Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

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Arch. We know that the temples of the gods and the theatres
required an immense apparatus of machinery for the ce¬
lebration of some of their mysteries; and that the Diony¬
siacs contracted for those jobs, even at far distant places,
where they had not the privilege of building the edifice
which was to contain them. This is the most likely way
of explaining the very small quantity of mechanical know¬
ledge that is to be met with in the writings of the an¬
cients. Even Vitruvius does not appear to have been of
the fraternity, and speaks of the Greek architects in terms
of respect next to veneration. The Collegium Murario-
rum, or incorporation of masons at Rome, does not seem
to have shared the secrets ol the Dionysiacs.
11. The art of building arches has been most assidu¬
ously cultivated by the associated builders of the middle
ages of the Christian church, both Saracens and Christians,
and they seem to have indulged in it with fondness: they
multiplied and combined arches without end, placing them
in every possible situation.
12. Having studied this branch of the art of building
with so much attention, they were able to erect the most
magnificent buildings with materials which a Greek or
Roman architect could have made little or no use of.
There is infinitely more scientific skill displayed in a
Gothic cathedral than in all the buildings of Greece and
Rome. Indeed these last exhibit very little knowledge
of the mutual balance of arches, and are full .of gross
blunders in this respect; nor could they have resisted the
shock of time so long, had they not been almost solid
masses of stone, with no more cavity than was indispensa¬
bly necessary.
13. Anthemius and Isidorus, whom the Emperor Justi¬
nian had selected as the most eminent architects of Greece,
for building the celebrated church of St Sophia at Con¬
stantinople, seem to have known very little of this matter.
Anthemius had boasted to Justinian that he would outdo
the magnificence of the Roman Pantheon, for he would
hang a greater dome than it aloft in the air. Accord¬
ingly he attempted to raise it on the heads of four piers,
distant from each other about 115 feet, and about the same
height. He had probably seen the magnificent vault¬
ings of the temple of Mars the Avenger, and the temple
of Peace at Rome, the thrusts of which are withstood by
two masses of solid wall, which join the side walls of the
temple at right angles, and extend sidewise to a great dis¬
tance. It was evident that the walls of the temple could
not yield to the pressure of the vaulting without pushing
these immense buttresses along their foundations. He
therefore placed four buttresses to aid his piers. They
are almost solid masses of stone, extending at least ninety
feet from the piers to the north and to the south, form¬
ing as it were the side walls of the cross. They effectu¬
ally secured them from the thrusts of the two great arches
of the nave which support the dome; but there was no
such provision against the push of the great north and
south arches. Anthemius trusted for this to the half
dome which covered the semicircular east end of the
church, and occupied the whole eastern arch of the great
dome. But when the dome was finished, and had stood
a few months, it pushed the two eastern piers with their
buttresses from the perpendicular, making them lean to
the eastward, and the dome and half dome fell in. Isido¬
rus, who succeeded to the charge on the death of Anthe¬
mius, strengthened the piers on the east side by filling
up some hollows, and again raised the dome. But things
gave way before it was closed; and while they were build¬
ing in one part, it was falling in in another. The pillars
and walls of the eastern semicircular end were much shat¬
tered by this time. Isidorus seeing that they could give
no resistance to the push which was so evidently direct-
C H.
ed that way, erected some clumsy buttresses on the east Arch,
wall of the square which surrounded the whole Greek
cross, and was roofed in with it, forming a sort of cloister
round the whole. These buttresses, spanning over this
cloister, leaned against the piers of the dome, and thus
opposed the thrusts of the great north and south arches.
The dome was now turned for the third time, and many
contrivances were adopted for making it extremely light.
It was made offensively flat, and, except the ribs, was
roofed with pumice stone ; but, notwithstanding these pre¬
cautions, the arches settled so as to alarm the architects,
and they made all sure by filling up the whole from top to
bottom with arcades in three stories. The lowest arcade
was very lofty, supported by four noble marble columns,
and thus preserved in some measure the church in the
form of a Greek cross. The story above formed a gallery
for the women, and had six columns in front, so that
they did not bear fair on those below. The third story
was a dead wall filling up the arch, and pierced with three
rows of small, ill-shaped windows. In this unworkman¬
like shape it has stood till now, and is the oldest church
in the world; but it is an ugly mis-shapen mass, more re¬
sembling an overgrown potter’s kiln, surrounded with fur¬
naces pierced and patched, than a magnificent temple.
We have been thus particular in our account of it, because
this history of the building shows that the ancient archi¬
tects had acquired no distinct notions of the action of
arches. Almost any mason of our time would know,
that as the south arch would push the pier to the east¬
ward, while the east arch pushed it to the southward, the
buttress which was to withstand these thrusts must not
be placed on the south side of the pier, but on the south¬
east side, or that there must be an eastern as well as a
southern buttress.
14. No such blunders are to be seen in a Gothic cathe¬
dral. Some of them appear, to a careless spectator, to
be very massive and clumsy; but when judiciously ex¬
amined, they will be found very bold and light, being
pierced in every direction by arcades; and the walls are
divided into cells like a honeycomb, so that they are very
stiff, while they are very light.
15. About the middle, or rather towards the end, ofbrHooke’s
last century, when the Newtonian mathematics opened P™“Ple of
the road to true mechanical science, the construction ofau *
arches engrossed the attention of the first mathematicians.
The first hint of a principle that we have met with is Dr
Hooke’s assertion, that the figure into which a chain or
rope, perfectly flexible, will arrange itself when suspended
from two hooks, is, when inverted, the proper form for an
arch composed of stones of uniform weight. This he af¬
firmed on the principle, that the figure which a flexible
festoon of heavy bodies assumes, when suspended from
two points, is, when inverted, the proper form of an arch
of the same bodies, touching each other in the same
points ; because the force with which they mutually press
on each other in this last case are equal and opposite to
the forces with which they pull at each other in the case
of suspension.
This principle is strictly just, and may be extended to
every case which can be proposed. We recollect seeing
it proposed in very general terms in 1759, when plans
were forming for Blackfriars Bridge in London ; and since
it is perhaps equal in practical utility to the most elaborate
investigations of the mathematicians, our readers will not
be displeased with a more particular account of it in this
place.
16. Let ABC (fig. 6) be a parcel of magnets of any explainc
size and shape, and let us suppose that they adhere with
great force by any points of contact. They will compose
such a flexible festoon as we have been speaking of if