This stove had a development which lasted about a half century before it was stopped by the preponderating advantages of the brick stove, and in this interval a type of construction was evolved which was almost universally used. This consisted of a large red-brick chamber lined with firebrick, beneath the brick floor of which passed horizontal pipes with nozzles or connections on their top surface, and a vertical partition in their central plane. On these were arranged U-shaped pipes reaching practically to the top of the chamber, through which the air had to pass on its way from the blowing engine to the furnace; short-circuiting of the U-pipes was prevented by the central vertical partition. Around these pipes burned the furnace gas entering at one end and passing through the rows of pipes and through the baffle walls to the other end, where at last it passed out of the stack.

An excellent design of this type of stove is shown by Figs. 126, 127, 128 and 129, which illustrate a stove built by the Longdate Iron Company at its plant at Longdate, Virginia, under the designs of Mr. H. Firmstone, the president of the company. Fig. 126 is a vertical longitudinal section, Fig. 127 a vertical transverse section, Fig. 128 a sectional plan, Fig. 129 a detail of the U-pipes.

It will be seen that upon a masonry foundation rests a series of cast-iron bedplates strongly bolted together and planed on their top surfaces. Upon these rest the bed pipes, square in cross section, with a central vertical partition and with three lateral nozzles on each side, in addition to the nozzles for the pipes on top.

None of these bed pipes have any connections on their ends except that the outer chamber of the bed pipe at one end is prolonged into an inlet nozzle, and the outer chamber of the pipe at the opposite end is similarly prolonged into an outlet nozzle. Of course, these end chambers are without the lateral nozzles which connect the chambers to one another. The U-pipes are all of elliptical section in order to get a maximum of heating surface for a given area, and the nozzles on the bed pipes are similarly elliptical in section and of the same size as the end of the U-pipes, these ends being enlarged by a sort of collar cast on them which projects outside the body of the pipe about a half an inch, and which fills the spandrel between the two ellipses to a straight line, this is the shape of the exterior of the nozzles on the bedplates also.

It is perfectly obvious, on account of the central vertical partition in the bed pipes, that there is no passageway for the air except through the U-pipes, and the bed pipes being joined in series, with the U-pipes in parallel, the air passes through an entire set of U-pipes on one bed pipe as a single unit, thence to the next bed pipe, and through all the U-pipes on that simultaneously, and so on.