OBLIQUE 
arc du b, peer ae nies the front of the arch, reach 
from ¢ to d, viz e abutment on the fide, where it forms 
an obtufe spe wit ich at front, whilft the fame number of 
fimilar {paces in the lke direétion extend only from ¢ to 4, in 
the arc x v y, reprefenting the internal end or extreme extent 
of the voifloirs feen on the face of the arch. ‘The fame cir- 
cumftances are reverfed on the fide where the front forms an 
acute angle with the abutment, as y the fimilar 
references on the other fide of ¢#: confequently the joint 4, 
in front of the arch, will fall into the impoft at y on the 
hefe divergencies would be more 
wee oe fimilar lines upon t rvs; 
which eee et that of x vy, upon the fame level as the 
front arc du b, gall an equal lateral diftance between 
them, which as y eafily conceived is left undone, 
becaufe they would inter fe at the lines requifite for the 
further explanation of the fubjeé&. 
upon the laft defcribed arc, correfpond with x and y 
that for which it is fubttituted, every ‘ai letter or 
figure of reference being the fame in 
The vertical {paces on the a 
cepted between 4 s, 
oF the arch inter- 
joints they pues 3 and on the other fide, dto 4, on the 
lower dotted arc; 3 3, &c. thew the rife on each voiffoir 
erie on fiche points. The horizontal bafe between the 
voifloir at ¢f#, and es of 
the arch: & &, fig. 5, ¢ correfpondent with 
a larger {cale, expre(s the bafe of t 
ha of face, or its width 
afflumed to be fimilar in the others, vis. and 42 
The rife cf the curve in the foffit ea : and &, 
ig. 4, becaufe the crown of the arch 
fhewn by the line vz, mutt be interfeéted at half-way be- 
tween #, é, the extremes of the voiffoir; one end of the ftone 
being on that fide cf the crown defcending to the left, and 
the other ota equidiftant on the fide defcending to the 
rea{t-wall is fhewn rifing reangular from this 
rig upon the principles already explained, 
fhews on the fide where the front line of the arch makes an 
fpringing point at d.to 4, on the pofterior arc which cor- 
i i mity ; and jig. 7. exhibits, on 
oa _ ea the front is acute with its abutment, the eae 
thefe diagrams it appears, that under 
d the fpace between the intrados and 
extrados increafes upon the face of the arch as the haunches 
ARCHES. 
of the arch are aba areee therefore the reat blocks for 
the voiffoirs muft be shan their breadth of face to 
a rectangle with-a line ary 
each extremity of their foffit: viz. as dx is to 
or 47 to 42 In fg. And if the twift of the emacs 
lines be attended to, the long voiffoirs muft have a ftill 
greater width of block between their beds than the fhort 
ones ; 
individual place, as 
eafily be done than giving due allowance to the firft of thefe 
variations, which will be {ufficient in praCtice where the arch, 
excepting its quoins, is formed of brick ; becaufe the facility 
haunches on each joint of it, ve se lal of fitting each 
a to its place will not be t. 
. fufficiently fhews the uae of the breaft or face- 
outwards on t f the foflfit ; 
Finlay bri 
the breaft-wall fhou 
angle with the abutment, or the voifloir fhould have sa 
which may be of the breadth of a brick, if the wall be built 
of that material, to form a ftop to its flipping forward, as 
fhewn by the dotted lines ae it in fig. 6. 
We fhall now point out fome anomalies from the leading 
principle, which are neceflary to be adverted to, for a due 
If it be fuppofed, in either inft 
that the cylindric fegment, lying horizontally on its plane, 
with its axis in the direétion of its abutments, is longer than 
neceflary for the road-way over it, in any given oblique d1- 
rection ; we then, to obtain an rc arch, have only to 
conceive the convex face of the cylindric fegment to be cut 
wn between two parallel vertical dey in the oa on 
rom the nature of cylin 
ich have already been Hone hen ; and 
by vioufly ie not onl it tk c form, bu 
alfo aA fiear approach to a femicircle. 
We have obferved that the leading principle of thefe. 
oblique seer is, that each courfe of voiffoir fhould run 
rectangular to the face of the arch. This, however, muit 
be taken in a limited fenfe; becaule if the fheeting, or bed 
= the arch, were unfolded, or laid into a plane, its faces 
n the t itl ounding the paflage over it, would 
act be age lines par allel to each other, hke the parapet 
walls ; but would form two curved lines, each convex where 
the line of the arch is acute with the abutment, and concave 
an 
ina divetion cbaue 7 gh, ( the a 
verfe abutments, } mult aie in its fpiral gyration 
along the ee of the cylindric fegment on each fide o 
§ Ps! 
