The foregoing are the two leading arrangements involved in the construction of blast machines; both have their advantages and disadvantages. Where the propelling power is a waterwheel, and where it is contemplated to use but one or two cylinders, the horizontal cylinder may be considered to present many advantages; for, in such cases, it is always more or less troublesome to balance the weight of the piston, rod, &c. If the motive power is steam, the vertical position of the cylinder is decidedly preferable; for, in this case, the weight of the piston and its accompaniments can be balanced by the weight of the steam piston and its associated parts. A machine consisting of one blast cylinder and a receiver may here be considered the most simple and advantageous. Where a waterwheel is the propeller, it is less advantageous to employ a single cylinder, for, as the stroke is made by a crank, a great irregularity in the blast ensues, and a comparatively large receiver is therefore required to regulate the inequalities of the pressure. This is one of those instances in which a crank works to the disadvantage of the power applied, which is seldom the case. For these reasons, various forms of blast machines, propelled by waterwheels, have been tried. In this country, however, only those with two cylinders and double stroke are used. This makes a useful, but not an excellent blast, even though the cranks work at right angles to each other. A receiver is almost indispensable, in this case, to equalize the blast, and to make the best possible use of the water power. Blast machines with three cylinders and double stroke have been applied; and this arrangement may be considered the most advantageous where water is the moving power. Such a machine produces a very steady blast, without a receiver, and gives the best. effect of the waterwheel.

The expense of

c. There is no reason whatever for employing water power in the propelling of blast machines at blast furnaces. There is abundance of waste heat for the generation of steam. erecting a steam-engine will be found less, in most cases, than that incurred in the erection of a waterwheel. For these reasons, we shall not dwell any longer upon the application of water power to blast machines, and shall confine our subsequent remarks to those propelled by steam alone.

IV. General Remarks on Cylinder Blast Machines.

There is no doubt that the application of one cylinder to a blast machine is accompanied with great advantages. Such an arrange

ment is in conformity with sound principles of mechanics, because, by this means, the least friction commensurate with the same effect is produced. Weight and surface, the two most important causes of friction, are very greatly reduced. If the blast cylinder is on one end of a balance beam, and the steam cylinder on the other, the regularity of the blast is much greater. But this is no reason why a balance beam should be applied; because any inequality in the pressure of the blast can be regulated by applying a large receiver. If, therefore, it is found advantageous to abandon the balance beam, and still to retain the vertical position of the cylinders, the unbalanced weight of the pistons and piston rods is no obstacle. A piston rod, to connect blast and steam cylinders, has been applied where horizontal cylinders have been used-in cases, however, in which the blunder of making the piston rod too short was committed. The hot part of the piston rod, playing in the steam cylinder, is thus cooled when in the blast cylinder, and the adherent oil dried when playing there. By this means, the hemp of the stuffing box of the former is very soon worn out. Besides this disadvantage, the close proximity of the steam apparatus and the blast cylinder is very injurious to the operations in the blast furnace. It is impossible to keep the steam out of the blast cylinder, if the latter is too close to the steam cylinder or the steam boilers, or even if it is in a very warm place. We know that moisture introduced into the hearth of a blast furnace is very injurious.

a. The size of a blast cylinder depends partly on the amount of air needed, and the number of strokes made, and partly upon the purposes for which it is designed. A charcoal forge requires from 400 to 500 cubic feet per minute; a finery from 800 to 1000; a charcoal furnace from 1000 to 2000; and an anthracite or coke furnace from 3000 to 5000. The number of strokes that can be made by a machine depends chiefly on the length of the stroke and the construction of the valves. In cylinders of four feet diameter, the piston can move with the speed of three feet; in smaller cylinders with greater, and in larger ones with less speed. If the motion is regulated by a flywheel and crank, more speed can be given than where a flywheel is not employed.

b. The size, form, and weight of the valves have a highly important influence upon the speed of the piston, loss of power, and quality of blast. The smaller the valves are made, the greater is the increase in the velocity of the air which is to pass through them.

