Arruvur.—On Skew Arches. 269 
AB. In the same way draw face line CD, and then the heading joints of 
skewbacks, and face quoins at right angles to coursing joints. The other 
heading joints of soffit may be left to the builder to arrange, care being 
taken that a good bond is maintained throughout. This is the usual 
method of designing the development of the intrados, and, taken in con- 
nection with plan and development of extrados on same drawing, it is very 
convenient, so far as showing the relation of the different parts. But it is 
reversed from its normal position, or turned upside down: this will be 
referred to again. 
Face Joints. 
The elevation on skew or elevation of face, must have the face joints, 
or rather their chords, drawn from a centre lower than the axis of the 
cylinder and from which point they radiate. This property in skew arches 
was discovered by Mr. Buck, and is known as the focal eccentricity, or 
the difference between centre of cylinder and centre from which chords of 
face-joints radiate. The formula for calculating this is 60 = Rad. x tan. 0 
x tan. 9; and a geometrical method is given by diagram in ‘‘ Masonry and 
Stone-cutting," by Dobson. 
Extrados. 
The development of the extrados should next be drawn. This is done 
similarly to that of the intrados, but with this difference, that the coursing 
spirals are not at right angles to the heading spirals. The reason of this is 
that the length of the coursing spirals, measured or projected on the axis of 
the cylinder is the same for intrados and extrados, but the length of the 
heading spirals is greater in the extrados than the intrados. Another con- 
sequence is that the angle of extrados Œ A’ H or 4, is greater than the 
angle of intrados 8. The figure 4’ B' C D’, Drawing No. 2, represents a de- 
velopment of the extrados. 
Elevation, etc. 
The more essential designs having now been completed, from these the 
elevation, plan and cross section, shewn in Drawing No. 1, are drawn. 
In the elevation, the only matters further to be noted are that the back of 
the arch-stones should be stopped, and the courses of spandril walls built 
to correspond, thus securing greater stability than if these precautions were 
neglected. The plan shews the peculiar serrated appearance which the 
springers or skewbacks present, owing to the obliquity of the courses ; and 
it will also be perceived that the boss stones at the obtuse and acute angles, 
are greater and less respectively than the other skewbacks, for the same 
reason. 
Bathlin Burn Bridge. 
The remaining peculiarities of skew arches, and the rules which are 
