Chapter 3

3½ 1 3-3/8 3½ 1½ 3-1/8 3½ 2 2-7/8 3½ 2½ 2½ 3½ 3 1¾

4 2 3½ 4 2½ 3-1/8 4 3 2-5/8 4 3½ 2

5 3 4 5 4 3

For larger ovals multiples of these numbers may be taken; thus for 7 and 4, take from the table twice the width corresponding to 3½ and 2, which is twice 2-7/8, or 5¾. It will be noticed also that columns 2 and 3 are interchangeable.

To use the apparatus in connection with the table: Find the length of the desired oval in the first column of the table, and the width most nearly corresponding to that desired in the third column. The corresponding number in the middle column tells which hole the needle must be passed through. The tack D, _around_ which the string must pass, is so placed that the total length of the string AD + DC, or its equal AE + EC, shall equal the greater diameter of the ellipse. In the figure it is placed 6½ inches from A, and 1½ inches from C, making the total length of string 8 inches. The oval described will then be 8 inches long and 6¼ inches wide.

The above table will be found equally useful in describing ovals by fastening the ends of the string to the drawing as is recommended in all the text books on the subject. On the other hand, the instrument may be set "by guess" when no particular accuracy is required.

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THE MANUFACTURE OF CHARCOAL IN KILNS.

The manufacture of charcoal in kilns was declared many years ago, after a series of experiments made in poorly constructed furnaces, to be unprofitable, and the subject is dismissed by most writers with the remark, that in order to use the method economically the products of distillation, both liquid and gaseous, must be collected. T. Egleston, Ph.D., of the School of Mines, New York, has read a paper on the subject before the American Institute of Mining Engineers, from which we extract as follows: As there are many SILVER DISTRICTS IN THE WEST where coke cannot be had at such a price as will allow of its being used, and where the ores are of such a nature that wood cannot be used in a reverberatory furnace, the most economical method of making charcoal is an important question.

Kilns for the manufacture of charcoal are made of almost any shape and size, determined in most cases by the fancy of the builder or by the necessities of the shape of the ground selected. They do not differ from each other in any principle of manufacture, nor does there seem to be any appreciable difference in the quality of the fuel they produce, when the process is conducted with equal care in the different varieties; but there is a considerable difference in the yield and in the cost of the process in favor of small over large kilns. The different varieties have come into and gone out of use mainly on account of the cost of construction and of repairs. The object of a kiln is to replace the cover of a meiler by a permanent structure. Intermediate between the meiler and the kiln is the Foucauld system, the object of which is to replace the cover by a structure more or less permanent, which has all the disadvantages of both systems, with no advantages peculiar to itself.

The kilns which are used may be divided into the rectangular, the round, and the conical, but the first two seem to be disappearing before the last, which is as readily built and much more easily managed.

ALL VARIETIES OF KILNS

Are usually built of red brick, or, rarely, of brick and stone together. Occasionally, refractory brick is used, but it is not necessary. The foundations are usually made of stone. There are several precautions necessary in constructing the walls. The brick should be sufficiently hard to resist the fire, and should therefore be tested before using. It is an unnecessary expense to use either second or third quality fire-brick. As the pyroligneous acid which results from the distillation of the wood attacks lime mortar, it is best to lay up the brick with fire-clay mortar, to which a little salt has been added; sometimes loam mixed with coal-tar, to which a little salt is also added, is used. As the principal office of this mortar is to fill the joints, special care must be taken in laying the bricks that every joint is broken, and frequent headers put in to tie the bricks together. It is especially necessary that all the joints should be carefully filled, as any small open spaces would admit air, and would materially decrease the yield of the kiln. The floor of the kiln was formerly made of two rows of brick set edgewise and carefully laid, but latterly it is found to be best made of clay. Any material, however, that will pack hard may be used. It must be well beaten down with paving mauls. The center must be about six inches higher than the sides, which are brought up to the bottom of the lower vents. Most kilns are carefully pointed, and are then painted on the outside with a wash of clay suspended in water, and covered with a coating of coal-tar, which makes them waterproof, and does not require to be renewed for several years.

The kilns were formerly roofed over with rough boards to protect the masonry from the weather, but as no special advantage was found to result from so doing, since of late years they have been made water-proof, the practice has been discontinued.

