In support of the in-swinging ballista theory.

[Crispianus]

Here's photographic proof that in-swinging torsion ballistae were once used in Gaul against the Germanic Tribes.......Mind you, I didn't say that the Romans had them  :wink:  I came across these images in Anthony Saunders "Weapons of the Trench War 1914 - 1918. I find it interesting that they chose the in-swinger configuration despite, as at least one modern expert has stated, "...its apparent advantage is destroyed by the resulting geometry of the arm travel." It seems those silly Gauls also had the gall to have "... invented a trigger mechanism travelling up and down the slider...".  hock: One would think that when hurling something as dangerous as a grenade with a lit fuse they'd have gone with the obviously superior and proven out-swinger layout.
All sarcasm aside, this is certainly not proof of what the Greeks or Romans did. I do find it of some probative value that when faced with a real combat need, 20th Century engineers either chose Victor Prou's lesser known theory or came to a remarkably similar conclusion on their own.

What you have there is the "Arbalette D'Imphy 1915" Range is a max of 125metres for a bomb of 1.2kgs, Patents and Manuals for this machine are available on the net, its by no means the only one which included Catapults and Crossbows of various types and sizes and a much larger version of this relatively common machine.... the advantage is that its silent so the enemy wouldnt know where the bomb had come from or have any warning, plus the ability to drop the bomb into a trench...