BOILER, in its most extended sense, implies any vessel employed for pro ducing the ebullition of liquids; thus the ordinary domestic pots and kettles, brewing and washing coppers, as well as all kinds of vessels used for heating liquids in various manufacturing operations, come under this denomination. Our business in this place is, however, the description of that important apparatus wherein is generated the source of that power which is regulated and applied by the steam engine ; previous to which, we shall make a few observations on the requisite properties of steam boilers in general. The first and most important quality we consider to be safety from explosion, for unless this be attained, so as to render personal danger improbable, such boilers should not be used at all.
The next in importance is eectiveness, or that which will produce the required quantity of steam by the consumption of the least quantity of fuel. Cheapness in the cost of construction, and durability in the wear, may be ranked as the third requisite. The fourth is, perhaps, convenience, or that which can be worked with the utmost facility, that will require the least personal attention, and will occupy the least space. That boiler which combines the four qualities just mentioned, and possesses each in the most eminent degree, may be deemed a perfect one. The materials principally used at present for the construction of boilers are iron and copper, though boilers have been made of wood and stone. In those formed of wood the furnace was of necessity placed internally, sur rounded by the water; and as wood is an extremely bad conductor of heat, but little loss of it was sustained by radiation therefrom, while a considerable economy of fuel was consequently effected. The cost of such was also small, compared to those of they were introduced in America by Chancellor Livingstone and Mr Anderson. Boilers of stone were introduced by Mr. Brindley, who, in 1756, erected a steam engine near Newcastle-under-Lyne, with a boiler of this description. It was composed of brick and stone firmly cemented together, and the water was heated by iron flues. The material of which a boiler is constructed is of more consequence, in an economical point of view, than is usually supposed, as the heat cannot be communicated to the water without being first transmitted through the substance of which it is formed. M. Despretz, who made a series of very accurate experiments to determine the comparative power of different substances for conducting heat, obtained the following results: Gold 1000.0 Silver 973.0 Platina 981.0 Copper 898.2 Iron 374.3 Zinc 363.0 Tin 303.9 Lead 179.6 Marble 23.6 Porcelain 12.2 Fire bricks 11.4 The conducting power of copper being thus more than double that of iron, offers very great advantages; but there are other considerations to be entered into before determining to which the preference is due, especially their comparative cohesive strength and cost. A ccording to some experiments the copper was found to be the strongest, but that was probably owing to the iron being of inferior quality, as most philosophers have agreed in attributing superior cohesive strength to iron. Notwithstanding this circumstance, from the greater uniformity of the texture of sheet copper over that of iron, manufacturers usually construct copper boilers of thinner plates than those of iron, that have to withstand the same pressure of steam. Experience has, we believe, established this as a rule, pro observing that when a copper boiler bursts, it only tears open, while a boiler of wrought iron plates is often blown to pieces. The cost of coppers, however, four times that of iron ; but if it be admitted that the quantity of heat passing through iron in a given time can be doubled in copper, it follows that a copper boiler having only half the superficies of one of iron, exposed to the action of the fire, will be adequate to the generation of the same quantity or force of steam. This circumstance, therefore, greatly reduces the weight of the copper boiler, and, consequently, its first cost is made to approximate more to that of the iron boiler of double the size. Increased strength is likewise acquired by the reduced dimensions of the copper boiler, so as to permit of a decrease of the pre-supposed thickness of metal ; and thus the greatly enhanced price per pound of a copper boiler over that of an iron one, which alarms many steam engine proprietors from ordering them, is very much disproportioned to the cost of the entire vessel. When an iron boiler is worn out, the old metal is scarcely worth the expense of removal; but when one of copper is decayed, the old metal is worth three-fourths of its original cost. These considerations, together with the increased safety and reduced bulk of copper boilers, inclines us to believe that in a course of years their use will be found more economical than those of iron. In the construction of boilers of the ordinary form (that is, such as consist of a capacious single chamber), the bottom surface should be of suffi cient extent to be capable of absorbing as much heat as will be necessary to pro duce the required quantity of steam, what little heat may be given out laterally serving to prevent condensation in the upper part of the vessel; and the smoke, before it enters the chimney, should be robbed as much as possible of its heat by being brought into contact with the conduit pipe, by which the boiler is supplied with cold water. It has been stated by a scientific authority that there is a con siderable waste of fuel in producing steam by intensity of heat upon a small surface, and that the application of a moderate heat (800° Fehr.) is far more economical. A cubic foot of water converted into steam per hour was reckoned by Mr. Watt as equivalent to one horse's power, who observed that this quantity of steam could be raised per hour by 8 feet of surface of boiler and flue, in a judiciously constructed furnace. In practice, it is usual to allow from 4 to 5 feet of bottom surface of boiler to raise 1 cubic foot of water into steam per hour. It is considered essential that a boiler should contain four or five times as much water as it boils off per hour; and it is obvious that it should have a space above the water capable of containing as much steam as will supply the engine at each stroke, without materially diminishing its elastic force. For this, the steam room (or space above the water) should hold a volume equal to of eight or ten strokes of the engine : in large engines it is not unusual to employ two, three, or more boilers, to supply them with steam ; one of them being reserved for use, in case of repairs being required to the others. In fact, a spare boiler should be provided wherever stoppages are of serious importance in a concern. The strength of low-pressure boilers should be twice the regulated pressure on the safety-valve ; but high-pressure boilers should be proved to at least three times their working pressure. Upon the form of boilers much of their strength and their efficacy in the generation of steam depend. The earliest boilers employed for generating steam, and applied as a motive force, with which we are acquainted, were either globular or hemispherical, with flat or concave bottoms ; and many of these are still in use for the supply of high-pressure engines. In situations where fuel is so abundant as to render needless any economy of it, they may continue to be used with advantage, as their form adapts them to withstand steam of great elastic force. The waste of fuel caused by them, elsewhere, led to a very general substitution of boilers of an oblong form; and those known by the term of " waggon boilers," from their shape, formed one of the many improvements of the steam engine introduced by Watt. A longitudinal
