n! A R C H I T 
the return, moulding of an open pediment. Divide the 
contour of a into four or more equal parts, as at i, 2, 3, 
Sec. through which draw right lines parallel with the rake, 
and from the points 1, 2, 3, 4, draw lines parallel with the 
level moulding; then draw ed at b perpendicular to the 
rake, and take the projections 4, 4, &c. and place them on 
the rake at b, as (hewn in the figure ; and through thefe 
points deferibe the contour of the cima recta moulding of 
the raking cornice. Laltly, at C, divide Jg in the fame 
manner as before at a, and let fall perpendiculars, which 
by interfering their refpedtive raking-lines, will give the 
true curve, as will evidently appear by infpeCting Jg. 2, 
where obferve, that the projections of the level moulding 
a are taken from level lines; but, at b, the fame projec¬ 
tions are laid on the raking-lines. Again alio at C, 1, 2, 
3, are on a level line, taken from 1, 2, 3, on the rake; and 
the fame points may be found by taking the feveral lengths 
sit 1, 2, 3, 4, on the raking-lines, of a, and placing them 
on the fimilar raking-lines at C. Some architects perform 
this operation by biledting the hypothenufal lines of each 
moulding, which anfvvering as chords to each curve, the 
centre at L, Jig. 3, is found by the common method of 
deferibing a circle that will pafs through three given points. 
Of BRICK and PLASTER GROINS. 
Groins are the angular curves made by the interfeffions 
of femi-cylinders or arches ; and are either regular, or ir¬ 
regular. A regular groin is when the interfering arches, 
whether femicircular or femi-elliptical, are of the fame 
diameters and heights; an irregular groin is where .one of 
the arches is femicircular, and the other femi-elliptical. 
Fig. 4, in the preceding Plate, is the plan of an irregular 
brick groin, whofe body-arch A is a femicircle, and whofe 
interfering or fide arches BB are femi-elliptical. In Jig .5, 
is given a perfperive view of a regular brick groin, whofe 
arches are femicircular, and of the fame diameter, inter¬ 
fering each other at right angles; and, fince any oblique 
ferion of a cylinder produces a regular elliptic curve, it 
mull be evident that the angular ribs of fuch groins will 
be femi-ellipfes, having their tranfverfe axes horizontal, 
and their femi-conjugate vertical. And this alio will be 
the cafe when the interfering or fide arches, as BB, Jig. 4, 
are elliptical; for, as 1,2, 3, 4, are evidently the bafe-lines 
of fuch ferions, it is eafy to conceive that their ordinates 
will coincide with thofe of the body-arch A, which is a 
femicircle, and confequently produces an elliptic arch, as 
iliewn at Jig. 6, having its horizontal axis equal to 1 2, or 
3 4, and its vertical to A b. The reader will by this time 
have noticed, that the interfering arches BB, are formed 
by the ererion of what workmen call th z jack ribs , a per¬ 
fperive view of which is given at Jig. 6, where 2,3,4, 
exhibit the manner of their being fixed on the body-arch 
A, after it is boarded over. To keep thefe jack ribs true, 
and in a right line at the top, workmen place a tranfverfe 
board upon the crown of the arch, as fhewn in Jig. 6, and 
fix it fufficiently low to receive the thicknefs of the cover¬ 
ing, that the body and interfering arches maybe perfer* 
ly even when the whole is covered with boards, as in fig.5, 
which reprefents the date of the groin ready for turning 
the brick-work over the arches. Let it therefore be ob- 
ferved, that the body-arch A, Jig. 6, is already completely 
covered, independent of the ererion of the jack ribs, 
whofe feat on the body-arch, and their feveral heights, 
may be found as fliewn at C 2, 3, 4, in Jig. 6, the bare in- 
fperion of which will afford a fufficient explanation. It 
may however be obferved, that as the feat of the feveral 
jack ribs on the covering of the body-arch, will produce 
an elliptic curve, which being deferibed on a level plane, 
as fliewn at Jig. 7, it may be found thus : Let a be be the 
body-arch, and ade the interfering elliptic arch ; draw 
fimilar ordinates to both arches, as 1, 2, 3, &c. make them 
interfer each other, and produce them each way at plea¬ 
sure. Then make gh equal to the girt or circumference 
of ab, and gk equal to the girt of ad%z 1. Divide gh 
and gk into four equal parts, for the quadrant of each 
3 
E C T U R E. 
arch was fo divided; then through each divifion draw per¬ 
pendicular lines cutting at right angles the ordinates which 
v/ere drawn out at pleafure; and through their interfec- 
tions, as at 5,6, 7, 8, 9, 10, if curves be drawn, they will 
produce corredt moulds, by which the mitering of each 
arch may be deferibed, or their refpeftive coverings. 
