See Chemistry, No. 2d 605, 606.

Lamp Furnace. Ibid 611.

Machines for Blowing Air into Furnaces. The earliest method of animating large fires in the furnaces where ores were melted, seems to have been by exposing them to the wind. Such was the practice of the Peruvians before the arrival of the Spaniards among them. Alonso Barba relates, that their furnaces, called guairas, were built on eminences, where the air was freest; that they were perforated on all sides with holes, through which the air was driven in when the wind blew, which was the only time when the work could be carried on; that under each hole was made a projection of the stone-work, on which were laid burning coals, to heat the air before it entered the furnace. Some authors speak of several thousands of these guairas burning at once on the sides and tops of the hills of Potosi; and several remains of this practice are to be found in different parts of Great Britain.

This method of supplying air being found excessively inefficient and precarious, the instruments called bellows succeeded. These were at first worked by the strength of men; but as this was found to be very laborious and expensive, the force of running water was employed to give motion to these machines. Thus a much greater quantity of metal could be procured than formerly, and the separation was likewise more complete; insomuch, that in many places the flags or cinders from which the iron had formerly been extracted were again used as fresh ore, and yielded plenty of metal.

But though this method was found to be greatly preferable to the others, yet great improvements were still wanted. In order to melt very large quantities of ore at a time, it was necessary to use bellows of an immense size; and in proportion to their size they stood in need of the more frequent and expensive repairs. The oil, also, which the bellows required in large quantity, becoming rancid, was found to generate a kind of inflammable vapour, which sometimes burst the bellows with explosion, and thus rendered them totally useless. A new method, therefore, of blowing up fires altogether free from the abovementioned inconveniences, was fallen upon by means of water. It depends on the following principle, viz. That a stream of water, running through a pipe, if by any means it is mixed with air at its entrance into the pipe, will carry that air along with it, and part with it again as soon as it comes out of the pipe; and if the air is then collected by a proper apparatus, it may with success be used for exciting the most violent degrees of heat.

Machines of this kind are represented on Plate CCIV. fig. 4. In the right-hand machine, AB represents a stream of water falling into the funnel, whose throat is contracted at B; after which the stream runs through the perpendicular pipe EF, in the upper part of which there are some small holes represented by cdgs. Through these holes the air has access to mix itself with the descending water, which being dashed against the sides of the pipe is reduced to froth, and thus fills the whole cavity of the pipe EF, which is considerably larger than the throat of the funnel B. When this frothy stream enters the vessel C, the air extricates itself from the water; and as it cannot return through the pipe EF because it is continually filled with a stream of liquid matter, it flies off with considerable force through the smaller pipe D, by which it is conveyed to the furnace.

From this description, it is evident that the principal thing to be kept in view in the construction of these machines is, to mix the descending stream of water with as great a quantity of air as possible. For this purpose the contrivance represented in the left-hand machines answers much better than the former. By this the water descending from the reservoir A, falls into a kind of culinder B, perforated with a great number of holes in its sides. Thus the water being forced out in a number of small streams is very effectually dashed against the sides of the wide descending pipe, when it enters the condensing vessel C, and is sent off by the pipe D, as in the former.

In some machines of this kind the constructors seem Phil. Com. to have been of opinion, that a great height was required in the water-fall; but Dr Lewis, who has hitherto made a great number of experiments upon the subject, shows, that an excess in height can never make up for a deficiency in the quantity of the water. Four or five feet, he thinks, is a sufficient height for the water-fall; where there is a greater height, however, it may be rendered useful, by joining two or more machines together in the manner represented in the plate; where the water, after having once emitted its air in the condensing vessel C, flows out into a new reservoir E. From thence it descends through another culinder F, and descending from it into a condensing vessel G, the air is extricated, and carried off through the pipe H. The upper figure represents the culinder, with the shapes of the holes and their proportional distances, according to Dr Lewis.

Thus, with very little expense, where there is a sufficient quantity of water, as strong a blast of air as can be desired may be readily obtained; for several machines may be constructed, and joined together in a manner somewhat similar to that above mentioned, until all the quantity of water is employed. It is proper to observe, however, that as by this method the air is loaded with moisture, it is proper to make the condensing vessel as high as conveniently may be, that the air may arrive at the furnace in as dry a state as possible.—The long slender pipes in the left-hand machines represent a gage filled with mercury or water, by which the strength of the blast may be determined.

In the large iron foundries another method is used for blowing up the fires by means of a kind of air-pumps. These consist of cast-iron cylinders of about three feet diameter, exactly fitted with a piston moved up and down by means of a water-wheel. In the bottom of the cylinder is a large valve like that of a bellows, which rises as the piston is lifted up, and thus admits the air into the cavity of the cylinder from below. Immediately above the bottom is a tube which goes to the furnace; and as it proceeds from the cylinder is furnished with a valve opening outward. Thus, when the piston is drawn up, the valve in the bottom rises and admits the air that way into the cylinder; while the lateral valve shuts, and prevents any air from getting into it through the pipe. When the piston is thrust down, the valve in the bottom shuts, while the air being compressed in the cavity of the cylinder is violently forced out through the lateral tube into the the furnace. In the great foundry at Carron, four of these large cylinders were a few years ago employed at their principal furnace, and so contrived that the strokes of the pistons, being made alternately, produced an almost uninterrupted blast. Some little intermission might indeed be perceived by the ear, but it was too trifling to produce any sensible effect on the heat of the furnace. Even this could have been prevented by means of a large reservoir into which all the four cylinders might discharge their blast. This should be furnished with an heavy piston; whose weight being supported by the air of the cylinder alone, would force it out through its lateral tube in a manner perfectly equable, without any of that puffing or interruption in the blast, perceptible though but in a small degree in the other.