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° or 400°, 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° 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