THIS engine is here shown ready for discharge with its bow-string drawn to its full 
extent by the windlass.

The heavy iron-tipped arrow rests in the shallow wooden trough or groove 
which travels along the stock.

The trough has a strip of wood, in the form of a keel, fixed beneath it. This 
keel travels to or fro in a dove-tailed slot cut along the upper surface of the stock
for the greater part of its length.  ( F, fig. 14.)

The arrow is laid in the trough before the bow-string is stretched.  (A, B, fig. 14.)

The  ballista  is  made  ready  for  use  by  turning  the  windlass.  The  windlass 
pulls  back  the  sliding  trough,  and  the  arrow  resting  in  it,  along  the  stock  of  the engine, till the bow-string is at its proper tension for discharging the projectile.
( Fig.13.).

As  the  trough  and  the  arrow  are  drawn  back  together,  the  arrow  can  be 
safely laid in position before the engine is prepared for action.

The  catch  for  holding  the bow-string,  and  the  trigger  for  releasing  it,  are 
fixed to the solid after-end of the wooden trough.  ( Fig. 14.)

The two ratchets at the sides of the after-end of the trough travel over and 
engage, as they pass along, the metal cogs fixed on either side of the stock.
( Fig.14. )¹

By this arrangement the trough can be securely retained, in transit, at any 
point between the one it started from and the one it attains when drawn back to its 
full extent by the windlass.

As the lock and trigger of the ballista are fixed to the after-end of the sliding 
trough ( fig. 14. ), it will be realized that the arrow could be discharged at any 
moment  required  in  warfare,  whether  the  bow-string  was  fully  or  only  partially stretched.

In  this  respect  the  ballista  differed  from  the  crossbow,  which  it  somewhat 
resembled, as in a crossbow the bow-string cannot be set free by the trigger at an 
intermediate point, but only when it is drawn to the lock of the weapon.
It will be seen that the ballista derives its power from two arms ; each with 
its separate skein of cord and pair of winches.

These  parts  of  the  ballista  are  the  same  in  their  action  and mechanism  as 
those of the catapult.

A.  Side view of the stock, with the arrow in the sliding trough before the 
bow-string is stretched.

B.  Surface view of  the stock, with the arrow in the sliding trough before  the bow-string is stretched.

C.  Section of the fore-end of the stock, and of the trough which slides in  and along it.

¹  When the bow-string has been released and the arrow discharged, the ratchets are lifted clear of  the cogs on the stock of the engine.  This allows the trough to be slid forward to its first position as shown in A, B, Fig. 14.  It is then ready to be drawn back again for the next shot.

D.  Surface  view  of  the  trough,  with  the  trigger  and  catch  for  the  bow-
string.

E.  Side  view, showing the  keel (F) which slides along the slot cut in the 
surface of the stock as the trough is drawn back by the windlass.

G.  Enlarged  view  of  the  solid  end  of  the  trough.  This  sketch  shows  the 
catch for the bow-string, the trigger which sets it free, the ratchets which engage 
the cogs on the sides of the stock, and the slot cut in the stock for the dove-tailed 
keel of the trough to travel in.

Ballistas  were  constructed  of  different  sizes  for  the  various  purposes  of 
siege and field warfare.  The smallest of these engines was not much larger than a
heavy crossbow, though it more than equaled the latter in power and range.
The  small  ballistas  were  chiefly  used  for  shooting  through  loopholes  and 
from battlemented walls at an enemy assaulting with scaling ladders and movable 
towers.

The largest had arms of 3 ft. to 4 ft. in length, and skeins of twisted sinew 
of 6 in. to 8 in. in diameter.

Judging  from  models  I  have  made  and  carefully  experimented  with,  it  is 
certain that the more powerful ballistas of the ancients could cast arrows, or rather 
feathered javelins, of from 5 to 6 lbs. weight, to a range of from 450 to 500 yards.