In cutting out the shape of the segments for a dome-covering (Fig. 192), no great skill is required. All the setting-out necessary is shown in Fig. 193. A half-section of the dome is drawn, and divided up into convenient parts, and numbered as seen. The dome being octagonal, the angle that the plan of the joint line will make with the base line will be -

360 / 8x2 = 22 1/2°

When the joint line is drawn in at the required angle, the figure below the base line, it should be noticed, will represent the plan of half a segment.

Fig. 192.

Lines are run down from the division points across the base to the joint line. The girth line is laid out in the usual way, and construction lines drawn across, these being cut off equal to the length of the corresponding lines drawn between the base and joint lines. Thus, to give one example, line 8 8° on the pattern is the same length as 8' 8" on the plan, and so with the rest of the lines.

If there is a ridge-roll at the joints, then the width of this must be allowed for in marking out the pattern, one-half the width of roll being set along inside the plan of joint line, which will give the required reduction.

The number of pieces of sheet metal required to make up a segment will, of course, depend upon its size. There should be no trouble, however, to determine this when once the shape of a complete segment is marked out.

If a roll or ridge-cap has to be bent to the shape of a joint, then this can be accomplished by first marking out the exact shape of a joint This is shown set out at the bottom of Fig. 193.

The joint-line plan and its division points are laid down, and construction lines run up from the intermediate points, the lengths of these being cut off equal to the heights of the corresponding line on the half-section. It should be observed that the three curves of the joint shape come out as quarters of ellipses, and, if desired, can be marked out by the trammel method, as explained in Chapter XXI (Elliptical Work. Construction Of Ellipse).

Fig. 193.