g, side openings for receiving the tuyères, of which there are six upon each side of the furnace. Each of them may be shut at pleasure, by means of a small cast iron plate h, made to slide horizontally in grooves sunk in the main plate, pierced with the holes g g.
k k, interior lining of the surface, made of sand, somewhat argillaceous, in the following way. After having laid at the bottom of the furnace a bed of sand a few inches thick, slightly sloped towards the orifice of discharge, there is set upright, in the axis of the cupola, a wooden cylinder of its whole height, and of a diameter a little less than that of the vacant space belonging to the top of the furnace. Sand is to be then rammed in so as to fill the whole of the furnace; after which the wooden cylinder is withdrawn, and the lining of sand is cut or shaved away, till it has received the proper form.
This lining lasts generally 5 or 6 weeks, when there are 6 meltings weekly.
i i, cast iron circular plate, through which the mouth of the furnace passes, for protecting the lining in k during the introduction of the charges.
N N, level of the floor of the foundry. The portion of it below the running out orifice consists of sand, so that it may be readily sunk when it is wished to receive the melted metal in ladles or pots of large dimensions.
The fan distributes the blast from the main pipe to three principal points, by three branch tubes of distribution. A register, consisting of a cast-iron plate sliding with friction in a frame, serves to intercept the blast at any moment, when it is not desirable to stop the moving power. A large main pipe of zinc or sheet iron is fitted to the orifice of the slide valve. It is square at the beginning, or only rounded at the angles; but at a little distance it becomes cylindrical, and conducts the blast to the divaricating points. There, each of the branches turns up vertically, and terminates at b b, [fig. 479.], where it presents a circular orifice of 71⁄2 inches. Upon each of the upright pipes b, the one end of an elbow-tube of zinc c c c c, [fig. 479.], is adjusted rather loosely, and the other end receives a tuyère of wrought iron d d, through the intervention of a shifting hose or collar of leather c c d, hooped with iron wire to both the tube and the tuyère. The portion c c c c may be raised or lowered, by sliding upon the pipe b, in order to bring the nozzle of the tuyère d d, to the requisite point of the furnace. The portion c c c c may be made also of wrought iron. A power of 4 horses is adequate to drive this fan, for supplying blast to 3 furnaces.
The founders have observed the efflux of air was not the same when blown into the atmosphere, as it was when blown into the furnaces; the velocity of the fan, with the same impulsive power, being considerably increased in the latter case. They imagine that this circumstance arises from the blast being sucked in, so to speak, by the draught of the furnace, and that the fan then supplied a greater quantity of air.
The following experimental researches show the fallacy of this opinion. Two water syphons, e e e, f f f, made of glass tubes, one-fifth of an inch in the bore, were inserted into the tuyère, containing water in the portions g g g, h h h. The one of these manometers for measuring the pressure of the air was inserted at k, the other in the centre of the nozzle. The size of this glass tube was too small to obstruct in any sensible degree the outlet of the air. It was found that when the tuyères of the fan discharged into the open air, the expenditure by a nozzle of a constant diameter was proportional to the number of the revolutions of the vanes. It was further found, that when the speed of the vanes was constant, the expenditure by one or by two nozzles was proportional to the total area of these nozzles. The following formulæ give the volume of air furnished by the fan, when the number of turns and the area of the nozzles are known.