968 Transactions. —Miscellaneous. 
Section on Square and Skews. 
A section on the square, of the arch with given rise and span, is then 
described as segment 4 B. A series of ordinates are drawn parallel to 
abutments from the chord Ab on plan, to the solid face line A B, and con- 
tinued where necessary to the extrados of section on square. From the 
points where these lines meet 4 B, ordinates are then drawn at right angles 
to AB, and on these heights are carefully marked corresponding to the 
heights measured between the chord Ab and the two surfaces of section on 
square, or its intrados and extrados. By this means, the elevation on skew 
or of face, can be accurately drawn ; the curved lines of this elevation are 
portions of ellipses. 
Intrados. 
The development of the intrados, or soffit of the arch, must next be. 
designed. A B C D on left of plan, represents the hollow surface or under 
side of arch, turned backwards round the impost line A C and spread out 
as a flat surface. It is drawn in this way :— Through A produce chord Ad, 
making its length equal length of segment. Draw bB at right angles to 
segmental length Ab from b and equal to bB on plan, and join A B as 
shewn by dotted lines; that lineis the development of a heading spiral. 
Complete the figure by drawing BD, DC, parallel and equal to the sides 
opposite to them. Draw 4 E from A at right angles to heading spirals 4 B 
or CD, and produce BD to meet it in E, and complete figure ALF F, 
making AFE a right angle. The length bE or, AF is readily calculated 
thus :—b EPI. Angle E AF or f is the angle of intrados, which in this 
arch is 21° 3’; and tan. B=. The number of courses of arch stones, 
always an odd number (in this case 27), is decided on and laid off along 
the heading spiral CD. If AE does not coincide at its intersection with 
CD with one of these divisions, it must be made to do so—in other words, 
the development requires adjustment. This is effected in several ways; 
such as altering the number of divisions of face line, altering span, radiùs, 
or length of impost. In Bathlin Bridge, it was done by ruling the line 4 Æ 
through the nearest face joint, and altering impost line 4 C to correspond ; 
thus avoiding any disturbance of angles or number of courses. On AC, 
mark off as many skewbacks as there are courses between C and A E, and 
through these points draw lines parallel to 4 # ; these lines will intersect 
the courses marked on the heading spiral between C and AF, and will 
represent coursing spirals. Complete drawing of courses through remain- 
ing divisions on CD, and draw development of face line 4 B by ordinates 
laid down at right angles to 4 b at distances equal to those between the 
ordinates along the segment or section on square, and of lengths equal to 
those of the corresponding ordinates between the chord 4 b and face line 
