The question may arise: What causes this? Is it for variation in the inclination of the tangents over the well-hole? It is simply owing to the tangents being used in handrailing to square the joints.
The inclination of the bottom tangent a" of the bottom wreath is clearly shown in the diagram to be determined by the inclination of the bottom flight. The joint at a" is made square to both the straight rail of the flight and to the bottom tangent of the wreath; the rail and tangent, therefore, must be equally inclined, otherwise the joint will not be a true butt-joint. The same remarks apply to the joint at 5, where the upper wreath is shown jointed to the straight rail of the upper flight. In this case, tangent d" must be fixed to incline conform ably to the in clination of the upper rail; other wise the joint at 5 will not be a true butt-joint.
The same principle is ap plied in deter mining the pitch or inclination over the crown tangents b" and c". Owing to the necessity of joint ing the two wreaths, as shown at h, these two tangents must have the same inclination, and therefore must be fixed, as shown from 2 to 4, over the crown of the well-hole.
The tangents as here presented are those of the elevation, not of the face-mould. Tangent a" is the elevation of the side plan tan gent a; tangents b" and c" are shown to be the elevations of the plan tangents b and c; so, also, is the tangent d" the elevation of the side plan tangent d.
If this diagram were folded, as Fig. 94 was shown to be in Fig. 03, the tangents of the elevation—namely, a", b", c", d"—would stand over and above the plan tangents a, b, c, d of the well-hole. In prac tical work, this diagram must be drawn full size. It gives the correct length to each tangent as required on the face-mould, and furnishes also the data for the lay-out of the mould.
Fig. 96 shows how to find the angle between the tangents of the face-mould for the bottom wreath, which, as shown in Fig. 95, is to span over the first plan quadrant a b. The elevation tangents a" and b", as shown, will be the tangents of the mould. To find the angle between the tangents, draw
the line a h in Fig. 96; and from a, measure to 2 the length of the bottom tangent a" in Fig. 95; the length from 2 to h, Fig 96, will equal the length of the upper tangent b", Fig. 95.
From 2 to 1, measure a distance equal to 2-1 in Fig.
95, the latter being found by dropping a perpendicular from w to meet the tangent b" extended. Upon 1, erect a perpendicular line; and placing the dividers on 2, extend to a; turn over to the perpendicular at a"; con nect this point with 2, and the line will be the bottom tangent as required on the face-mould. The upper tangent will be the line 2-h, and the angle between the two lines is shown at 2. Make the joint at h square to 2-h, and at a" square to a"-2.
The mould as it appears in Fig. 96 is complete, except the curve, which is comparatively a small matter to put on, as will be shown further on.
The main thing is to find the angle between the tan gents, which is shown at 2, to give them the direction to square the joints.
In Fig. 97 is shown how to find the angle between the tangents c" and d" shown in Fig. 95, as required on the face-mould. On the line h-5, make h--4 equal to the length of the bottom tangent of the wreath, as shown at h"-4 in Fig. 95; and 4-5 equal to the length of the upper tangent d". Measure from 4 the distance shown at 4-6 in Fig 95, and place it from 4 to 6 as shown in Fig. 97; upon 6 erect a perpendicular line. Now place the dividers on 4; extend to h; turn over to cut the perpendicular in h"; connect this point with 4, and the angle shown at 4 will be the angle required to square the joints of the wreath as shown at h" and 5, where the joint at 5 is shown drawn square to the line 4-5, and the joint at h" square to the line 4 h" .
Fig. 98 is a diagram of tangents and face-mould for a stairway having a well-hole at the top landing. The tangents in this example will be two equally i nclined tan gents for the bot tom wreath; and for the top wreath, one inclined and one lev el, the latter align ing with the level rail of the landing.