In Fig. 566 are shown the side and end elevations and the plan of a chimney top. ABCD of the plan represents the size of the article at the bottom to fit the chimney, and E F G H is the size of the opening in the top to fit a pipe or extension. An inspection of the drawings will show that the article consists of two flat triangular sides, of which A H D is the plan and A1 H1 D1 the elevation, two similar triangles, D G C, forming the ends, and four corner pieces, of which F C G is a plan, which are portions of an oblique cone. Further inspection of the plan will also show that the entire article consists of four symmetrical quarters, therefore in Fig. 567 is shown a quarter plan of the same in which corresponding points are lettered the same as in Fig. 566, from which the pattern for one-quarter of the article is obtained.

Fig. 566. - Plan and Elevations of Chimney Top

The quarter circle F G is the quarter plan of an oblique cone, of which C is the apex; therefore divide F G into any convenient number of equal parts, and from the points thus obtained draw lines to C. The next step is to construct a diagram of triangles of which the lines just drawn are the bases and of which the hight of the article is the altitude. Assuming O C as the base line of this diagram, place one foot of the compasses at C, and, bringing the pencil point to the various points in F G, strike arcs cutting O C, as shown. At right angles to O C erect C Q, equal in hight to J H1 of Fig. 566, and from Q draw lines to the several points in O C. These hypothenuses will then represent the true distances from C to the points in F G. From Q as center, and radii equal to the several hypothenuses, strike arcs indefinitely, as shown to the left. From any convenient point on arc 0, as G1, draw a line to Q, which will form one side of the pattern of the rounded corner. Set the dividers to the space used in stepping off the arc F G, and, commencing at the point G1, step to arc 1 and from that point to arc 2 and so on, reaching the last arc in the point F1. Trace a line through these points, as shown from F1 to G1, and draw F1 Q, which will complete the pattern of the corner piece.

Fig. 567. - One-Quarter Plan and Pattern of Chimney Top. .

From C set off on O C the distances O F and L G, as shown by M and N. Draw lines from these points to Q, then M Q and N Q will represent the true distances shown by O F and G L of the plan or J H1 and L G2 of the elevations in Fig. 566.

With Q as center, and C L as radius, describe an arc, L1, and from G1 as center, with radius equal to N Q, intersect the arc, as shown, thus establishing the point L1. Draw G1 L1 and L1 Q. In a similar manner, with Fl as center, and Q M as radius, describe the arc O1, and from Q as center, with a radius equal to C O of the plan, intersect the arc at the point O1. Draw Q O1 and O1 F1; then F1 O1 Q L1 G1 will form the pattern for one complete quarter of the chimney top. A duplicate of this pattern may be added to it if desired, joining the two upon the line F1 O, thus forming a pattern for one half, as shown in Fig. 568.

Fig. 568. - One-Half Pattern of Chimney Top.