The wood used is cut about one and a fifth meters long. The diameter is not considered of much importance, except in so far as it is desirable to have it as nearly uniform as possible. When most of the wood is small, and only a small part of it is large, the large pieces are usually split, to make it pack well. It has been found most satisfactory to have three rows of vents around the kiln, which should be provided with a cast-iron frame reaching to the inside of the furnace. The vents near the ground are generally five inches high--the size of two bricks--and four inches wide--the width of one--and the holes are closed by inserting one or two bricks in them. They are usually the size of one brick, and larger on the outside than on the inside. These holes are usually from 0.45 m. to 0.60 m. apart vertically, and from 0.80 m. to 0.90 m. apart horizontally. The lower vents start on the second row of the brickwork above the foundation, and are placed on the level with the floor, so that the fire can draw to the bottom. There is sometimes an additional opening near the top to allow of the rapid escape of the smoke and gas at the time of firing, which is then closed, and kept closed until the kiln is discharged. This applies mostly to the best types of conical kilns. In the circular and conical ones the top charging door is sometimes used for this purpose. Hard and soft woods are burned indifferently in the kilns. Hard-wood coal weighs more than soft, and the hard variety of charcoal is usually preferred for blast furnaces, and for such purposes there is an advantage of fully 33-1/3 per cent. or even more in using hard woods. For the direct process in the bloomaries, soft-wood charcoal is preferred. It is found that it is not usually advantageous to build kilns of over 160 to 180 cubic meters in capacity. Larger furnaces have been used, and give as good a yield, but they are much more cumbersome to manage. The largest yield got from kilns is from 50 to 60 bushels for hard wood to 50 for soft wood. The average yield, however, is about 45 bushels. In meilers, two and a half to three cords of wood are required for 100 bushels, or 30 to 40 bushels to the cord. The kiln charcoal is very large, so that the loss in fine coal is very much diminished. The pieces usually come out the whole size, and sometimes the whole length of the wood.

The rectangular kilns were those which were formerly exclusively in use. They are generally built to contain from 30 to 90 cords of wood. The usual sizes are given in the table below:

1 2 3 4 Length 50 40 40 48 Width 12 15 14 17 Height 12 15 18 18 Capacity, in cords 55 70 75 90

1 and 2. Used in New England. 3. Type of those used in Mexico. 4. Kiln at Lauton, Mich.

The arch is usually an arc of a circle. A kiln of the size of No. 4, as constructed at the Michigan Central Iron Works, with a good burn, will yield 4,000 bushels of charcoal.

The vertical walls in the best constructions are 12 to 13 feet high, and 1-½ brick thick, containing from 20 to 52 bricks to the cubic foot of wall. To insure sufficient strength to resist the expansion and contraction due to the heating and cooling, they should be provided with buttresses which are 1 brick thick and 2 wide, as at Wassaic, New York; but many of them are built without them, as at Lauton, Michigan, as shown in the engraving. In both cases they are supported with strong braces, from 3 to 4 feet apart, made of round or hewn wood, or of cast iron, which are buried in the ground below, and are tied above and below with iron rods, as in the engraving, and the lower end passing beneath the floor of the kiln. When made of wood they are usually 8 inches square or round, or sometimes by 8 inches placed edgewise. They are sometimes tied at the top with wooden braces of the same size, which are securely fastened by iron rods running through the corners, as shown. When a number of kilns are built together, as at the Michigan Central Iron Works, at Lauton, Michigan, shown in the plan view, only the end kilns are braced in this way. The intermediate ones are supported below by wooden braces, securely fastened at the bottom. The roof is always arched, is one brick, or eight inches, thick, and is laid in headers, fourteen being used in each superficial foot. Many of the kilns have in the center a round hole, from sixteen to eighteen inches in diameter, which is closed by a cast iron plate. It requires from 35 M. to 40 M. brick for a kiln of 45 cords, and 60 M. to 65 M. for one of 90 cords.

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The belief that population in the West Indies is stationary is so far from accurate that, as Sir Anthony Musgrave points out, it is increasing more rapidly than the population of the United Kingdom. The statistics of population show an increase of 16 per cent. on the last decennial period, while the increase in the United Kingdom in the ten years preceding the last census was under 11 per cent. This increase appears to be general, and is only slightly influenced by immigration. "The population of the West Indies," adds Sir A. Musgrave, "is now greater than that of any of the larger Australian colonies, and three times that of New Zealand."

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HEAT DEVELOPED IN FORGING.

M. Tresca has lately presented to the Academy of Sciences some very interesting experiments on the development and distribution of heat produced by a blow of the steam hammer in the process of forging. The method used was as follows: The bar was carefully polished on both sides, and this polished part covered with a thin layer of wax. It was then placed on an anvil and struck by a monkey of known weight, P, falling from a height, H. The faces of the monkey and anvil were exactly alike, and care was taken that the whole work, T = PH, should be expended upon the bar. A single blow was enough to melt the wax over a certain zone; and this indicated clearly how much of the lateral faces had been raised by the shock to the temperature of melting wax. The form of this melted part could be made to differ considerably, but approximated to that of an equilateral hyperbola. Let A be the area of this zone, b the width of the bar, d the density, C the heat capacity, and t-t0 the excess of temperature of melting wax over the temperature of the air. Then, assuming that the area, A, is the base of a horizontal prism, which is everywhere heated to the temperature, t, the heating effect produced will be expressed by

Ab x d x C(t-t0)

Multiplying this by 425, or Joule's equivalent for the metrical system, the energy developed in heat is given by

T1 = 425 AbdC(t-t0).