section of a boiler of this kind is represented by Fig. 1 in the subjoined engraving, fitted up with all the appendages now generally applied, and set in a furnace of the usual construction. a a a is the boiler, having a cylindrical return flue b throughout its length ; the form of these parts will be best understood by con sidering them with reference to Fig. 2, which shows the figure of the boiler iu its cross section, b being the return flue ; this portion of the boiler should be always placed near to the bottom, and be constantly kept covered with the water, as seen at c in the drawing. The flame and smoke from the furnace €1 first passes under the boiler, then returns through the flue b to the front, where the current is divided, and passing to the right and left through lateral flues in the brick work (one of which is brought into view at e e) before it enters the chimney ff. The water is supplied to the boiler by the which is made to contain a column of water equal to the amount of pressure in the boiler. On the top of this pipe is a cistern h; i is a float, made of stone, suspended by a wire passing through a stuffing-box j, which is attached to one extremity of a lever, whose fulcrum is at k, a weight I being suspended to the extremity of the other arm of the lever sufficient to counterbalance the difference between the specific gravity of the stone and the water, causing the former to lie on the surface of the latter. By this arrangement, when the water sinks below the proper level in the boiler, the stone float descends with it, causing the attached wire to operate upon the lever above, and lift the valve shown in the cistern A, which then affords a ftesh supply of water. The feed pipe likewise contains an iron bucket, hung by a chain that passes over two pulleys o o; the other end of the chain is attached to a plate of iron p, called the damper, which is used to enlarge or contract, and, when required, entirely close the throat of the chimney. By this contrivance, when the steam in the boiler is uwd to too great an extent, its pressure forces the water in the feed pipe upwar thereby raising the bucket es, and causing its counterbalance, the damper, to descend in the throat of the chimney, and reduce the intensity of the fire. At ? and r are two guage-cocks, by which the proper height of the water in the boiler may be always ascertained. If g dis charges water, and r steam, the water is at the proper height ; if both cocks discharge water, it is above its proper height ; and if both discharge steam, it is below its proper height This latter circumstance, which can only take place by the defective action of the last described apparatus, is of serious moment, and requires an immediate remedy; the most safe is, in our opinion, the reduction of the fire, by stopping up, by the readiest means at hand, all access of air to the grate, after which, attention may be paid with confidence to removing the cause of the defect. For the purpose of showing at all times the pressure acting upon the boiler, a steam-guage is employed. This is usually made of an iron tube, bent into the form represented, one end communicating with the steam room of the boiler, and the other open to the atmosphere. This tube is partly filled with mercury, and into the externally open end is put a slight rod of wood, which floats perpendicularly in the mercury, and shows by its altitude, on a divided scale of inches affixed to that leg of the tube, the pressure of the steam. If the steam raises the mercury or rod one inch, it proves that the pressure is one half pound per square inch on the internal surface of the boiler, tending to burst it; for if the section of the bore of the pipe was ,just one inch, the pressure would be supporting one cubic inch of mercury, which will be found to weigh nearly half a pound; therefore, for every two Inches rise, one pound pressure may be reckoned; and as condensing engines seldom work with more than three or four pounds pressure upon the inch, the scale need not be longer than eight or nine inches For preventing the pressure becoming greater than the boiler is calculated to sustain without incurring danger, a safety-valve u is pro vided. This is usually a circular piece of metal, with a conical periphery ground to fit into a conical seat, and kept there by the pressure of a lever, loaded with a determinate weight, that will not suffer it to rise until the steam has acquired an excess of force above that required for working the engine, or would endanger the bursting of the boiler. In large boilers, especially those in steam boats, it is usual to have another safety valve, inclosed in a box under lock, the key being kept by the chief manager, to prevent the possibility of improper interference with it, by ignorant or imprudent persons : by the neglect of which precaution many serious accidents have occurred. The steam which escapes from safety valves is generally conducted by a pipe from the valve boxes into the chimney. At vs is the steam-pipe by which the engine is supplied, the quantity being regulated by a throttle-valve at to, and a screw-down valve at r. For the of cleaning out or examining the condition of the boiler, a circular or ov hole g is made sufficiently large to admit a man to pass through, and therefore called the man-hole. 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, how ever, 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 dosed by flanged disks of great thickness, on account, we sup pose, of the greater facility of construction by ordinary workmen. These boilers are generally provided with an in ternal flue, through which the heated air and flames, after tra versing the length of the undeP 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. Saun ders, 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 boiler ; 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.