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

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. .

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.

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