ARCHITECTURE. 
fionre of the extrados of which is a portion that of the other, it is called an equal- pitched 
of a cylindric surface, terminating on the 
top of the walls which support it in a ho- 
rizontal plane, parallel to the axis of the 
cylinder. This is also called a cradle 
vault. 
A cylindroidal vault is a plain vault, the 
figure of the extrados of which springs from 
a horizontal plane ; its section perpendicular 
to those lines is every where a semi-ellipsis, 
equal and similar throughout, and its base is 
that of either axis ; or it is sometimes a seg- 
ment of an ellipsis, less than a semi-ellipsis, 
having an ordinate parallel to the axis for its 
base. 
A dome may be defined to be a vault 
rising from a circular, elliptical, or polygo- 
nal plan or base, such that all horizontal 
sections of the intrados are similar figures, 
having their centres in the same vertical 
line or axis, and such that the plans of any 
two sections may have the sides of similar 
inscribed figures parallel to each other, or 
that the figures of these plans may be concen- 
tric. If the dome is a portion of a sphere, 
that is, if its base be a circle, and its vertical 
section through the centre of its base the 
segment of a circle, then it is also called a 
cnpola. 
When the portion of a sphere, or cupola, 
springs from a wall on a polygonal plan, 
and the vertical axis of the sphere passes 
through the middle of the plan, then the 
spandrels, or triangular spheric portions, 
comprehended between the springing lines 
and a horizontal plane passing through the 
different summits of the walls, are called 
pendentives. 
When two or more plain vaults penetrate 
or intersect each other, the figure of the in- 
trados formed by the several branches is 
called a groin, or cross vault. 
When two opposite equal branches meet 
other two opposite equal branches in two 
intersecting vertical planes, passing through 
the diagonal lines, joining the four exterior 
angles of the plane, the groin may be called 
an equal pitched quadrilateral groin. 
If two opposite branches of an equal- 
pitched groin have cylindrical intradoes, 
and their plan of less breadth than that of 
the other two branches, the groin may be 
called cylindro-cylindroidal, or cylindroido- 
cylindric groin, according as the cylindric 
branches or the other two are of the greatest 
breadth. 
When a groin consisting of four branches 
is made by two equal portions of cylindric 
surfaces, with the axis of the one cutting 
cylindric groin. 
When two opposite branches of a cylindric 
groin are of less breadth than the other two, 
it may be called un unequal-pitched cylin- 
dric groin. This is called by workmen a 
Welsh groin. 
When the branches of a cylindric groin 
are of equal breadth in the plan, the groin 
may be called an equilateral cylindric groin. 
It is not easy to give a geometrical defi- 
nition that will extend to all properties of 
vaulting, called by writers of the first emi- 
nence, groins. The first given is almost 
universal. It applies not only to plain 
vaults intersecting each other, but also to 
those that are annular, or in the form of 
semi-cylindric rings, intersected by cylindric 
or cylindroidal plain vaults, the axis of 
which tends to that of the annulus. It does 
not, however, comprehend that species 
used in King Henry VII.’s 'chapel, West- 
minster, and King’s College chapel, Cam- 
bridge. 
This species of groins, instead of the hori- 
zontal sections of the curved surfaces pre- 
senting exterior right angles, as is generally 
the case, present convex arches of circles. 
There is yet one property that is common 
to every species of groins, that is, the seve- 
ral branches intersect and form arches upon 
each inclosing wall, and the perpendicular 
surface of the wall upon each side is con- 
tinued till it is intercepted by the entrados 
of the arches ; consequently the upright of 
each wall is equal in height to the summit 
of the arches. Hence the difference be- 
tween groins and domes. A groin is a 
branched vault, and each branch terminates 
against the enclosing walls ; whereas a dome 
is a vault without branches, and the curves 
spring from the wall, or walls, from all 
points around its bottom circumference, 
whether the walls stand upon a polygonal, 
circular, or elliptic plan. 
The Greeks, it would appear, had few or 
no arches or vaults much prior to the reign of 
Augustus, from which time they sometimes 
employed plain vaults with cylindrical in- 
tradoes ; we also find that they used qua- 
drilateral, equal-pitched groined vaults, with 
cylindrical or cylindroidal intradoes, or 
mixed of both, over the passages of the thea- 
tres and gymnasia. 
The Romans, as would appear also, did 
not employ vaults more early than the 
Greeks. The Pantheon, one of the earliest 
remaining structures with arches, was pro- 
bably built by Agrippa, the sou-in-law of 
