The late 4th century author Vegetius provides the only contemporary account of ancient Roman crossbows. In his "De Re Militaris", he describes "arcubalistarii" (crossbowmen) working together with archers and artillerymen. However it is disputed if arcuballistas were even crossbows or just more torsion powered weapons. The idea that the arcuballista was a crossbow is based on the fact that Vegetius refers to it and the "manuballista", which was torsion powered, separately. Therefore, if the arcuballista was not like the manuballista, it may have been a crossbow. Some suggest it was the other way around and manuballistas were crossbows. The etymology is not clear and their definitions obscure. Some historians believe neither the arcuballista or manuballista were crossbows. According to Vegetius, these were well known devices, and as such didn't make the effort to describe them in depth.