THE TORTOISE
1. A tortoise intended for the filling of ditches, and thereby to make
it possible to reach the wall, is to be made as follows. Let a base,
termed in Greek [Greek: eschara], be constructed, with each of its sides
twenty-one feet long, and with four crosspieces. Let these be held
together by two others, two thirds of a foot thick and half a foot
broad; let the crosspieces be about three feet and a half apart, and
beneath and in the spaces between them set the trees, termed in Greek
[Greek: hamaxopodes], in which the axles of the wheels turn in iron
hoops. Let the trees be provided with pivots, and also with holes
through which levers are passed to make them turn, so that the tortoise
can move forward or back or towards its right or left side, or if
necessary obliquely, all by the turning of the trees.
2. Let two beams be laid on the base, projecting for six feet on each
side, round the projections of which let two other beams be nailed,
projecting seven feet beyond the former, and of the thickness and
breadth prescribed in the case of the base. On this framework set up
posts mortised into it, nine feet high exclusive of their tenons, one
foot and a quarter square, and one foot and a half apart. Let the posts
be tied together at the top by mortised beams. Over the beams let the
rafters be set, tied one into another by means of tenons, and carried up
twelve feet high. Over the rafters set the square beam by which the
rafters are bound together.
3. Let the rafters themselves be held together by bridgings, and covered
with boards, preferably of holm oak, or, this failing, of any other
material which has the greatest strength, except pine or alder. For
these woods are weak and easily catch fire. Over the boardings let there
be placed wattles very closely woven of thin twigs as fresh as possible.
Let the entire machine be covered with rawhide sewed together double and
stuffed with seaweed or straw soaked in vinegar. In this way the blows
of ballistae and the force of fires will be repelled by them.