Since the above paragraph was written the author saw one of these steel chimneys which had been built by a most reliable firm, erected with a great deal of care, painted with the best materials to be had, that in a little over a year became a total wreck, owing to the rusting of the material around the joints and rivets. A new stack had to be erected. The combined cost of the two would have built a good and substantial brick chimney that would have endured for many years.

Again, in the effort to protect the metal they must be frequently painted, or coated with some of the numerous "cure-all" paints, "warranted to protect them perfectly inside and out"; and the use of any protective covering is a continual expense for maintenance to which the brick chimney is not subject.

The conclusion, therefore, must be that, if the life of the chimney is of less consideration than its first cost, we would adopt that constructed of sheet iron or steel; but if we regard permanency and the ultimate outlay, both for construction and maintenance and all the advantages derived, brick is evidently the material to be chosen.

The height of the chimney will depend somewhat upon surrounding hills, high buildings, and similar obstructions to the free course of the wind, but should never be less than the diameter of the internal flue multiplied by twenty. The diameter of the internal flue will depend on the aggregate areas of the smoke flues or "up-takes" leading from the boilers, and these necessarily depend upon the grate surface, allowing about 4.5 square feet per horse-power.

There are many methods of calculating the diameter of chimney flues, some of which are very complex and depend upon many assumed conditions at each step, which ofttimes have hardly more practical value than guesses. Others assume to calculate the volume of gases, the speed of their flow, the area of grate openings, etc., all of which might be changed with each sample of coal, or according to the condition of the weather.

Practical engineers will probably favor the following simple method, even with its arbitrary assumptions, and will be quite successful in the practical application of it as it is the result of much actual experience. The horsesay in this case 470 power being given and allowing 4.5 square feet of grate surface per horse-power, we have 104.4. At 5 pounds of coal per horsepower, which is quite liberal, we will burn 2,350 pounds of coal per hour. Our chimney is 100 feet high. We divide the pounds of coal burned per hour by the square root of the height multiplied by 12 (10X 12=120) and we have 19.58 as the area of the chimney flue, in square feet. Divide this by .7854 and extract the square root and we have the diameter, slightly less than 5 feet.

Having the diameter of flue and height given we may by inverse methods

obtain the horse-power, grate surface, etc. In making these calculations we should be sure to get capacity enough; for if the chimney is a little too large no harm is done, while if a little too small a serious expense is incurred for a supplementary one.

All chimneys over 75 feet high should be built with a central "core," or flue, preferably of circular form, surrounded by an outside casing sufficiently strong to properly support the inner core and to resist the pressure of the strongest winds.

The thickness of the walls of both the outer portion and the inner core should be sufficient to be very rigid near the ground and gradually thinner as the walls rise, the "breaks" being, of course, on the inside of the outer portion and the outside of the inner core. These breaks are usually four inches, or

the width of a brick at each step.

The "batter," or inclination of the outside face of the main structure should be a quarter of an inch per foot. In our case we are supposed to require a chimney 100 feet high, with a circular flue 5 feet in diameter.

In the illustrations, Fig. 21 shows a vertical section through the center of the chimney and its foundation. It shows the thickness of walls, special form of central flue, retaining caps at the top, and special arrangements for increasing the draft. Fig. 22 is an elevation of the exterior, showing its general form and appearance when completed. Fig. 23 shows a horizontal section through the octagonal portion on the line AA, Fig. 21. Fig. 24 shows a horizontal section through the square base, on the line BB, Fig. 21. Fig. 25 illustrates, on an enlarged scale, the number of bricks necessary for a course, if the square form were to be continued to the top. Fig. 26 shows the economy of adopting the octagonal form, as saving material and labor, and offering considerably less surface to wind pressure from certain directions.

In the Figs. 25 and 26 the upper half shows the laying of a "header" course and the lower half the laying of a "straight" course.

In these two sketches it will be seen that in a header course the octagonal form contains 52 bricks less than the square form; and in a straight course 32 bricks are saved. Assuming five courses per foot in height, and that in each foot we have one header course, we save by the octagonal form 180 bricks. This, of course, is less as we approach the top, but the average saving will be considerably over 100 bricks per foot, or over 8,000 for the whole work.

Actual experience shows that the extra labor cost of making the many corners is more than balanced by the smaller number of bricks laid in the octagonal form than in the square form. The appearance is much enhanced and the wind pressure is considerably diminished by getting rid of the projecting

corners.

The base of the chimney is of square form, this being more convenient

HEADER COURSE, 312 BRICKS

STRAIGHT COURSE, 280 BRICKS

FIG. 25. Horizontal Section, showing number of Bricks in the Square Form.

HEADER COURSE, 260 BRICKS

STRAIGHT COURSE, 248 BRICKS

FIG. 26. Horizontal Section, showing number of Bricks in the Octagonal Form.

for the introduction of the smoke flues or "up-takes" from the boilers, the placing of the ash doors and the general appearance.

The ash door is shown in Fig. 22. The opening should be arched, preferably of semicircular design, as affording the most strength to sustain the great weight of brickwork over it. It should be closed with a sheet iron door. The openings for smoke flues should also be strongly arched, similar to the ash doorways. From the square portion at the base, the main shaft of the chimney is of octagonal form, as indicated in Fig. 24.

For foundations the earth should be excavated to perfectly hard ground, making a pit 28 feet square; that is, twice the depth of the foundation, assuming that in consequence of the condition of the ground it is necessary to excavate to a depth of 14 feet. In this pit should be a bed, 4 feet thick of large stones laid in strong cement mortar. Upon this should be courses about 18 inches thick and gradually drawn in at the top to 16 feet square.

By strong cement mortar we mean that containing two parts cement, one part of lime, and about three parts of clean, sharp sand. The amount of sand will vary considerably with its fineness, sharpness, and its freedom from dirt; the finer the sand the greater the quantity that must be used.

In erecting the central core and the outward supporting structure great care should be used to make all joints of uniform thickness, and to see that as the courses are laid on they are frequently leveled.

"Batter plumbs" should be used for the outside; that is, the board on which the plumb line is attached should have the batter or inclination by being made narrower at the bottom. For instance, a plumb board 4 feet long should be 6 inches wide at the top and 4 inches at the bottom (the batter being equal on both edges).

Another matter that must be scrupulously attended to is that of properly supporting the inner core. It will not do to lay bricks from wall to wall so as to tie them together, as the expansion and contraction of the inner core would soon ruin the structure.

The support is given by building up into the outer wall inwardly projecting bricks reaching half way across the space, and against these, others projecting outwardly from the inner core. These should be placed on all eight sides, in the same course, and at intervals of not over eight feet through the entire height.

The form and thickness of walls and the heights of the "breaks" are shown in Fig. 21. The central flue is formed after the model of the wellknown student lamp and forms a very effective combustion chamber for escaping smoke. It is the form adopted by a prominent engineer who built a large number of chimneys of this design which have been in successful use for many years.