PRESSURE, TEMPERATURE, AND RELATIVE VOLUMES OF STEAM TO THE BY THE COUNT 85 STEAM BOILERS. Ir would be impossible to determine, with any degree of accuracy, the period at which a closed boiler was first employed for generating steam. Various allusions are made by ancient writers to steam; but it is a matter of doubt whether or not this expression signified watery vapour. About 150 years B.C., Hero of Alexandria wrote his treatise on the "Inventions of the Ancients," and, among other matters, described a fire-engine pump. The boiler employed appears to have consisted of a simple hollow sphere exposed to the action of an open fire. The merit of economising fuel by means of an improved boiler is apparently due to Papin, who from the year 1680 to 1709 was occupied in the application of steam to various useful purposes. This physician and engineer constructed a syphon boiler, and placed the fire in the shorter end, thus making the draught pass downward; but by this means the fire-bars were so rapidly destroyed, from the high degree of heat to which they were subjected, as to cause him to abandon this system. He subsequently made another boiler 8 feet by 5, with a tubular flue 24 feet in length by 10 inches square, so bent as to pass four times through the water, and by this means effected a saving of about 75 per cent. of fuel. The boilers employed both by Savery and Newcomen, were inferior to Papin's. In one of Savery's engines, erected at Kensington in 1711, the boiler was spherical; but in other machines it is shown with a flat bottom, having a circular or dome-shaped top. This was altered by Newcomen for a haystack boiler which was partially exposed to the air; but Beighton afterwards thoroughly encased the boiler in masonry. Subsequently to this period various new boilers and generators appeared. In 1741, Payne proposed a balloon-shaped steam-generator to produce spheroidal steam, which is said to have evaporated 8 lbs. of water by 1 b. of coal. About this time, also, Fitzgerald attempted so to combine steam and air in a boiler as to effect economy of fuel. In 1759, Brindley attempted to prevent the loss of heat from external radiation, by introducing boilers of stone and wood, with cast-iron fire-places and flue-tubes. The bottom was usually composed of stone, and the top of wood; whilst the arrangement of fire-tubes rendered its principle somewhat analogous to the modern locomotive boiler. The tubular boiler was invented by Blakey, in 1766. In this apparatus water was contained in a series of small tubes passing through the furnace, over the fire, and around which the hot air and gases acted in their passage to the chimney. Smeaton's experiments enabled him to evaporate 7.88 lbs. of water with 1b. of coal. This engineer determined the evaporative values of different varieties of coal, and placed the feed-pipes within the flues of the boiler. In addition to this, he attempted to improve the evaporative power of his boiler, by causing the hot gases to operate upon it through a long circuit of flues before entering the chimney; but he found the draught so impaired by this means, that more was lost by imperfect combustion of the fuel than could be gained by the greater length of flue. The boiler adopted by Watt was of a rectangular form, the upper portion being semi-cylindrical, the sides concave, and the bottom flat. The heated gases from the fire-place traversed the sides and bottom of this apparatus, whilst in the larger boilers a central flue passed through the middle. From 1784 to 1804, Oliver Evans was occupied, in America, in extending the usefulness of the steamengine, and promoting the employment of high-pressure steam. Among other inventions, he introduced the cylindrical boiler with an internal tube, and constructed a flue under the apparatus. He further economized heat by passing the exhaust-steam through a spiral pipe placed in the feed-cistern; thus elevating the temperature of the feed before it entered the boiler. In 1806, Count Rumford presented the model of a boiler to the French Institute, the body of which was drum-shaped and comprised a vertical cylinder 12 inches in diameter and 12 feet in height. This was closed at the extremity by circular plates of iron. In the centre of the upper plate was a cylindrical neck, 6 inches in diameter and 3 inches in height, closed at the top by a plate of copper, 3 inches in diameter and 1 inch in thickness, fastened down by screws. The flat circular bottom of the body of the boiler was also pierced by seven holes, each 3 inches in diameter. Seven cylindrical tubes of thin sheet copper, 3 inches in diameter and 9 inches long, closed at the lower ends by circular plates, were fixed in these holes, firmly riveted, and then soldered to the bottom of the boiler. As these tubes were charged with water, and descended from the bottom into the fire, they were consequently surrounded on all sides by flame it follows that the water contained in the boiler was evaporated by the consumption of a comparatively small proportion of fuel. When large volumes of steam were required, it was considered advisable to employ several of these boilers heated by separate fires, instead of using one large boiler heated by a single fire. This idea of constructing a boiler with cylindrical tubes, forming a : magazine for water and steam, was amplified by Woolf, who was occupied with this subject from the year 1796 to 1815. His first project embraced six, eight, or more metallic