[Illustration:  Fig. 6.]

2. Stoves heated by hot-water pipes.—­Let us first of all consider the principle on which these are constructed.  In Perkins’ apparatus for conveying heat through buildings by the circulation of water in small-bore hot-water pipes an endless tube or pipe is employed, the surface of which is occasionally increased by spiral or other turnings where the heat is to be given off or acquired:  the annexed figure may serve to illustrate this principle; it represents a strong wrought-iron tube of about one inch diameter completely filled with water; the spiral A passes through a furnace where it is highly heated, and the water is consequently put into motion in the direction of the arrows; the boiling of the water or formation of steam is prevented by the pressure, whence the necessity of the extreme perfection and strength of the tube.  B represents a second coil which is supposed to be in an apartment where the heat is to be given out.  C is a screw stopper by which the water may be occasionally replenished.  By this form of apparatus the water may be heated to 300 deg. or 400 deg., or even higher, so as occasionally to singe paper.  A larger tube and lower temperature are, however, generally preferable.[1]

[Illustration:  Fig. 7.—­Enamelling Stove—­in a Tin-plate Printing Factory—­heated by Perkins’ Hot-water Pipes.]

The principle of Perkins’ invention has, during the last eighty years, i.e. since the date of the invention in 1831, been very extensively applied not only for the heating of buildings of every description, but it has also been utilized for numerous industrial purposes which require an atmosphere heated up to 600 deg.  F. The principle lends itself specially to the design of apparatus for raising and maintaining heat evenly and uniformly, and also very economically for such purposes as enamelling, japanning, and lacquering.

The distinctive feature of this apparatus when applied to moderate temperatures lies in the adoption of a closed system of piping of small bore, a certain portion of which is wound into a coil and placed in a furnace situated in any convenient position outside the drying chamber or hot closet.  The circulation is thus hermetically sealed and so proportioned that while a much higher temperature can be attained than is possible with a system of pipes open to the atmosphere, yet a certain and perfectly safe maximum cannot by any possibility be exceeded.

The efficiency of the apparatus increases within certain limits in proportion to the pressure employed, which fact explains the exceedingly economical results obtained, while the fact that, owing to the high temperature used, a small-bore pipe can be made more effective than the larger pipes used in any open system, accounts for the lower first cost of the Perkins’ apparatus.

[Illustration:  Fig. 8.—­Japanning and Enamelling Oven Heated by Single Hot-water Pipes sealed at both ends with Furnace in Rear.]