The boiler is always supplied with water by the action of the engine; the hot water, pumped out of the condenser, is raised into a cistern placed at a sufficient height above the boiler, by the foree-pump men tioned in the general description of Watt's engine ; from this cistern a pipe passes through the top of the boiler, and reaches nearly to the bottom, where it is bent at right angles ; the upper orifice in the cistern is closed by a valve connected by a spindle-rod with one end of a simple lever, from the cud of the other arm of which a wire is suspended, sup porting a stone float in the boiler, the valve being weighted just to counterbalance the specific gravity of the float ; as the water subsides in the boiler in consequence of its evaporation, the float falls and raises the valve, allowing sufficient water to descend into the boiler till the float, rising again, causes the valve to close ; the weight of the column of water in the pipe prevents the steam from escaping in that direction 'during the action of the valves ; hence the necessity for the feeding cistern being raised sufficiently above the boiler. The bend at the bottom of the feeding-pipe is intended to cause the water, when issuing near to the bottom of the boiler, to wash away the sediment which would otherwise collect there.
The locomotive-engine, as has been stated, requires a boiler of a form and principle totally different from those of an ordinary one.
The boiler A (fig. 13) is a cylinder made of wrought-iron plates, riveted together in the usual way, but it is covered with a wooden casing, to prevent, as far as possible, the great waste of heat which would radiate from a metal surface moving through the air with great velocity. At one end of the boiler is the furnace ir, consisting of a double case, the outer one of iron with a semi-cylindrical head, but quite open at bottom ; within this is an inner square case of sheet copper, riveted all round the bottom edge to the outer one, but leaving on its three sides a space of three inches between them, which is filled with water, and indeed forms a continuation of the boiler. The bottom of this inner case is the grating on which the fuel is laid. r is the feeding door in front, opening of course through both cases, which arc therefore riveted together, so as to be steam-tight all round. A series of upwards of 100 brass tubes of small diameter pass from the back of the furnace to cylinder, or in other words to adapt the force it has to transmit to the resistance it has to overcome.
the further end of the boiler, where they open into the chimney. These tubes, which are entirely immersed in the water, constitute the flue, and thus increase considerably the heated surface in contact with the water, and therefore promote the rapid generation of steam. The introduction of these tubes forma an epoch in the construction of boilers, and was the suggestion of Mr. R. Stephenson, and simultaneously of M. Seguin, in 1S29. [Rsitwev.]
The cylinders of the engine, of which one only can be seen in tho section, are fixed at the bottom of the chimney. The steam passes to the slide-valve by the pipe 8, s, while the waste steam escapes up the funnel at T. The cylindrical vessel with the spherical head v, is called the steam-chest, the steam-pipe terminating in it at s. The object of this arrangement is to prevent the water, which is agitated by the motion, from passing through the steain-pipe to the cylinder, where it would be highly detrimental; the main steam-pipe divides into two branches in the chimney, one passing to each cylinder.
The boiler is supplied with water from the tender by two force pumps worked by the engine, and has a gauge, try-pipes, safety valves, &e., in common with other engines, though all peculiar in their construction.
This brief account of the boiler, and of its various arrangements, will hardly be considered complete without some notice of the explo sions to which it is liable.
The bursting of boilers presents very different phenomena, being sometimes a simple rent in the metal, allowing the harmless escape of steam and water ; and at others accompanied by an explosion in its violence equal to that produced by gunpowder; it has hence been conjectured that on these occasions some explosive gases are formed in the boiler ; but this does not appear probable, nor is it reconcileable with any known physical laws, while the elastic force of steam is capable of indefinite increase, and is quite adequate to produce any mechanical effect whatever. It is always difficult to get any satisfactory evidence as to the facts of an explosion of a boiler; the terror of the moment prevents the survivors from accurately recalling the phenomena imme diately antecedent, while those who from their proximity would be best capable of affording this evidence, are either killed, or are too interested in exculpating themselves to be impartial witnesses. With regard to the formation of explosive gaseous compounds in the boiler, it is generally admitted that hydrogen gas is the only ingredient of such that can be formed ; and that is obtained by the decomposition of the steam when in contact with the red-hot iron ; but pure hydrogen is not explosive ; and to render it so it must be mixed with oxygen or atmospheric air. It has been suggested that the latter may be intro duced along with the water by a defect in the feeding-pump ; but the proportions of the air and of the hydrogen must be definite to produce an explosion ; and it is difficult to suppose that in such a situation either should continue to accumulate till the quantity is exactly that necessary to produce an explosion. In short this explanation of the subject is beset with difficulties which have not yet been removed, though the attention of scientific men both in Europe and America has been frequently directed to it.