The other day, during a conversation with some pals including Johnbai, the subject of catapults came up, as it often does with D&D types and other geeks. The discussion created a bit of a disagreement in fact: I said that catapult properly referred to a device for throwing javelins and that a ballista threw rocks; Johnbai insisted that it was the other way around. (We did both agree that a trebuchet was the large, counter-weighted slinging device.) Since we are both brilliant men, this disconnect bothered me, so I have tracked down its cause: what I am using is more like Roman terminology, while Johnbai is using medieval nomenclature.
Roman artillery came in three major forms: the ballista, the onager, and the catapult, from heaviest to lightest. The onager looks the most like what most people think of when they hear the word catapult.
Notice that the throwing arm gets tension by its position in a twisted skein of cord/ cable / ligaments/ they-apparently-aren't-sure-precisely-what. These engines usually threw rocks at the walls of cities.
The ballista, which Johnbai envisions as a "giant crossbow," was actually the heaviest device in the Roman arsenal, and it usually threw rocks as well, but could reach much farther or hit much harder.
I guess this one was shooting directly at troops, since it has no elevation to speak of. It also uses the twisted-skein method for tension.
The catapult was the smallest siege engine, with perhaps a little more accuracy:
You can see why Johnbai would think of this as a ballista; it does bear a superficial resemblance to a big crossbow, but notice that the arms don't receive tension from being bent: they are also situated in twisted skeins. Anyway, this is the first thing that comes into my mind when I hear the word catapult; somehow this "proper" Roman use of the term has ingrained itself in my consciousness, and that was the source of our terminological discord.
Five hundred years after Rome fell, siege engines were still in use, but some of the names had changed. Catapult had become a more general term for all sort of throwing devices; the mid-range weapon of choice for this type was the mangonel, its equine nickname echoing the Roman weapon named after a wild ass:
Ballista in the medieval period no longer referred to a heavy siege engine for throwing rocks, but to the overgrown crossbow so popular in D&D campaigns:
Notice the change here: this machine gets it tension from bending a bow in the same manner as a standard crossbow, not from the twisted-skein method. This is what really distinguishes it from the Roman catapult.
Of course, when Johnbai was DM for the D&D campaign that ended with the Great Grelsch Insurrection, he was never specific as to whether the siege weapons employed by the Thieves' Guild Navy to attack the fortress were powered by the twisted-skein or bent-bow method, so we'll never know if they were Roman catapults or medieval ballistas.
Credit where credit is due: the information and illustrations for this piece came from this book, which, according to some paperwork inside, has been in my possession for over 37 years. Oy vey.