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 conformably to the inclination of the upper rail; otherwise the joint at 5 will not be a true butt-joint.
Fig. 98. Diagram of Tangents and Face-Mould
for Stair with Well-Hole at Upper Landing.
The same principle is applied in determining the pitch or inclination over the crown tangents b″ and c″. Owing to the necessity of jointing 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 tangent 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.
Fig. 99. Drawing
Mould when
One Tangent
Is Level and
One Inclined
over Right-
Angled Plan.