backward and forward. When
this machine was further perfected by rigging it upon wheels, and
constructing over it a roof, so as to form a _testudo_, which protected
the besieging party from the assaults of the besieged, there was no
tower so strong, no wall so thick, as to resist a long-continued attack,
the great length of the beam enabling the soldiers to work across the
defensive ditch, and as many as one hundred men being often employed
upon it. The Romans learned from the Greeks the art of building this
formidable engine, which was used with great effect by Alexander, but
with still greater by Titus in the siege of Jerusalem; it was first used
by the Romans in the siege of Syracuse. The _vinea_ was a sort of roof
under which the soldiers protected themselves when they undermined
walls. The _helepolis_, also used in the attack on cities, was a square
tower furnished with all the means of assault. This also was a Greek
invention; and the one used by Demetrius at the siege of Rhodes, B. C.
306, was one hundred and thirty-five feet high and sixty-eight wide,
divided into nine stories. The _turris_, a tower of the same class, was
used both by Greeks and Romans, and even by Asiatics. Mithridates used
one at the siege of Cyzicus one hundred and fifty feet in height. These
most formidable engines were generally made of beams of wood covered on
three sides with iron and sometimes with rawhides. They were higher than
the walls and all the other fortifications of a besieged place, and
divided into stories pierced with windows; in and upon them were
stationed archers and slingers, and in the lower story was a
battering-ram. The soldiers in the turris were also provided with
scaling-ladders, sometimes on wheels; so that when the top of the wall
was cleared by means of the turris, it might be scaled by means of the
ladders. It was impossible to resist these powerful engines except by
burning them, or by undermining the ground upon which they stood, or by
overturning them with stones or iron-shod beams hung from a mast on the
wall, or by increasing the height of the wall, or by erecting temporary
towers on the wall beside them.

Thus there was no ancient fortification capable of withstanding a long
siege when the besieged city was short of defenders or provisions. With
forces equal between the combatants an attack was generally a failure,
for the defenders had always a great advantage; but when the number of
defenders was reduce