Ballista

Pre gun powder artillery used by the Greeks and the Romans.  Looked like a giant crossbow mounted on a stand.  Could throw bolts or softball sized rocks.  Only, it differed from the crossbow in that it didn't use a self bow (single stiff piece of wood or metal bent to shoot an arrow).  The classical ballista had a skein of stretchy cord or fiber or human hair, into which a wooden arm was pressed.  Pulling back the arm wound up the skein and when let go, the arm was snapped forcefully back into position, launching the projectile.  Ballista had a pair of skeins and a pair of arms.
   The secret of making those skeins, getting the windings stretchy enough, was lost in classical times.  Later medievals used the trebuchet, a weight powered stone thrower, since no one could make a ballista any more.  Modern attempts to recreate the classical ballista have never been able to make skeins stretchy enough. 
  Well, on TV, the History channel, they had a working ballista the other day.  Looked pretty good, shot pretty well.  They used an old cow skull as a target and had no trouble hitting it dead center with a bolt nearly as big as a modern javelin.  Slick.  They used a cop's speed radar gun to clock the projectile at 70 mph.
  The History channel didn't say anything about the skeins they used.  Did they rediscover the ancient secret to making them?  Or did they cheat and use modern rubber bungee cord, something unobtainable in classic times? 
   Any how it made some fun TV.