abutment to
counterbalance the outward pressure of the vault covering the highest
and central parts of the building in cases where that vault rested
upon and abutted against walls which themselves were carried by
arches, and were virtually internal walls, so that no buttress could
be carried up from the ground to steady them.

A pier of masonry, sometimes standing alone, sometimes thrown out from
the aisle wall opposite the point to be propped, formed the solid part
of this buttress; it was carried to the requisite height and a flying
arch spanning the whole width of the aisles was thrown across from it
to the wall at the point whence the vault sprung. The pier itself was
in many cases loaded by an enormous pinnacle, so that its weight might
combine with the pressure transmitted along the slope of the flying
arch to give a resultant which should fall within the base of the
buttress. The back of such an arch was generally used as a water
channel.

The forest of flying buttresses round many French cathedrals produces
an almost bewildering effect, as, for instance, at the east end of
Notre Dame;--our English specimens, at Westminster Abbey for example,
are comparatively simple.

_Towers._

The gable and the tower are developments of the walls of the building.
Gothic is _par excellence_ the style of towers. Many towers were
built detached from all other buildings, but no great Gothic building
is complete without one main tower and some subordinate ones.

In the E. E. style church towers were often crowned by low spires,
becoming more lofty as the style advanced. In the Dec. style lofty
spires were almost universal. In the Perp. the tower rarely has a
visible roof.[12]

The artistic value of towers in giving unity coupled with variety to a
group of buildings can hardly be exaggerated.

The positions which towers occupy are various. They produce the
greatest effect when central, _i.e._ placed over the crossing of the
nave and transepts. Lichfield, Chichester, and Salisbury may be
referred to as examples of cathedrals with towers in this position and
surmounted by spires. Canterbury, York, Lincoln, and Gloucester are
specimens of the effectiveness of the tower similarly placed, but
without a spire (Fig. 12). At Wells a fine central octagon occupies
the crossing, and is remarkable for the skill with which it is fitted
to the nave and aisles internally. Next to central towers rank a pair
of towers at the western end