obstruction is removed, falls with a ringing sound into a bronze vessel.

7. So, when a ship is making headway, whether under oars or under a gale
of wind, the floatboards on the wheels will strike against the water and
be driven violently back, thus turning the wheels; and they, revolving,
will move the axle, and the axle the drum, the tooth of which, as it
goes round, strikes one of the teeth of the second drum at each
revolution, and makes it turn a little. So, when the floatboards have
caused the wheels to revolve four hundred times, this drum, having
turned round once, will strike a tooth of the horizontal drum with the
tooth that is fixed to its side. Hence, every time the turning of the
horizontal drum brings a stone to a hole, it will let the stone out
through the pipe. Thus by the sound and the number, the length of the
voyage will be shown in miles.

I have described how to make things that may be provided for use and
amusement in times that are peaceful and without fear.

CHAPTER X

CATAPULTS OR SCORPIONES

1. I shall next explain the symmetrical principles on which scorpiones
and ballistae may be constructed, inventions devised for defence against
danger, and in the interest of self-preservation.

The proportions of these engines are all computed from the given length
of the arrow which the engine is intended to throw, and the size of the
holes in the capitals, through which the twisted sinews that hold the
arms are stretched, is one ninth of that length.

2. The height and breadth of the capital itself must then conform to the
size of the holes. The boards at the top and bottom of the capital,
which are called "peritreti," should be in thickness equal to one hole,
and in breadth to one and three quarters, except at their extremities,
where they equal one hole and a half. The sideposts on the right and
left should be four holes high, excluding the tenons, and five twelfths
of a hole thick; the tenons, half a hole. The distance from a sidepost
to the hole is one quarter of a hole, and it is also one quarter of a
hole from the hole to the post in the middle. The breadth of the post
in the middle is equal to one hole and one eighth, the thickness, to one
hole.

3. The opening in the middle post, where the arrow is laid, is equal to
one fourth of the hole. The four surrounding corners should have iron
plates nailed to their sides and faces, or should be studded with bronze
pins and nails. The pi