we must provide such a system of driving power as not to interfere with them. Shafting may be used near the side walls for driving machines. under it or near to it, but the main dependence will have to be the electric system, which, with separate motors for each machine, or one motor for a group of machines located closely together, seems to be the favorite method of driving.

Fig. 16 shows the most approved form of truss for supporting this type of roof, and Fig. 17 gives the form of girders used to support the ridges of the roof where the glazed portion joins the roof proper. This construction is of light structural steel and no more members are used than is absolutely necessary, hence the entrance of light is very slightly interrupted and still there is sufficient strength for all practical purposes.

The glazing of the light portions should receive much attention, in order to avoid leakages, as this is always one of the drawbacks of any kind of inclined windows.

For purposes of ventilation the sashes may be hinged at the top and opened by any convenient means. A simple device is to run a light shaft along inside the building and near the bottoms of the sashes, and fix to it cast iron arms, in the form of cranks, for each sash. From these arms connecting rods run to the sashes. This shaft may be operated by fixing to it a grooved pulley, over which a small rope runs, and reaches down near the floor, from which it may be operated. Levers standing in opposite direction on this shaft, and having a cord attached to each end, may be used for the same purpose. These shafts ought not to be over 50 feet in length, operated by one set of ropes. Pieces of shafting 15 or 20 feet long can be easily coupled together. The plainest possible construction is sufficient for all practical purposes.

Ribbed glass should be used in these sashes as it prevents the glaring effect of direct and unimpeded sunlight, and diffuses a soft and agreeable light over the whole area to be lighted.

In Fig. 18 is shown a longitudinal section, and in Fig. 19 a cross-section, of this same style of roof, but in this case it is constructed principally of wood instead of steel. The form of the inclined truss, and the method of securing the timbers by bolts is clearly shown. Wall plates of the form shown should be used, as well as proper resting plates on the tops of the central posts, or columns, so constructed as to not only properly support the truss for holding the window frames and sashes, but also to furnish a proper abutment for the lower end of the rafter braces.

The roof timbers should be secured to the walls and to the posts by anchor bolts, to prevent the roof from being lifted by high winds.

In putting on the roof planking for this type of roof different methods

CAST IRON

-30 ft:

CAST IRON

FIG. 20.

Perspective View of Machine Shop with Saw-tooth Roof.

must be adopted for the steel construction from that used for wood trusses and roof timbers.

In the case of the steel trusses, the supporting bars are shown in solid section in the longitudinal section, Fig. 16, and to these the planks may be secured by bolts, or lag screws, if steel supports are used, or by spikes, if these supports are of wood. In either case the planks will run in the direction of the pitch of the roof.

If the wood construction is used, the rafters will be not over 10 feet centers, and the planks long enough to reach at least two spaces, or twenty feet, and are spiked directly to the rafters, consequently they will run at right angles to them. The planks should be 3 inches thick and 6 inches wide and be grooved in both edges, and have separate splines put in to connect them.

In case of either steel or wood construction, the roof planks should be covered with good rosin-sized roofing paper, mopped with hot tar, and upon this a heavy quality of roofing tin, or some form of the modern sheet steel roofing. All All sheet metal should be painted on the under side before it is laid.

The gutters or valleys of these roofs should be the subject of careful attention. They need not be of sharp pitch, as a quarter of an inch to the foot will be

sufficient. Conductor pipes at each valley should carry off the water. In the case of the shop shown in perspective in Fig. 20, the valleys on the high central portion should incline each way from the center and from each side, conductor pipes carrying the water to the valleys in the lower part of the building, and from thence it flows to the conductor pipes at the sides.

Either of the methods above described for constructing the roof will be found economical to build and well adapted to the purposes for which they are to be used.

CHAPTER IX

DESIGN AND CONSTRUCTION OF CHIMNEY, OR STACK

Peculiarities of chimney design. Brick construction. Foundations. The various types of chimneys compared. Steel chimneys. The proportions of the chimney. Height of chimney. Its dimensions to suit certain boiler capacity. Methods of calculation. Limits to the heights of single wall chimneys. Vertical and cross-sections. The outer walls. Octagonal versus square forms of construction. Relative numbers of bricks used. The chimney base and foundation. Quality of mortar. The central core. Care in construction. Form of central flue. The cap. Provisions for reaching the top. Lightning rods. The chimney of strength and the chimney of expediency.

THUS far our methods of construction and the necessary materials for them have been such as are encountered daily by the architect and the builder. We now come to the erection of the chimney or stack, which has many peculiarities and restrictions on its design and construction, resulting from its narrow foundation, great height, and the necessity of its resisting not only the high-wind pressures and great changes in temperature at different seasons, but also the great difference of temperature on the inside and on the outside. It seems necessary, therefore, to treat this subject of chimney construction in a separate chapter wherein we will consider the respective merits of and the objections to chimneys of the more common forms and materials.

Regarding the chimney built of brick, the principal objections would appear to be its first cost, which is considerable, and the fact that owing to its narrow base and great height very firm and solid foundations must be prepared. This, of course, becomes more difficult and expensive where the ground is soft and excavations must be made at great depth, or where piles have to be driven to build the foundation upon.

At the present time many sheet iron or steel chimneys are erected, and it is the prevailing idea that they are the more economical. About the only advantage they seem to possess, however, is that owing to their comparatively light weight they may be erected on superstructures upon which a brick chimney could not. Then, too, their first cost is much less than for a brick chimney of equal capacity.

Some of their disadvantages are, that they are very liable to rust at the seams and rivets, owing to the impossibility of keeping these points properly protected from water. Therefore they are comparatively short lived.