Passing now under the seventh cylinder, the course of the flue is over the sixth, under the fifth, over the fourth, under the third, over the second, and partly over and partly under the first, when it enters the chimney. To produce this long serpentine reverberation, the upper and under side of the brick-work is formed in arches, alternately reversed, their extremities abutting against the tubes, covering about a sixth part of their circumferences, but exposing the rest to the action of the current of heated matters from the fire. Each of the tubes is provided with a flanged disk at one end, fastened on by screw bolts, that they may bo easily removed, and the tubes cleaned out at pleasure from sediment and incrustations. The water carried oft* by evaporation is replaced by the usual means of a force pump, and the steam generated is conducted to the engine or other object, by a tube connected to the steam reservoir. "

It may not be improper," says Mr. Woolf, " to call the attention of those who may hereafter wish to construct such apparatus, to one circumstance: namely, that in every case, the tubes composing the boiler should be so combined and arranged, and the furnace so constructed, as to make the fire, the flame, and the heated air, to act around, over, and among, the tubes, embracing the largest possible quantity of their surface. It must be obvious to any one that the tubes may be made of any kind of metal; but l prefer cast iron as the most convenient The size of the tubes may be varied; but in every case care should be taken not to make their diameter too great; and it must be remembered, that the larger the diameter of any single tube in such a boiler, the stronger it must be made in proportion, to enable it to bear the same expansive force as the smaller cylinders." Mr. Woolf also directs that the lower tubes should be always kept filled, and the upper, or steam cylinders, half filled with water, that is, as high as the fire is allowed to reach, and that in no case the water ought to be allowed to get so low as not to keep full the branches which join the lower tubes to the upper cylinder.

The annexed engraving exhibits an arrangement of parts combining the leading features of several previous inventions, and was patented by Mr. Thomas Tippet, of Gwennap, in Cornwall, in 1828. Fig. 1 represents an end view, and Fig.2 a side view of the boiler; the same letters of reference in each indicating similar parts, a is a large cylindrical boiler, containing an internal cylinder, which constitutes the fire place and principal flue. From the external cylinder, which contains water, proceed three rows of open vertical pipes b b b, which support a semi-cylindrical steam vessel c. At the farthest extremity of the cylinder a there proceeds horizontally a short open pipe d, communicating with a small supplementary boiler, which is a cylinder of the same area as a, but very short. This boiler is built in a furnace, in which the flues are so arranged that the heated air, in passing out at the end f of the furnace flue, shall impinge against the flat side of the supplementary boiler; the flue thence proceeds upwards, and along the underneath flat side of the semi-cylindrical vessel between the vertical tubes to the front of the boiler, where it descends, passing under the bottom of the latter, then round the back of the supplementary boiler, and over the top of the semi-cylinder to the chimney, which is in front, nearly over the furnace doors.

Fig !.

Fig. 2.

An ingenious attempt to expose water in thin sheets over an extended surface of metal, was made by Mr. John M'Curdy, from the United States, who patented it in this country. It consisted of a series of cylinders, with spherical ends, arranged horizontally like retorts, in a pyramidal form, in a furnace, so as to cause the heat as much as possible to impinge against their surfaces, in its ascent amongst them to the flue above. Each of these cylinders contained within it another cylinder, of so much less diameter than the outer, as to leave between them throughout a very narrow space, the uniformity of which space was preserved by coiling a spiral band upon the outside of the inner cylinder, or other suitable contrivance. These latter cylinders were hermetically closed at each of their ends, and were placed inside the former to produce between them hollow cylindrical sheets of water. The water was forced into one of the lower cylinders, and made, by the action of the pump, to circulate through the others of the series.

By this arrangement of disposing the water in thin sheets, it was presumed that steam of a very high pressure would be generated with extraordinary rapidity.

We have, however, never been informed of the cause of this boiler not having been brought into practical operation, and are therefore left to conjecture that it was probably owing to the great expense of construction; the liability to deposits and incrustations in the narrow spaces between the internal and external cylinders; the difficulty of cleansing them; and, by the neglect of the latter operation, causing an irregular generation of steam, the heating red hot the incrusted parts, and, as a consequence, the blowing out of the water, and the rapid destruction of the metal. To these causes may be added the waste of fuel by the shortness of the flue. The contrivance is, nevertheless, not devoid of merit, and may afford a useful hint to succeeding inventors The superior strength and safety of boilers made of small tubes have within the last few years led to their introduction in almost every possible variety of form, a great many of which have been the subjects of unproductive patents.

Those which possess the most distinct character from each other, and have been more or less brought into public use, we shall proceed to notice.