottom of the coil at a, thus making a continued circuit. In this form of apparatus, allowance must be made for the expansion, not only of the tubes, but also for that of the water; a larger pipe is therefore attached to the highest point, which is generally vertical, as e f. This tube being at first open, is employed for filling the apparatus; the water being poured in till it rises a little way in it, as, for instance, to the dotted line i k, after which the aperture is secured by a cap and screw. Instead of filling the apparatus through the expansion tube, a small one is sometimes attached to the highest part, as n o, after which the screw on it and on the safety-pipe are put on. By using this smaller tube, there is no risk of making the water too high, as it cannot rise above the dotted line i k.

In an apparatus of this kind, due attention must be paid to the strength of the tubes; because, as a strong heat is applied to the coil to raise the water beyond 212, were the tubes not of sufficient strength, they would be burst by the tendency to form steam of high elastic force. Thus the temperature of the coil is sometimes 500, at which the pressure on the inside of the pipes is upwards of 1000 lb. on the square inch; hence the necessity of having the pipes to be used proved to a pressure beyond this. Those employed are generally warranted to upwards of 2000 lbs. When an accident does occur, it is usually owing to the coil giving way, partly from its resisting force being weakened by the high temperature; partly from the action of the water on the iron, by which its properties are altered.

This high-pressure apparatus, though it has many advantages over the other forms, has also some disadvantages. The water in the coil can be heated to a greater degree than in a boiler, consequently more heat can be taken from the fire and conveyed to the apartments; but this, it is generally allowed, is done at a comparatively greater cost of fuel.

The system of heating by the transport of water from one place to another, as now described, has advantages which it is supposed recommend it above all others. It is well adapted for warming the air in large and extensive buildings, such as manufactories; and also for creating a requisite degree of heat for particular purposes, as in paper-making, drying of gunpowder, and many others. As yet, however, it remains to be proved whether it is equally economical with the more easy method of throwing into the apartment a large supply of air, moderately warmed, as is done by the means already described under the article Stove. The apparatus itself is costly, and is more troublesome to fit up. At the same time, however, it must be admitted that a uniform temperature can be more easily and more permanently maintained by it than when air or steam is used as the heating medium, owing to the quantity of heat in water being great compared to that in the others. When the water is once warmed, it retains its heat for a long time. When uniformity of temperature is a great desideratum, the water system has therefore its advantages; such as in hot-houses, where it is of the utmost importance that, after the temperature is once raised, it should not be allowed again to sink, or to sink rapidly, which may be the case when an irregularity occurs in the fire of a hot-air or steam apparatus. With regard to the expense, we may state, that the cost of erecting a high-pressure apparatus used for heating a large ware-room amounted to L70. The furnace is placed in a room below; and from the coil the ascending tube proceeds directly to the apartment above, and from it two tubes pass horizontally in different directions, and after traversing the room, return in the same direction, and join near where they took their origin, and then descend to the coil. The apartment is ninety-four feet in length, twenty-five in breadth at one place and forty-four at another, and varies in height at different places from fourteen to twenty-five feet. The cubic contents are in all very nearly 50,000 feet. The door opens directly to the street. Small coal is used, at five shillings and sixpence per ton, which lasts for two weeks, say for twelve days, at an expense therefore of about sixpence per day, by which the temperature is kept easily at 57°. Though this is a trifling expense, yet it is not less than would be required for a hot-air stove for the same apartment, while the original cost of the apparatus is much greater.