The boiler is entirely composed of stout wrought-iron tubes, having an inter nal diameter of an inch and three-quarters. The length or number of the tubes are, of course, arbitrary, depending upon the capacity of the boiler; in the instance before us, the boiler consists of 12 tubes; each individual tube is bent into a spiral figure of three turns, which are of equal diameter or breadth.
The first spiral tube thus formed, would contain within its coils a cylinder of 1 foot in diameter ; the second spiral tube is curved in a parallel line to the first, but of about four inches greater diameter, so that it will lie outside of the first, and exactly circumscribe it, leaving between the two a space of about an eighth of an inch. Each successive tube of the whole twelve is curved in like manner, the coils of the whole being equidistant, but the diameter of each separate spiral is in succession 4 inches greater than the preceding one. By this arrangement, it will be perceived, is produced a spirally inclined plane of tubes, winch are inclosed in a cylindrical case ; at the bottom of this the fur nace is situated, about one foot beneath the lowest ends of the tubes, and occupying the whole area of the circle. The upper ends of the range of tubes open into a strong receptacle, being secured in both in a thoroughly steam-tight manner, by means of hollow screwed bolts, with nuts and collars, in the follow ing manner :—A small tube is fixed to the end of each of the spiral tubes, and each of these small tubes is passed through the receptacle, and the shoulder formed by the ends of the large tubes are, with suitable packing interposed, brought, by means of screwed nuts, close up to the side of the receptacles : these receptacles are strong tubes, elliptical in their transverse section, and flattened at their conjugate axes, for the convenience of screwing up irmly. For removing the deposit from the water at pleasure, solid plugs are screwed into the ends of the small tubes, which can be taken out whenever required for that purpose. By the arrangement described, it will appear that the heat from the circular fire, about 4 feet 6 inches in diameter, impinges vertically upon a similar extent of the boiler above ; thence ascending the current winds round between the coils of the inclined plane of tubes, which form ing the fines as well as the boiler, the heat is abstracted in its progress, by an economical consumption of fire', through the small spaces between the con centric spirals, the heated air and flames escape out of the spiral current, and by completely enveloping the tubes, materially augment the production of steam.
To increase the combustion, an exhausting fan-wheel is placed immediately over the boiler, revolving on a vertical spindle, which passes through the centre of the boiler and furnace, and is actuated by suitable gear put in motion by the engine. To avoid the inconvenience that might be experienced from the escape of the gaseous products of combustion upwards, the patentees propose to dis pense with the use of a chimney, and by enclosing the upper part of the fan-wheel, and surrounding the boiler with an external casing, cause the vapours to pass downwards against the ground, underneath the vehicle. The patentees likewise propose to force a mixture of highly rarefied steam and heated air through the ignited air in the &mace along a pipe, that makes several coils around the ash-pit, before entering a chamber immediately beneath the great bars ; into this chamber by another pipe is also introduced steam, the mixed air and steam from this chamber proceed through a series of short vertical tubes and the hollow bare of the grate, and thence through certain perforated nozzles into the fire in minute jets.
In the centre of the boiler is situated the float chamber; this is of a cylindrical form as far as the boiler extends ; but the lower portion, which passes through the furnace, and the bottom of the grate, is tapered off to a reduced diameter, making the figure of an inverted frustrum of a cone ; to the lower end of this vessel is screwed an iron cap, and the upper end is closed in like manner ; passing through both these caps and the mmdle of the chamber, is a straight piece of tube fixed " stanch " to the caps by screwed nuts and packing.
tube is left open on the outside of the vessel at both ends, and through it passes the vertical spindle of the fan-wheel, at the lower ends of which ie a spur wheel, to which the motion is communicated. By this tubular passage, there fore, the central situation of the float chamber becomes no impediment to the last-mentioned operation, and the tube itself serves as a guide for the float in its ascent and descent. The float I. a hollow air-tight copper vessel, having an opening, to let the tube pass to its centrgto which it is kept ba vertical rod (fixed to the bottom of the float) that passes through a in the lower cap, beyond which it is connected to a lever that operates upon the steam-cock of the engine, by which the pump is worked that feeds the supply pipe.