Ammianus writes of the catapult ¹:

`In the middle of the ropes² rises a wooden arm like a chariot pole . . . to the 
top of the arm hangs a sling . . . when the battle is commenced a round stone is set 
in the sling . . . four soldiers on each side of the engine wind the arm down till it is 
almost level with the ground . . . when the arm is set free it springs up and hurls 
forth from its sling the stone, which is certain to crush whatever it strikes.  This 
engine was formerly called the “scorpion,” because it has it sting erect,³ but later 
ages have given it the name of Onager, or wild ass, for when wild asses are chased 
they kick the stones behind them.’

Fig. 7. – Catapult  (with  a  sling) (See previuos page)

A.  The arm at rest, ready to be wound down by the rope attached to it and 
also to the wooden roller of the windlass.  The stone may be seen in the sling.
The upper end of the pulley rope is hitched by a metal slip-hook (fig.6) to a ring-bolt secured to the arm just below the sling.

B.  The  position  of  the  arm  when  fully  wound  down  by  means  of  the 
windlass and rope.  See also EE, fig. 8.

C.  The  position  of  the  arm  at  the  moment  the  stone D leaves  the  sling, 
which it does at an angle of about 45 degrees.

E.  By pulling the cord E the arm B is at once released from the slip-hook 
and, taking an upward sweep of 90 degrees, returns to its original position at A.

F.  Its fixed end which passes through a hole near the top of the arm.

G.  The leather pocket for the stone.

H.  The loop which is hitched over the iron pin at the top of the arm when 
the stone is in position in the sling, as shown at A and B, fig. 7.

¹   Roman History, Book XXIII., Chapter IV.

²  i.e. in the middle of the twisted skein formed of ropes of sinew or hair.

³ The upright and tapering arm of a catapult, with the iron pin on its top for the loop of the sling, is here fancifully likened to the erected tail of an angry scorpion with its sting protruding.

I. I.    }
II. II. }  The side pieces
III. IV. The large cross-pieces
V. The small cross-pieces

The ends of the cross-piece beams are stepped into the side pieces.

AA. The skein of twisted cord.

BB. The  large  winding  wheels.  The  skein  is  stretched  between  these 
wheels,  its  ends  passing  through  the  sides  of  the  frame,  and  then  through  the 
wheels and over their cross-bars.  (fg. 12.)

By  turning  with  a  long  spanner  (fig.  6.)  the  squared  ends  of  the 
spindles DD, the pinion wheels CC rotate the large wheels BB and cause the latter 
to twist the skein AA, between the halves of which the arm EE is placed.

FF. The wooden roller which winds down the arm EE.  (fig. 6.)

The roller is revolved by four men (two on each side of the engine) who fit 
long spanners on the squared ends of the iron spindle GG.