ole.

6. These engines are constructed according to these proportions or with
additions or diminutions. For, if the height of the capitals is greater
than their width--when they are called "high-tensioned,"--something
should be taken from the arms, so that the more the tension is weakened
by height of the capitals, the more the strength of the blow is
increased by shortness of the arms. But if the capital is less
high,--when the term "low-tensioned" is used,--the arms, on account of
their strength, should be made a little longer, so that they may be
drawn easily. Just as it takes four men to raise a load with a lever
five feet long, and only two men to lift the same load with a ten-foot
lever, so the longer the arms, the easier they are to draw, and the
shorter, the harder.

I have now spoken of the principles applicable to the parts and
proportions of catapults.

CHAPTER XI

BALLISTAE

1. Ballistae are constructed on varying principles to produce an
identical result. Some are worked by handspikes and windlasses, some by
blocks and pulleys, others by capstans, others again by means of drums.
No ballista, however, is made without regard to the given amount of
weight of the stone which the engine is intended to throw. Hence their
principle is not easy for everybody, but only for those who have
knowledge of the geometrical principles employed in calculation and in
multiplication.

2. For the holes made in the capitals through the openings of which are
stretched the strings made of twisted hair, generally women's, or of
sinew, are proportionate to the amount of weight in the stone which the
ballista is intended to throw, and to the principle of mass, as in
catapults the principle is that of the length of the arrow. Therefore,
in order that those who do not understand geometry may be prepared
beforehand, so as not to be delayed by having to think the matter out at
a moment of peril in war, I will set forth what I myself know by
experience can be depended upon, and what I have in part gathered from
the rules of my teachers, and wherever Greek weights bear a relation to
the measures, I shall reduce and explain them so that they will express
the same corresponding relation in our weights.

3. A ballista intended to throw a two-pound stone will have a hole of
five digits in its capital; four pounds, six digits; and six pounds,
seven digits; ten pounds, eight digits; twenty pounds, ten digits; forty
pounds, twel