Dividing T1 by T, we obtain the ratio which the energy developed in heat bears to the total energy of the blow.

With regard to the form of the zone of melting, it was found always to extend round the edges of the indent produced in the bar by the blow. We are speaking for the present of cases where the faces of the monkey and anvil were sharp. On the sides of the bar the zone took the form of a sort of cross with curved arms, the arms being thinner or thicker according to the greater or less energy of the shock. These forms are shown in Figs. 1 to 6. It will be seen that these zones correspond to the zones of greatest sliding in the deformation of a bar forged with a sharp edged hammer, showing in fact that it is the mechanical work done in this sliding which is afterward transformed into heat.

With regard to the ratio, above mentioned, between the heat developed and the energy of the blow, it is very much greater than had been expected when the other sources of loss were taken into consideration. In some cases it reached 80 per cent., and in a table given the limits vary for an iron bar between 68.4 per cent. with an energy of 40 kilogram-meters, and 83.6 per cent. with an energy of 90 kilogram-meters. With copper the energy is nearly constant at 70 per cent. It will be seen that the proportion is less when the energy is less, and it also diminishes with the section of the bar. This is no doubt due to the fact that the heat is then conducted away more rapidly. On the whole, the results are summed up by M. Tresca as follows:

(1) The development of heat depends on the form of the faces and the energy of the blow.

(2) In the case of faces with sharp edges, the process described allows this heat to be clearly indicated.

(3) The development of heat is greatest where the shearing of the material is strongest. This shearing is therefore the mechanical cause which produces the heating effect.

(4) With a blow of sufficient energy and a bar of sufficient size, about 80 per cent. of the energy reappears in the heat.

(5) The figures formed by the melted wax give a sort of diagram, showing the distribution of the heat and the character of the deformation in the bar.

(6) Where the energy is small the calculation of the percentage is not reliable.

So far we have spoken only of cases where the anvil and monkey have sharp faces. Where the faces are rounded the phenomena are somewhat different. Figs. 7 to 12 give the area of melted wax in the case of bars struck with blows gradually increasing in energy. It will be seen that, instead of commencing at the edges of the indent, the fusion begins near the middle, and appears in small triangular figures, which gradually increase in width and depth until at last they meet at the apex, as in Fig. 12. The explanation is that with the rounded edges the compression at first takes place only in the outer layers of the bar, the inner remaining comparatively unaffected. Hence the development of heat is concentrated on these outer layers, so long as the blows are moderate in intensity. The same thing had already been remarked in cases of holes punched with a rounded punch, where the burr, when examined, was found to have suffered the greatest compression just below the punch. With regard to the percentage of energy developed as heat, it was about the same as in the previous experiments, reaching in one case, with an iron bar and with an energy of 110 kilogram-meters, the exceedingly high figure of 91 per cent. With copper, the same figure varied between 50 and 60 per cent.--_Iron_.

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A NOVEL PROPELLER ENGINE.

By Prof. C.W. MacCord.

The accompanying engravings illustrate the arrangement of a propeller engine of 20 inch bore and 22 inch stroke, whose cylinder and valve gear were recently designed by the writer, and are in process of construction by Messrs. Valk & Murdoch, of Charleston, S.C.

In the principal features of the engine, taken as a whole, as will be perceived, there is no new departure. The main slide valve, following nearly full stroke, is of the ordinary form, and reversed by a shifting link actuated by two eccentrics, in the usual manner; and the expansion valves are of the well known Meyer type, consisting of two plates on the back of the main valve, driven by a third eccentric, and connected by a right and left handed screw, the turning of which alters the distance between the plates and the point of cutting off.

The details of this mechanism, however, present several novel features, of which the following description will be understood by reference to the detached cuts, which are drawn upon a larger scale than the general plan shown in Figs. 1 and 2.

The first of these relates to the arrangement of the right and left handed screw, above mentioned, and of the device by which it is rotated.

Usually, the threads, both right handed and left handed, are cut upon the cut-off valve stem itself, which must be so connected with the eccentric rod as to admit of being turned; and in most cases the valve stem extends through both ends of the steam chest, so that it must both slide endwise and turn upon its axis in two stuffing boxes, necessarily of comparatively large size.

