1ſet with their Heads one againſt the other, and
their Feet ſet wide, would, if faſtened at Top,
ſtand, very firm, by means of the Equalneſs
of their Weight: They were pleaſed with this
Invention, and began to make their Roofs
in the ſame Manner, to throw off the Rain,
both Ways. Afterwards, perhaps, not being
able to cover a wider Space for want of Beams
long enough, they put between the Heads of
theſe two Beams another croſſways at Top,
ſo that they made a Figure much like that of
the Greek Letter π, and this middle Beam
they might call a Wedge; and as this ſuc
ceeded very well, they multiplyed the Wedges,
and thus made a Kind of Arch, whoſe Figure
mightily delighted them. Then transferring
the ſame Method to their Works of Stone, con
tinuing to multiply the Wedges, they made
an entire Arch, which muſt be allowed to be
nothing elſe but a Conjunction of a Number
of Wedges, whereof ſome ſtanding with their
Heads below the Arch, are called the Foot of
the Arch, thoſe in the Middle above, the Key
of the Arch, and thoſe on the Sides, the Turn,
or Ribs of the Arch. It will not be improper
here to repeat what we ſaid in the firſt Book
upon this Subject: There are different Sorts
of Arches, the Entire, is the full half of a
Circle, or that whoſe Chord runs through the
Centre of the Circle; there is another which
approaches more to the Nature of a Beam than
of an Arch, which we call the Imperfect, or
diminiſhed Arch, becauſe it is not a compleat
Semi-circle; but a determinate Part leſs,
having its Chord above the Centre, and at
ſome Diſtance from it. There is alſo the
Compoſite Arch, called by ſome the Angular,
and by others an Arch compſed of two Arches
leſs than Semi-circles; and its Chord has the
two Centres of two Curve Lines, which
mutually interſect each other. That the Entire
Arch is the Strongeſt of all, appears not only
from Experience, but Reaſon; for I do not
ſee how it can poſſibly diſunite of itſelf, unleſs
one Wedge ſhoves out another, which they are
ſo far from doing, that they aſſiſt and ſupport
one another. And indeed, if they were to go
about any ſuch Violence, they would be pre
vented by the very Nature of Ponderoſity, by
which they are preſſed downwards, either by
ſome Superſtructure, or by that which is in the
Wedges themſelves. This makes Varro ſay,
that in Arches, the Work on the right Hand
is keptup no leſs by that on the Left, than the
Work on the Left is by that on the Right. And
if we look only into the Thing itſelf; how is
it poſſible for the middle Wedge at Top, which
is the Key-ſtone to the Whole, to thruſt out
either of the two next Side Wedges, or how
can that be driven out of its Place by them?
The next Wedges alſo in the Turn of the
Arch, being juſtly counterpoiſed, will ſurely
ſtand to their Duty; and laſtly, how can the
two Wedges under the two Feet of the Arch,
ever be moved while the upper ones ſtand firm?
Therefore we have no need of a Cord, or Bar
in an entire Arch, becauſe it ſupports itſelf
by its own Strength; but in diminiſh'd
Arches there is Occaſion either for an Iron
Chain or Bar, or for an Extenſion of Wall on
both Sides, that may have the Effect of a Bar
to ſupply the Want of Strength, that there is
in the diminiſh'd Arch, and make it equal to
the Entire. The ancient Architects always
uſe theſe Precautions, and where-ever it was
poſſible, conſtantly ſecured their diminiſh'd
Arches, by ſetting them in a good Body of
Wall. They alſo endeavour'd, if they had an
Opportunity, to turn their imperfect Arches
upon a ſtrait Beam; and over theſe imperfect
ones, they uſed to turn entire Arches, which
protected the diminiſhed ones which were
within them, and took upon themſelves the
Burthen of the Superſtructure. As for Com
poſite Arches, we do not find any of them in
the Buildings of the Ancients; ſome think
them not amiſs for the Apertures in Towers;
becauſe they ſuppoſe they will cleave
the great Weight that is laid upon
them, as the Prow of a Ship does the Water,
and that they are rather ſtrengthened than op
preſs'd by it.
