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

Fig. 21. Vertical Section of Chimney and Foundation.

Elevation of the Completed Chimney.

Fig. 22. Elevation of the Completed Chimney.

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

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

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

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

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 well-known 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.

The smoke flues enter the chimney below the constricted portion of the inner flue. The top caps are of cast iron and may be made in sections and bolted together, as well for convenience in erecting as for economy of pattern making.

That on the inner core terminates 7 feet 2 inches below the top of the main cap. At four sides of the outer structure are openings, as shown in Fig. 21, the bottom of each opening being on a level with the top of the inner cap. By this means the current of air which always rises along any high wall is taken advantage of, as it passes up the side of the chimney, into these openings, and out the top of the chimney, and creates a partial vacuum over the top of the central flue, thus considerably increasing the draft.

Means should be provided for reaching the top of the chimney, as the iron caps will need painting, or lightning rods may have to be placed or repaired. Iron ladders up the side may be fastened to the wall, or a permanent block may be attached to the main cap and provided with a wire rope, for this purpose.

As a matter of safety from lightning it is well to provide lightning conductors. A round copper rod of not less than ⅝-inch diameter, or one of equal area of cross-section, may be run up outside of the chimney, through heavy glass insulators, and terminate four feet above the main cap in four points, 5/16Vinch diameter or equivalent area. The lower end of the rod should go into moist earth and be attached to a cast iron plate 30 inches square and «-inch thick.

It is often the case that chimneys are erected that are much higher in proportion to their width than the example here shown, and in situations where the structure is well protected from the pressure of the wind, this may be safely done. But when erected in an exposed situation it will be well to consider, as of first importance, the factor of stability.

It will not be proper or fair, however, to confound the appearance of a chimney built in the substantial form herein described, with some of the smaller and more slender structures sometimes seen, which have no central core, either because their moderate height does not render it necessary, because the temperature of the gases is not so high as to endanger a chimney of single walls, or from considerations of economy.

The chimney here shown is in every way substantial and reliable, and from the dimensions and proportions given, chimneys of any required capacity, or to suit any condition of surroundings, may be successfully designed and constructed.