All this involves considerable friction, and in the engine under consideration an attempt has been made to reduce the amount of this friction, and to make the whole of this part of the gear neater and more compact, in the following manner:

Two small valve stems are used, which are connected at their lower ends by a crosstail actuated directly by the eccentric rod, and at their upper ends by a transverse yoke. This yoke, filling snugly between two collars formed upon a sleeve which it embraces, imparts a longitudinal motion to the latter, while at the same time leaving it free to rotate.

This sleeve has cut upon it the right and left handed screws for adjusting the cut-off valves; and it slides freely upon a central spindle which has no longitudinal motion, but, projecting through the upper end of the valve chest, can be turned at pleasure by means of a bevel wheel and pinion. The rotation of the spindle is communicated to the sleeve by means of two steel keys fixed in the body of the latter and projecting inwardly so as to slide in corresponding longitudinal grooves in the spindle.

Thus the point of cutting off is varied at will while the engine is running, by means of the hand wheel on the horizontal axis of the bevel pinion, and a small worm on the same axis turns the index, which points out upon the dial the distance followed. These details are shown in Figs. 3, 4, and 5; in further explanation of which it may be added that Fig. 3 is a front view of the valve chest and its contents, the cover, and also the balance plate for relieving the pressure on the back of the main valve (in the arrangement of which there is nothing new), being removed in order to show the valve stems, transverse yoke, sleeve, and spindle above described. Fig. 4 is a longitudinal section, and Fig. 5 is a transverse section, the right hand side showing the cylinder cut by a plane through the middle of the exhaust port, the left hand side being a section by a plane above, for the purpose of exhibiting more clearly the manner in which the steam is admitted to the valve chest; the latter having no pipes for this service, the steam enters below the valve, at each end of the chest, just as it escapes in the center.

The second noteworthy feature consists in this: that the cut-off eccentric is not keyed fast, as is customary when valve gear of this kind is employed, but is loose upon the shaft, the angular position in relation to the crank being changed when the engine is reversed; two strong lugs are bolted on the shaft, one driving the eccentric in one direction, the other in the opposite, by acting against the reverse faces of a projection from the side of The eccentric pulley.

The loose eccentric is of course a familiar arrangement in connection with poppet valves, as well as for the purpose of reversing an engine when driving a single slide valve. Its use in connection with the Meyer cut-off valves, however, is believed to be new; and the reason for its employment will be understood by the aid of Fig. 6.

For the purposes of this explanation we may neglect the angular vibrations of the connecting rod and eccentric rod, considering them both as of infinite length. Let O be the center of the shaft; let L O M represent the face of the main valve seat, in which is shown the port leading to the cylinder; and let A be the edge of the main valve, at the beginning of a stroke of the piston. It will then be apparent that the center of the eccentric must at that instant be at the point, C, if the engine turn to the left, as shown by the arrow, and at G, if the rotation be in the opposite direction; C and G then may be taken as the centers of the "go-ahead" and the "backing" eccentrics respectively, which operate the main valve through the intervention of the link.

Now, in each revolution of the engine, the cut-off eccentric in effect revolves in the same direction about the center of the main eccentric. Consequently, we may let R C S, parallel to L O M, represent the face of the cut-off valve seat, or, in other words, the back of the main valve, in which the port, C N, corresponds to one of those shown in Fig. 4; and the motion of the cut-off valve over this seat will be precisely, the same as though it were driven directly by an eccentric revolving around the center, C.

In determining the position of this eccentric, we proceed upon the assumption that the best results will be effected by such an arrangement that when cutting off at the earliest point required, the cut-off valve shall, at the instant of closing the port, be moving over it at its highest speed. And this requires that the center of the eccentric shall at the instant in question lie in the vertical line through C.

Next, the least distance to be followed being assigned, the angle through which the crank will turn while the piston is traveling that distance is readily found; then, drawing an indefinite line C T, making with the vertical line, G O, an angle, G C T. equal to the one thus determined, any point upon that line may be assumed as the position of the required center of the cut-off eccentric, at the beginning of the stroke.

But again, in order that the cut-off may operate in the same manner when backing as when going ahead, this eccentric must be symmetrically situated with respect to both C and G; and since L O M bisects and is perpendicular to G C, it follows that if the cut-off eccentric be fixed on the shaft, its center must be located at H, the intersection of C T with L M. This would require the edge of the cut-off valve at the given instant to be at Q, perpendicularly over H; and the travel over the main valve would be equal to twice C H, the virtual lever arm of the eccentric, the actual traverse in the valve chest being twice O H, the real eccentricity.

This being clearly excessive, let us next see what will occur if the lever arm, CH, be reduced as in the diagram to CK. The edge of the cut-off valve will then be at N; it instantly begins to close the port. CN, but not so rapidly as the main valve opens the port, AB.