Friction of the air and valves, besides a direct loss of pressure and air in proportion to the pressure in the valve, is thus occasioned. One-twelfth of the surface of the piston is sufficient for the passage of the blast; but no disadvantage results if the valves are larger. The form of the latter has an influence upon the effect of the machine. Trap valves are the most practicable. Semicircular valves, with the hinges in the diameter, deserve to be more extensively employed than they are at the present time. The semicircle has less outline in proportion to the same surface than the square or parallelogram, the usual form of valves, and for this reason diminishes the friction of the air. Quadrilateral valves are seldom used. In general, the oblong shape is preferred, in which case, the hinges are put to one of the longest sides. It is obvious, from reasons which will be subsequently given, that the longer the valve, the more perfect will be its form. The weight of the valve is an important object, for, if neglected, it may seriously injure the effect of a blast machine. It is easily understood that this weight may be so increased, that the effect of a blast apparatus amounts scarcely to anything. The weight of the valve causes an expansion of the air in suction; consequently, the pressure on the suction side of the piston in proportion to this weight will be less than that of the atmosphere. A loss of power and blast on the compressing side of the piston, proportional to the weight of the valve, is also occasioned. The air which remains in the dead space of the cylinder is of greater pressure than that in the blast pipes or receiver, in the ratio of the weight of the valve. To diminish the influence of this weight, the valves are generally placed in a vertical position, and are made entirely of a light material, such as wood and leather; they are also made as oblong as circumstances will admit. In this respect, the horizontal blast cylinder possesses great advantages. The location of the valves is best secured by vertical heads, and if the friction, or rather the weight, of the piston and piston rod could be balanced, the horizontal cylinder would be the best form of the blast machine. Their position and weight, also, have considerable influence upon the effect of a blast machine; but of still more consequence is the dead space left at the heads of a cylinder. Dead space is that which is not filled by the piston head, in its alternate motions, and from which the air that is compressed is not forced by the piston. In the best blast machines, the loss which this occasions amounts to at least ten per cent., and in some cylinders is as great as twenty-five per cent. The loss in power and blast increases with the size of the

dead space. In this respect, the horizontal has the advantage over the vertical cylinder.

c. There are advantages connected with the vertical which cannot be reached by the horizontal cylinder, namely, a closer fit of the piston head to the cylinder, and less friction, as well as smaller loss by leakage. But as there are serious objections to it, on account of dead space, and of the position of the valves, we shall propose an improvement to the vertical cylinder which may render it more acceptable. In a vertical cylinder, we cannot well place the valves at the top and bottom, because of their horizontal position; horizontal valves are thus rendered necessary. In small cylinders the valves are frequently horizontal; but large valves will not work well, if thus laid. To secure a vertical position for the valves, in a vertical cylinder, we are compelled to add valve boxes to the heads of the cylinder. Such boxes, of course, obstruct the passage of the blast, and occasion dead space-disadvantages, if possible, to be avoided. These may be obviated by the following arrangement: If the stroke of the piston is six feet, and the thickness of the piston head four inches, a cylinder six feet six inches long is required, to secure one inch space at each end. If we make the cylinder seven feet and a half long, instead of six and a half, a dead space of fourteen inches, or seven inches at each end, would be left. Around this space, and in the metal of the cylinder, a series of valves may be placed. In this way, the number of valves may be multiplied to any extent we choose. The blast pipe is formed by screwing sheet iron or boiler-plate to the flanches of the cylinders, and by covering as many outlet valves as we choose to put in. The dead space caused by the excessive length of the cylinder is occupied by an increased thickness of the piston head; this thickness can be produced by a filling of light pine wood, or any other light material. The heads of such a cylinder will then be quite smooth. In the following illustration, Fig. 135, the arrangement is so clearly exhibited that no further explanation is required.

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d. The piston, in wooden cylinders, is generally packed with leather, or hemp, or by a mixture of both; also, by filling leather hose with horse hair or wool, and by fastening this around the piston head into a groove, which is turned in the circumference of it. Packing in iron cylinders is performed like packing in steam cylinders. A steel or wrought iron hoop, a quarter of an inch thick, is laid around the piston head, and the space between the hoop and the head is stuffed with hemp or woollen material. In machines with

vertical cylinders, we often see the piston rod playing from above; but we quite as frequently see an arrangement by which the piston

is made to move from below. As far as the effect of the machine is concerned, this is merely a consideration of expediency and economy.

V. Various Forms of Blast Machines.

In no branch of human industry have more ingenuity and talent been displayed than in the construction of blast machines. Still, the greater part of such inventions were made with a limited knowledge of their purposes. Hence, an imperfection in most of the plans, though apparently well conceived, has been the consequence. The leading principles in this invention were generally reduced to the mechanical effect of the apparatus, that is, to obtaining the greatest effect from a given power, or producing the greatest amount of blast by the smallest means. Of the numberless variety of blast machines thus invented, there is but one which deserves our attention, in addition to the cylinder machines already described. This is the Cagniardelle, or screw blast machine. Other machines, however extensively they may be employed, are, for our purpose, scarcely worth notice; they may serve in other metallurgical operations, but are not available in iron manufactories. Among these, are the Trompe, the Rosary or Chain-trompe, the Water column machines, the Gasometer bellows, and Barrel machines. These