The tubes are provided with screw plugs, so as to be conveniently opened when it is required to fill the pipes with water, and closed again after being filled. This can be done with facility by a servant. The circulation of the water is produced by the application of heat to the coil in the furnace; and as the small size of the pipes admits of presenting the largest possible amount of surface to the action of the fire, it is clear that a greater economy of fuel is effected by it than by the ordinary system of boilers. As the water becomes heated it rises immediately to the highest level of the circulating pipes, and thus forms a column of heated water, specifically lighter than the colder water, which descends to the lower part of the coil. Thus a circulation is effected throughout the whole course of the pipes,[D] which eventually become heated, and the whole may be regulated exactly to that degree of temperature which is most conducive to a beneficial effect.
To regulate the degree of heat to be given to the tubes, without requiring the necessity of an attendant, advantage has been taken of the expansive property of the iron pipe when heated. There are three multiplying levers fixed in a box, and so placed that the short arm of one of the levers rests upon a regulating screw attached to the flow pipe. On the other end of the series of levers a rod so rests that upon the slightest movement of the levers, the damper in the flue, which is attached to the rod, is opened or closed, as the case may be. The box of levers is suspended from the hot pipe, so as to leave about two feet in length between the point of suspension and the point of contact with the short arm of the lever.
The operation of this arrangement is obvious, for the instant the pipe becomes heated, it expands and presses the short arm of the lever; and as the fulcrum within the box cannot move, by reason of the rod which suspends it being cold, it follows that the lever must be depressed, by which action a sufficient motion is given to the damper, to close it at any given temperature at which it may be originally fixed.
The great advantage in the use of this apparatus is the saving of time in obtaining the requisite degree of heat. It often happens that the time occupied in heating the water of an ordinary hot-water apparatus completely defeats the object of getting warmth in any reasonable time, particularly in greenhouses, where it is frequently desirable to get up the heat quickly, to prevent the effect of frost. It has been said that this property of generating the heat rapidly has the disadvantage of not being able to retain it: this, however, is not the case, for, on the contrary, an equal temperature may be maintained for any length of time that may be desired. It is only necessary to make the fireplace sufficiently large to contain fuel enough to last the time the heat is required to be continued, and the damper will regulate the combustion of the fuel and the heat of the pipes, so that there will be no variation for twelve hours together.
There being no boiler to the apparatus, it is free from the ordinary danger of explosion; if a pipe by possibility should burst, no harm ensues, for the water escapes from so small an aperture that it becomes absolutely cool by its expansion and mixture with atmospheric air.
So little fear of fire exists with the apparatus, that the directors of the principal fire offices readily accept, at the lowest rate of premium, all proposals for the insurance of buildings in which the system is adopted, not requiring even the customary inspection.
The author made drawings of one of these apparatus put up in an ornamental greenhouse in Kew Gardens in 1844; and fourteen years after, the director of the garden, Sir W. J. Hooker, publicly allowed it to be stated in print that no hot-water apparatus in any of their houses had given so much satisfaction; that the heat was given out after lighting the fires more rapidly than in any other of their houses, and steadily maintained at any degree of temperature required. The two systems of the high and low temperature can readily be combined, and the temperature of both large and small tubes nearly equalized. This may be done by using one furnace. A diagram given by Dr. Arnott in a lecture delivered by him at the Royal Institution in March, 1836, with his explanation, will show the principle upon which the combination is effected. Suppose A, fig. 1, is a cistern full of cold water, and B a cistern full of hot water: if the two cocks c c are unturned, it is a fact that the water at d will be one degree of warmth only above the water at
Fig. 1.
A, and the water at e will be of one degree less temperature than the water in B. If, therefore, on this principle, some of the pipes of the high-temperature system are passed through the large tubing of the low temperature one, the desired effect is obtained: the large pipes or tablets of one apparatus remain at their full heat, while an additional quantity of inch pipe of sufficiently warm temperature is obtained, that can be carried into rooms and placed in situations into which the warming surfaces of the low-temperature system could not be made to approach.