their Feet ſet wide, would, if faſtened at Top,
ſtand, very firm, by means of the Equalneſs
of their Weight: They were pleaſed with this
Invention, and began to make their Roofs
in the ſame Manner, to throw off the Rain,
both Ways. Afterwards, perhaps, not being
able to cover a wider Space for want of Beams
long enough, they put between the Heads of
theſe two Beams another croſſways at Top,
ſo that they made a Figure much like that of
the Greek Letter π, and this middle Beam
they might call a Wedge; and as this ſuc
ceeded very well, they multiplyed the Wedges,
and thus made a Kind of Arch, whoſe Figure
mightily delighted them. Then transferring
the ſame Method to their Works of Stone, con
tinuing to multiply the Wedges, they made
an entire Arch, which muſt be allowed to be
nothing elſe but a Conjunction of a Number
of Wedges, whereof ſome ſtanding with their
Heads below the Arch, are called the Foot of
the Arch, thoſe in the Middle above, the Key
of the Arch, and thoſe on the Sides, the Turn,
or Ribs of the Arch. It will not be improper
here to repeat what we ſaid in the firſt Book
upon this Subject: There are different Sorts
of Arches, the Entire, is the full half of a
Circle, or that whoſe Chord runs through the
Centre of the Circle; there is another which
approaches more to the Nature of a Beam than
of an Arch, which we call the Imperfect, or
diminiſhed Arch, becauſe it is not a compleat
Semi-circle; but a determinate Part leſs,
having its Chord above the Centre, and at
ſome Diſtance from it. There is alſo the
Compoſite Arch, called by ſome the Angular,
and by others an Arch compſed of two Arches
leſs than Semi-circles; and its Chord has the
two Centres of two Curve Lines, which
mutually interſect each other. That the Entire
Arch is the Strongeſt of all, appears not only
from Experience, but Reaſon; for I do not
ſee how it can poſſibly diſunite of itſelf, unleſs
one Wedge ſhoves out another, which they are
ſo far from doing, that they aſſiſt and ſupport
one another. And indeed, if they were to go
about any ſuch Violence, they would be pre
vented by the very Nature of Ponderoſity, by
which they are preſſed downwards, either by
ſome Superſtructure, or by that which is in the
Wedges themſelves. This makes Varro ſay,
that in Arches, the Work on the right Hand
is keptup no leſs by that on the Left, than the
Work on the Left is by that on the Right. And
if we look only into the Thing itſelf; how is
it poſſible for the middle Wedge at Top, which
is the Key-ſtone to the Whole, to thruſt out
either of the two next Side Wedges, or how
can that be driven out of its Place by them?
The next Wedges alſo in the Turn of the
Arch, being juſtly counterpoiſed, will ſurely
ſtand to their Duty; and laſtly, how can the
two Wedges under the two Feet of the Arch,
ever be moved while the upper ones ſtand firm?
Therefore we have no need of a Cord, or Bar
in an entire Arch, becauſe it ſupports itſelf
by its own Strength; but in diminiſh'd
Arches there is Occaſion either for an Iron
Chain or Bar, or for an Extenſion of Wall on
both Sides, that may have the Effect of a Bar
to ſupply the Want of Strength, that there is
in the diminiſh'd Arch, and make it equal to
the Entire. The ancient Architects always
uſe theſe Precautions, and where-ever it was
poſſible, conſtantly ſecured their diminiſh'd
Arches, by ſetting them in a good Body of
Wall. They alſo endeavour'd, if they had an
Opportunity, to turn their imperfect Arches
upon a ſtrait Beam; and over theſe imperfect
ones, they uſed to turn entire Arches, which
protected the diminiſhed ones which were
within them, and took upon themſelves the
Burthen of the Superſtructure. As for Com
poſite Arches, we do not find any of them in
the Buildings of the Ancients; ſome think
them not amiſs for the Apertures in Towers;
becauſe they ſuppoſe they will cleave
the great Weight that is laid upon
them, as the Prow of a Ship does the Water,
and that they are rather ſtrengthened than op
preſs'd by it.
THE Stones uſed in Building an Arch,
ſhould be every Way the biggeſt that can be
got; becauſe the Parts of any Body that are
united and compacted by Nature, are more
inſeparable than thoſe which are join'd and
cemented by Art. The Stones alſo ought to
be equal on both Sides, as if they were balan
ced with reſpect to their Fronts, Sizes, Weight,
and the like. If you are to make a Portico, and
to draw ſeveral Arches over continued Aper
tures, from the Capitals of Columns, never let
the Seat from which two or more Arches are
to riſe, be made of two Pieces, or of as many
as there are to be Arches, but only of one
ſingle Stone, and that as ſtrong as may be, to
hold together the Feet of all the Arches. The
ſecond Stones in the Arch, which riſe next to
theſe, if they are large Pieces, muſt be ſet
ſhould be every Way the biggeſt that can be
got; becauſe the Parts of any Body that are
united and compacted by Nature, are more
inſeparable than thoſe which are join'd and
cemented by Art. The Stones alſo ought to
be equal on both Sides, as if they were balan
ced with reſpect to their Fronts, Sizes, Weight,
and the like. If you are to make a Portico, and
to draw ſeveral Arches over continued Aper
tures, from the Capitals of Columns, never let
the Seat from which two or more Arches are
to riſe, be made of two Pieces, or of as many
as there are to be Arches, but only of one
ſingle Stone, and that as ſtrong as may be, to
hold together the Feet of all the Arches. The
ſecond Stones in the Arch, which riſe next to
theſe, if they are large Pieces, muſt be ſet