OBLIQUE ARCHES. 
gh, advance through equal ratios of the axis, in equal por- 
tions of the arc, but fimply as the ref{peétive horizontal bafes 
: aaa ore its deviation from a line rec- 
s becomes progreflively lefs in its ap- 
rch, w ie. 
or fete pitch of the arch, 
t d are the abutments of the bridge, and, confequently, rsdt 
the bafe of the arch. The angle of obliquity is here 51°, 
i the line g 4 paffes along the 
gmand hn, terminated 
lf the arch of Finlay bridgé, 
al plane; mn and ¢d being paaallel 
to, and re€tangular from each other; and their diftances 
ec rom ie crown oe the arch, g 4, in their refpedctive lines, 
the line ¢ 4, the curved lines, oo and 4g, con 
the right line pq, fhew the form of half a imicireular arch 
of the fame bafe, drawn out into a plane. In both exhibi- 
tions, it is clear that lines drawn perpendicular to different 
parts of the face of the arch, to ferve as guide-lines to the 
joints of the foffit, would in one part converge to each 
other, and form triangles; and in the other part, o the 
face-line extended on a plane is concave, would di 
the face 
ment, gre ouput up, 
ith their plank hheeting, ese fer off 
their bounding-lines, m p and gn, as in any other arch; fo 
that every part of thefe Face-lines 
y iz 
is divided into only fix ie as the purpofe of extn 0 
will be equally well attai 
gures I, 2, 3; eo. in aes from / to g, fhew 
the divifions on that line; an » 6, &e, sara corre- 
{pondent {paces clofe to each apace. Thefe divifions 
eing made, fet out, on the fheeting, fo many parallel lines 
to the outer faces, m g p, and nh g 
&e.: AI reprefents a line of joint near the crown of the 
arch, and rectangular to its face ; which commencing at the 
diftance of ££, will interfe& 4g, the line of the crown, at 
the diftance of 41, from the Ce of the arch; and will re- 
prefent half the line, ##, in diagram, jig. 4. 
By the theory laid down for thele arches, the joints on 
their foffit are to run parallel to each other ; or rather they 
are to interfe& equal portions of parallel equidiftant arcs, 
é r common axis. is en 
may obvioufly be bear by fetting off on eae face of 
A 
other from the points 41, 2, 3, 4, 5, and g, towards each 
abutment ; and then to mark upon the fheeting ftrong lines, 
to dire&t the courfe of the joints, fuch as fhewn by 41, 14, 
zv, &c. which will form a polygonic curve, more or lefs 
approaching to a regular curve, according to the obliquity 
of the arch, and its a siege to a femicircle, and th 
alee of divifions between 4 a The pee aad 
the 
an acute internal eel with the line of the arch, and from 
the abutment on the oppofite fide, as fhewn by thofe fee 
Thefe excentricities, eee they render it difficult to 
form all the voiffoirs prior to the commencement of the arch, 
= eafily got over by orang them in fucceffion, as de- 
cribed. 
As the lines, a1 a, 624, &c. over the curve of the arch 
will be rather troublefome to form correGly, the beft mode 
in practice will be to mark off on each face, and each fide of 
the arch, a Sean con of the {fpaces, gz and 4&, t ds 
each abutm ent. Then as the progreffive correfpondent 
points oa, » Viz. 2 f, will have right lines 
between them cage ears can eafily be ttruck ; 
found the fame a n the lines a1 a, ree * hich, ae 
fore, have no Be natG te) 
the number of cvifions on the line g been greater, 
Ha 
the curvature Hs th would hav 
a 
eCtangle wit ent to occafion 
fiderable irregularity, if the joint-lines be continued acrofs, 
eit ace. 
requent epporiis 0 of {eeing, for feveral years 
after . was built, the bridge from which moft of the dia- 
grams have been drawn, and never obferved erack in it 3 
but yet from the uncertainty of obtaining careful and intel- 
ligent — an eae terials, and ile becaufe 
Grand Canal of ia. which w 
obliquely, and in other ridges, Gece, over wide 
drains in the Eaft Riding of Yorkfhire, venture to exceed 
40° of ee - ae 30", acy oe an angle of in- 
terfeftion of t rally more con- 
fiderable. 
would often be highly ufeful, as it would rarely leave much 
to be made up by the deviation of the road from its ufual 
direftion. The wing-walls of Finlay bridge were curved 
one more than the other on the fame fide, to give a paflage 
alorg each bank of the canal; but where not over a canal, 
if the line of direétion of the ‘road, coincide with that of the 
¥ 
