AnrHUR.—ÜOn Skew Arches. 271 
measured on the laggings between a perpendicular plane in line of face and 
the coursing spiral at the particular point. A convenient method to find this 
angle is with a board, one edge of which is dressed to the curve of the 
soffit in the direction of spiral courses, applied at a point marked at the 
i centre, or any convenient point, of this 
curved edge to the face-line on laggings, 
and resting on laggings in the true 
position of a coursing spiral. A plumb- 
line run then carefully along the upper 
edge of board till the weight points 
exactly on the face line, will give a point 
Fig. 2. on that edge, which joined by a pencil 
line with centre-point on curved edge below, will represent the actual angle 
and its complement at the particular course where the measurement is 
taken. From this board the templets may be made. 
The obliquity of the arch causes a curious result in the length of the 
top and bottom of quoins. That is, on the acute side of arch the soffit of 
the quoins is longer than the top or extrados, while on the obtuse side the 
soffit of the quoins is shorter than the extrados. 
Working of Arch Stones. 
In the operation of dressing or working the arch stones, such as the 
quoins, a plain surface is first prepared approximating in size and direction 
a coursing joint or bed, and on this the curve of the arch soffit, in the same 
direction, is drafted. The twisting rules are next applied, and the plane 
surface is worked off to the proper twist, right handed or left, as the case 
may be. Then the soffit is worked to the arch square—(a rule made by 
joining a straight-edge at right angles to a rule having the curve of the arch) 
—the heading joint is dressed at right angles to soffit, and the face to a rule 
or templet forming the correct angle of the face. There are also the neces- 
sary dimensions of quoins to be taken from diagram on platform and from 
drawings as required. 
Though not all mathematically accurate, these rules as I have now 
given them, for the building of a skew arch, are those commonly followed 
in practice, and give sufficiently good results to justify their use. 
* Skew Arches of other forms. 
In the above description of the design and construction of a skew arch, 
it will be observed that a segmental arch only has been treated of. There are 
other forms of skew arches rendered sometimes unavoidable by the peculiar 
circumstances of the case, such as an elliptical skew arch, and a skew arch 
the plan of which is on a curve. 
