A sketch of the steam boiler is represented in the annexed drawing, divided into three portions, a, b, and c; and steam is generated in one or more of these divisions, according to the supply required. When the steam in a is raised to a pressure of twenty-five pounds on the square inch, that in b will be twelve, and that in a five pounds. When only one of the divisions, a, of a steam boiler about seventeen feet by ten is employed, it will heat pans to the extent of 2400 square feet up to 184° Fahr. • and when the three divisions, a, b, and e, are used together, an extent of 4300 square feet will be heated to the same temperature.
The steam is conveyed in a pipe from the boiler to a steam-vessel e, under the fine salt-pan f. This pan is made steam-tight, and the steam arising therein is conveyed by a pipe to a similar vessel under the broad alt-pan k. Over the broad salt-pan k is placed the bay salt-pan m, and the space between them is enclosed by thin boards, or other light material, to confine the vapour arising from k in order to produce the required heat in the pan in. This pan is made lowest in the middle, as represented in the drawing, so that water con densed on its lowest surface may be collected in one place, where it is received and carried off in a spout, to prevent its return into the brine in the lower pan considers it of great importance to keep the bottom of the pans clear of salt ; and for that purpose employs rakes, which are kept constantly in motion by a steam engine. These rakes deposit the salt in receptacles at the sides of the pans. The rods by which the rakes are moved pass through stuffing boxes in the pans, to prevent the escape of steam. When there are many rakes in a row to put in motion, it is effected by a horizontal shaft from the steam engine, which, by means of bevel gearing, puts in motion a series of nuts, screwed upon the rake-rods, by which the various rakes are made alternately to advance and recede.
For the purpose of making the interior surface of the pans smooth, to facilitate the removal of the salt, the right of which it is composed are united by basing down the edges at right angles, and rivetting them together The steam vessels under the pans are strengthened by a number of link-bobs attached to the joinings underneath the pans by a forked end, taking in the bent joinings, and to the bottom of the steam vessel, by being hooked to staples.
Mr. Furnival, a spirited manufacturer of salt on an extensive scale, has taken out several patents for improvements in the mechanical arran emenb of the process. His last patent, which embodies the leading characteristies of Ins previous plans, may be explained with reference to the annexed diagram, which represents a vertical section of the apparatus, with two tiers of pans. adze are four furnaces, the flues from which areextended under a considerable range or surface of the pans, which are of the shape represented at b e, the deep part o being made to receive the salt thrown over by the ebullition, and also such portion as may be scraped from the surface of b, by means of the instruments shown at d d. The deep chambers being removed from the direct action of the fire, prevent the salt deposited therein from becoming burned ; and these recep tacles being at the sides, the salt is conveniently scooped out. The steam raised from the lower range of pans is then employed to heat an upper range se of less area, supported upon suitable framing, lined interiorly, to confine the steam, with boards. In order that the water resulting from the oondessation of the steam against the bottoms of the upper pans, may not fall back into the lower pans, two inclined planes are formed, which receive the condensed water, and conduct it into a pipe, whence it is carried off by a gutter underneath. In these inclined planes suitable apertures are made for the passage of II"' ascending steam from the lower to the upper boilers.
The patentee also proposes to heat a third set of pans above the second; fir this purpose there is a central aperture to conduct the steam to them; this aperture is covered with a cap.
The object of Messrs. Braithwaite and Ericson, who specified a patent in 1830, for "an improved method of manufacturing salt" is the manufacture at better salt with a less expenditure of fuel, which theypropose to effect by heating the brine considerably above the boiling point, any is permitted to take place. This is done by confining the brine, while heating in a close holier, and then permitting it to pass from a pipe of large dimensions, extending considerably above the top of the boiler into a shallow evaporating Teasel, where the process goes on till it ceases, from the reduction of the temperature ; when the brine is conveyed to the boiler, entering at the bottom, and again heated; and thus a circulation is kept up on the principle of the methods which we have before described, for communicating heat by the circulation of hot water. The lower end of the pipe which extends from the lap of 'the boiler is enlarged for the reception of a pair'of Vanes, fixed on a ter acid axis extending through a stuffing box at the top, where a pulley is attached to communicate motion to the vanes, by which, according to the patentee's statement, the circulation of the fluid is promoted. The brine is made to pass through a deep vessel, with a vertical partition, before it enters the evaporator, and ffirough another in its return to the boiler. The evaporator is a long vessel, of the form of a parallelogram ; and a vertical partition extending from the end next the boiler to within a small distance of the other end, causes the fluid to pass along one aide and return by the other. As the water evaporates the salt is deposited on the bottom of the evaporating vessel, and is removed therefrom by scrapers, in the usual manner. The principal advantage of this arrangement is the entire removal from the fire of the surface on which the salt is deposited ;. and, consequently, the formation of pan-crust, or scratch-salt, is completely prevented; for, as no evaporation takes place in the boiler, no salt can be depo sited there. There are a set of covers for diminishing at pleasure the evapd. rating surface, by which the fineness of the crystals of salt can be regulated with the greatest facility. This arrangement evidently contains many advan tages, as far as regards the quality of the salt ; and when it is considered that the heat communicated to the brine can only escape by evaporation (as the shallow vessel is made of non-conducting materials), its economical application, will, likewise, be found an inducement for its employment.
The uses and importance of salt as a culinary article are pretty well under stood ; but there are some particulars relating to its combination with foreign substances, and its several varieties which thence arise, for which we must seek information of the chemist. Dr. Henry has favoured us with the result of his very able and elaborate investigations for determining, with accuracy, the dif ferent varieties of common salt. (See the Philosophical Transactions for 1810.) " In sea-salt prepared by rapid evaporation, the insoluble portion," says Dr. Henry, "is a mixture of carbonate of lime with carbonate of magnesia, and a fine siliceous sand ; and in the salt prepared from Cheshire brine, it is almost entirely carbonate of lime. The insoluble part of the less pure pieces of rock salt is chiefly a manly earth, with some sulphate of lime." The kind of sat which according to Dr. Henry, most eminently the combinedproperties hardness, compactness, and perfection of crystas, will be best adapted to the purpose of packing fish and other provision, bemuse it will remain permanently between the different layers, or will be very gni. dually dissolved by the fluidsthat exude from the provisions; thus ftirsishing a slow but constant supply of saturated brine. On the other hand, for the pirpom of preparing the pickle, or of striking the meat, which is done by immersion in e maturated solution of sat, the smaller grained varieties answer equally well, or, on account of their greater stability, even better.