On account of the great deposition of salts, and other earthy matters, on the bottom and sides of boilers employed in steam boats at sea, it becomes expedient, in long voyages, to stop the progress of the vessel, in order to discharge the contents of the boilers, and fill them anew; for if the heat be continued after a considerable deposition has taken place, the steam can only be raised by a greatly increased expenditure of fuel, and the augmentation of the heat materially injures the tenacity of the metal of which the boilers are composed. To obviate so great an inconvenience, Messrs. Maudslay and Field have proposed an arrangement of apparatus, by which the water is continually being changed, and for which they took out letters patent in 1824. These gentlemen state that from 20 to 30 per cent, of the quantity of water evaporated, being taken from the concentrated brine, will keep the water within a degree of saltness from which no practical evils will result, however long the boiling be continued; the quantity thus abstracted from the boiler being of course replaced by a like quantity of sea-water in its natural state.

The abstraction of the brine is made by means of a small pump, with a loaded discharge valve, worked by the engine, and so proportioned as to draw from the lowest part of the boiler the quantity determined on, which may be regulated by a meter, shewing the quantity of water driven off in the form of steam. The operation of the pump is, however, not to commence until the brine has attained a considerable degree of concentration; it should for instance contain five times as much salt as common sea water does; after this, every stroke may be made by means of the pump, to take as much salt out of the boiler as is deposited in the boiler by the separation of the steam used in that stroke. By these means, the water in the boiler can never exceed a certain predetermined degree of saturation; and whether the engine be working quickly or slowly, the quantity withdrawn may always be made to bear the same proportion to the quantity left in, thus avoiding one of the greatest evils to which steam vessels in making long voyages have been subjected. To economise the heat and consequent expenditure of fuel, Messrs.

Maudslay and Field further propose that the hot brine extracted by the pump be discharged into a vessel containing a series of metal pipes of small calibre, similar to a refrigeratory.

Through these pipes, which lie immersed in hot brine, the supply water is to be made to pass in order to abstract the heat in its progress, and deliver the sea-water into the boiler in a heated state.

Boiler 226

In the year 1824, Mr. Smith introduced, in some of the salt works of Lancashire, a mode of evaporating brine, by the application of high-pressure steam under the salt pans; and as the surfaces of these vessels are very extensive, they are incapable of sustaining much pressure. Mr. Smith, therefore, tied the bottom of the boiler to the bottom of the pan (which also formed the top of the boiler), by means of screw bolts and nuts, in the manner shewn at b in the subjoined sectional figure. Finding this arrangement productive of a safe and efficient generator of high pressure steam, he subsequently took out a patent for a modification of it, to be applied to steam engines. This modification chiefly consisted in the addition of the upper vessel a. The plan of these vessels is supposed to be a parallelogram, and the screw bolts about 9 inches apart in each tier, over their whole surfaces. The water is supplied by a force pump, as represented, and a number of guage cocks are fixed at different elevations, as shewn in the drawing, to ascertain the height of the water, and the state of the steam in each vessel, e e are steam pipes; f is a safety valve to the lower chamber b, and g another to the upper chamber a.

The" patentee states, that "about two inches of water are put into the lower vessel, and the other being half filled, the fire is lighted, which quickly raises the water in the lower vessel to ebullition, the steam of which acts upon the lower surface of the upper boiler, giving out its heat to the water contained therein, and is thereby itself condensed; and being thus alternately vapourized and condensed, the upper vessel is converted into a steam chamber of uniform temperature." Although this boiler is calculated to generate steam with rapidity, owing to the extensive surface of metal exposed to the direct action of the fire; and notwithstanding it muse be deemed safer than most others of equal capacity and effect, by reason of the numerous tie bolts; it must, we think, be expensive in construction, and very difficult to preserve free from leakages.

Boiler 227

In the generality of boilers the flue first takes a horizontal direction, more or less extended, and afterwards ascends the chimney; sometimes it is also made to descend; and in almost every way that ingenuity could devise, the flues have been made to encompass the water, for the purpose of transmitting their heat thereto. Mr. Joseph Gibbs, of Crayford, in Kent, has, however, invented a boiler, (patented in 1830,) which possesses some claims to originality of arrangement; and as it probably confers some advantages, we shall here notice it. The form of the upper portion of this boiler is circular, with a descending cylindrical branch of considerable magnitude and length, the latter being quite full of water, and the upper filled to about four-fifths its depth, the remaining one-fifth being reserved for steam room. In the middle of the upper vessel is the fireplace, the air for combustion being supplied by a vertical pipe, passing upwards through the descending cylindrical branch of the boiler.

The products of combustion first act upon the water in the upper vessel, whence the flue descends in a curvilinear direction around the vertical air pipe in the midst of the water contained in the descending branch of the boiler, to the bottom thereof, and thence into the chimney.