Suppofe it be required to mark a line on the body-arch at 
fig. fi, to touch the extremity of each jack rib at the bale} 
we take the mould 5, 6, 7, and, imagining it to be made of 
very thin pliable wood, we place the end h to the crown 
of the body-arch at h,Jg. 6, which being fecured to that 
point, we prefs the other end 5, to the foot of the jack rib 
at 5, and by a pencil deferibe the curve fought. By the 
like procefs the other mould 8,9, 10, may be applied to 
the interfeffions of the elliptic arch ; and, fuppofing it to 
be covered, as at Jg. 5, and we imagine the arch to be 
drawn back and feparated from the body, the mould 8, 9, 
10, bent over the boards, would'Coincide with their ends, 
provided the arches were of the fame dimenfions, which, 
however, is not the cafe in this example, although in theory 
the principle is precifely the fame. Yet we cannot, with 
fome architedls, affirm, that the mould, 8, 9, 10, is of any 
utility in covering the jack ribs with boards; for, if we 
would cover the ribs 2, 3, 4, at Jg. 6, the boards muff: be 
applied fingly, and each made to join clofe on the boards 
of the body-arch. It is evident, therefore, that to mark 
each board by the mould 8, 9, 10, and particularly if the 
boards are confidered as retaining their natural level fur- 
face, it would be a blind and uncertain method, and quite 
inadequate to the purpofe. 
In fixing the ribs of the body-arch, at Jg. 5, ed are 
ffrong wooden pods, and ii are blocks, or the ends of 
beams extending the whole length of the groin, and are 
fupported by polls undereach rib. Between is the girder 
of each rib, which is omitted at e, in order to give an in- 
fide view of the arch. Thefe long beams alfo adt on the 
principle of wedges, as may be feen at e ; fo that, when the 
brick-work is properly fet, they are gradual ly*eafed, and 
the whole of the wooden ribs, beams, and pods, are by this 
means ffruck, and readily cleared away. 
Fig. 8, is the plan of an ajeending or defeending groin, the 
fide-arches of which being given, it is required to find the 
interfeffions of tlie angles upon the plan, and the moulds 
for deferibing the curvature of the interfering arches, in 
a fimilar manner as on the coverings above deferibed. 
After what has been Hated in the foregoing example, 
the folution of the prefent queftion will require but little 
explanation, becaufe the general principles are the fame 
in both cafes, as well as in all thofe which follow. To 
projedt the prefent figure, divide one quarter of the body 
rib B into four equal parts, as at 1, 2, 3 ; from which draw 
lines to the perpendiculars 4, 5, and continue them round 
to 4, 6 ; then draw the lines of defeent 48, 67, &c. and 
make C equal to the given fide arches in the defeent; it 
matters not how many there are. From the feveral points 
1, 2, 3, &c. of the arch C, where the lines of defeent cut 
the periphery of C, draw lines perpendicular to the lines 
of defeent, and continue them at pleafure. Again, from 
the fame points 1, 2, 3, &c. at C, draw lines at right angles 
to the fides of the plan A, and from the points at B do the 
fame, and their interfedlions will give the curves of the 
arches on the plan, as is obvious from infpeffing the figure. 
Having thus drawn lines from C, perpendicular to the 
lines of defeent, take the girt of the quarter arch at B, 
and transfer it to the centre line at C, as 4, a,b, c,d , through 
which divifions draw lines parallel to the defeent, and their 
interfedlions will produce the curve for the mould, which 
is to be applied exaffiy in thdfame manner as deferibed in 
the preceding example. Groins of this defeription are to be 
feen in the utmoft perfection under the Adelphi buildings, 
in the Strand, London, which have a very rapid defeent. 
Fig. 9, reprefents a groin, whofe interfering or fide 
arch is Gothic, and under pitch, that is, when the perpen¬ 
dicular height of the fide arch, as B, is lower than the 
body arch, as A, which is a femicircle. At B, the inter¬ 
fering 