tubes, placed transversely across the fire-place and flues, connected by short necks with a main receiving-pipe, set immediately over them at right angles. The fire acted directly on the first three tubes, and the products of combustion then passed alternately over one tube and below the next into flues through which the transverse tubes extended, before entering the stack. Where high-pressure steam was required, he fixed two other smaller cylinders, one on each side, parallel to the larger one, and made the main cylinder communicate with the smaller one. Another form adopted by Woolf consisted in substituting longitudinal for transverse water-tubes, making the fire to act under them, and the heated gases to pass between the principal cylinder and upper surface of the tubes. The main generator did not usually exceed 4 feet in diameter, whilst the tubes ranged from 10 to 14 inches. This boiler is known in the manufacturing districts as the " French boiler," and is represented in the drawing of apparatus for superheating steam. Another form of boiler ascribed to Woolf, consists of two cylinders above each other, the upper being partially filled with water, whilst the lower is entirely filled. About 1804, Trevithick proposed the use of the tubular boiler for generating high-pressure steam, and towards the close of the year arranged to erect one of them in South Wales. It was intended to be 24 feet long; outer shell 7 feet in diameter, fire-tube 4 feet 4 inches in diameter at the wide end, diminishing to 1 foot 9 inches at the smaller end. In this form of boiler, the fire is introduced into the large end of the tube, and the heated air made to circulate through it, then under the boiler, and lastly along its sides. In 1812, a boiler of this kind was made for Wheal Prosper, and since that period its use has not only become general in Cornwall, but also in London and the manufacturing districts. Where fuel is abundant an "egg-end" boiler is sometimes employed. Its form is that of a cylinder with spherical terminations. The fire is applied beneath, and flues are formed around it for the circulation of the heated products of combustion. A modification of Trevithick's boiler has been patented by Mr. Fairbairn of Manchester, in his "double-flued and double-furnaced boiler." Two fire-tubes are inserted into the shell, which has a semi-concentric flue divided at the bottom by a partition of brickwork assisting to support the apparatus. A combination of the tubular and flue boiler has also been invented by Messrs. W. and J. Galloway. This is formed of concentric cylinders with a series of one of vertical water-tubes, slightly conical, joining the top and bottom of the smaller cylinder. Watt justly remarked that boilers should be so constructed as to economize the fuel as much as possible, and that it is not either the shallowness or depth of the apparatus that produces this effect; but that they should be constructed of such a shape as to absorb the greatest possible amount of the heat generated by combustion, previous to its entering the stack. This inventor also gave the following rule for determining the value of boilers:- "To determine practically the superior economy boiler over another, the quantity of water evaporated per pound of coals should be ascertained, and, where experiments are made upon the same boiler, the commercial value of different varieties of coal may be most accurately determined." Although numerous modifications have been made in the construction of boilers, yet two or three only have been extensively employed in mining practice. It is well known that Watt was an advocate for the employment of low-pressure steam, and adhered to the waggon-shaped boiler; whilst Trevithick and Woolf preferred steam of a higher tension, and adopted boilers with cylindrical tubes. The economic advantage resulting from employing high-pressure steam is now admitted by a large number of engineers, and consequently the waggon-shaped boiler is gradually getting into disuse, and will probably be ultimately abandoned. The dimensions of a plain 20-horse boiler of this construction, without internal flue, and under which the consumption of fuel was very moderate, were, length 20 feet, width 5 feet, depth 6 feet 8 inches, fire-grate 5 feet long and 4 feet wide, firebars about 2 inches thick on the upper surface and 6 inches deep in the middle, draft spaces from -in. to 8-in. wide, according to the flow of air and kind of coal used. A boiler of these dimensions contains about 20 cubic yards; and, allowing something less than one-half of the space for water, as well as a little for stay-bars, &c., about 12 cubic feet of water-room will be given per horse-power, whilst its effective heating surface will be about 20 square yards. In Cornwall, where slow combustion is considered desirable, boilers are usually made of large dimensions. At East Wheal Rose, Mitchell's 85-inch cylinder engine, 10 feet stroke, is furnished with four boilers, of the following dimensions:--two 36 feet by 6 feet, one 38 feet by 6 feet, and one 34 feet by 6 feet; weighing together 45 tons. The steam is raised to a maximum pressure of 30 lbs. per square inch, and the consumption of coal in December last, when making 4 strokes per minute, was 3.6 lbs. per horse-power per hour. From the year 1835 to 1838, Mr. Wicksteed was at intervals occupied in making experiments on cylindrical boilers. Relative to the advantages of quick and slow combustion, he states:-"My |