Boiler, in its most extended sense, implies any vessel employed for producing 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 effectiveness, 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, surrounded 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 metal; 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

Tinn..........................................

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. According 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, probably from 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 copper is, 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 sufficient extent to be capable of absorbing as much heat as will be necessary to produce 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° Fahr.) 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 i 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 purpose, the steam room (or space above the water) should hold a volume equal to the supply 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.