It is covered by a strong plate of iron bolted down, in which plate is usually fitted the atmospheric or vacuum safety valve z, which opens inwards by the pressure of the atmosphere, preventing the latter force from injuring the boiler, should a vacuum be formed by condensation within. A cock and pipe leading from the bottom of the boiler is employed for discharging it of its contents. Next to the waggon-shaped boilers of Watt, those of a long cylindrical form have been the most extensively employed, especially for high-pressure engines, on account of their superior strength; besides as affording a considerable surface for the direct action of the fire and heated gases. They are usually known in this country by the term " Trevithick boilers,' from the supposition that Mr. Trevithick was the inventor. We, however, observe, that Mr. Oliver Evans, of America, describes them as being used by him in his high-pressure engine, prior to the patents of Mr. Trevithick, and as Mr. Evans does not claim them, we may suppose they were in use before his time.

Cylindrical boilers are frequently made of a great length, ten, twelve, or more times their diameter, and are preferable of such proportions, where their situation will admit of it, for the reasons before mentioned.

In this country their extremities are usually made hemispherical; but in America the ends are usually closed by flanged disks of great thickness, on account, we suppose, of the greater facility of construction by ordinary workmen. These boilers are generally provided with an internal flue, through which the heated air and flames, after traversing the length of the under side of the boiler, pass before entering the chimney. The annexed diagram represents a transverse section of a small high-pressure cylindrical boiler, as manufactured by Mr. Saunders, late of Sheffield, but now of New York, U. S. The furnace door is at a; b the furnace , c the ash pit; d the hoiler; e the stone float (shown by dotted lines), for regulating the supply of water, by means of the counterpoise g; f is the internal flue, which returns the flame and heated air through the boiler; h the steam pipe leading to the engine.

Fig. 1.

The boiler figured in the annexed cut, is of a cylindrical figure in the lower part but combines some peculiarities that are deserving of notice. It is a patented arrangement by Messrs. Horton and Fisher, who are large boiler manufacturers, near Birmingham. The object of it is to form a reservoir of steam within the boiler, surrounded by the hot water, in order that the pressure of the steam may not be reduced by radiation, which the inventors presume to be the case in a greater degree in boilers of the ordinary construction. Fig. 1 fig. 2.

Fig:1.

represents a longitudinal section of the boiler, and Fig. 2 a transverse section of the same: the letters of reference apply to similar parts in each view, a a a shows the external form around which, the furnace and flues are to be constructed; b b b is the internal vessel or reservoir, for containing the steam generated, surrounded by the water which is supplied by the tube o o from another reservoir placed above, but not introduced into the drawing. The heat having caused the steam to fill the upper part of the boiler d, it passes thence through the bent tube c into the steam reservoir b below, from whence it is conducted to the engine by the steam pipe e, the top of which n is designed for the situation of the safety valve. At f is a cock for drawing off whatever water may be condensed in the steam chamber, and at k may be placed another for discharging the boiler. At 11 are man-holes for gaining access into the interior. We do not ourselves perceive how the intention of the patentees to make the internal vessel b b a store of high steam for the supply of the engine, can be effected by this arrangement, for the hottest steam will be rather disposed to obey the laws of nature, and ascend into the upper vessel through the bent tube, than descend through the same to the lower vessel, according to the desire of the contrivers.

It must, however, be admitted that there is a considerable degree of originality in this boiler, and it may prove a useful and effective generator of steam. In 1803, Mr. Woolf patented his boiler, which has obtained much deserved celebrity.

It has been for many years very extensively and successfully employed in Cornwall, for the production of steam for the large mining engines there. We have already observed that the long cylindrical boilers possess great advantages over those of a cubiform or rotund figure. To increase their safety, and their capability of producing steam at very high pressures, Mr. Woolf greatly extended the principle of the cylindrical form. One of the most simple of this gentleman's construction consists of eight tubes of cast iron (of six or more inches in diameter), connected to each other by a bent tube at their extremities, with communications to a larger cylinder above them, employed as a reservoir for the steam. The furnace is divided by a wall longitudinally into two parts, and the eight tubes are fixed horizontally across both these. The fuel chamber is at one extremity of one of the divisions, and arched above the two first tubes «o as to reverberate the flames and heated vapours, which then pass under the third tube, over the fourth, under the fifth, over the sixth, under the seventh, and partly over and partly under the eighth tube, when the flue turns into the second division of the furnace, on the other side of the wall, built under, and in the direction of, the large steam cylinder before mentioned.