970 Transactions.— Miscellaneous. 
necessary to be observed in construction, may perhaps be more conveniently 
laid before you if, during the rest of this paper, I adhere as nearly as 
possible to a description of the methods actually adopted in the Bathlin 
Bridge. Drawing No. 1 shows the ordinary designs that were used, and 
the principles above explained and represented by Drawing No. 2, were 
carried out in a modified form. 
Skewbacks or Springers. 
First, the skewbacks were cut to sizes, and templets taken from impost 
line AC (as def templet for soffit), and built into position, the face of 
quoins being cut to lie in perpendicular plane; the stones dressed to 
thickness of arch-ring, and the extrados left rough and quarry-faced—tem- 
plet, á g h, not being used. When skewbacks were fixed, centreing and 
laggings were erected carefully. The intrados was drawn on paper on a 
scale of two feet to an inch, but instead of being turned upside down, as in 
drawing No. 2, it was designed in its normal position ; an elevation on skew 
and section on square, on same scale, and a full-sized drawing of twisting 
rules, as sketched in No. 2, were made; but no development of extrados 
was drawn, it was found sufficient to calculate angle of extrados ¢. In 
using the normal development of intrados for this arch, it was found to 
secure much convenience to the builder, who was thus saved the awkward- 
ness of always treating the arch-stones as inverted. A large wooden plat- 
form was constructed adjoining site of bridge, and on it a full-size drawing 
was made of development of intrados in its natural position, and the sizes 
and joints of all the stones arranged on it. On the laggings the coursing 
spirals were laid down or marked off from the development on the plat- 
form. : 
Angle of Twist. 
The angle of twist (¢—/) having been found by calculation, the winding 
strip and parallel rule were drawn full size, and from these the actual rules 
were cut, which the builder used in dressing the beds of the arch-stones to 
the proper twist. The divergent portion of winding strip shown is that 
applicable to stones of three feet in length, applied at right angles to cours- 
ing spirals. These twisting rules may be used either as rules or templets. 
Face angle of Quoins. 
The next thing in importance, requiring great care, was the working the 
faces of the quoins to the proper angle with the soffit. From the springing 
to the keystone the faces of quoins make different angles with the coursing 
joints. On the side next the acute angle these angles are acute, and on 
the side next the obtuse angle they are obtuse; and the angles equi-distant 
from keystone are complements of one another. The angle in each case was 
