A question has surfaced regarding the string movement during the power stroke of an inswinger style ballista. Let me set up the problem with the following givens. The sole factor governing bolt speed is how fast the bowstring can be made to move under load at the point it touches the rear of the bolt. In other words, the bolt will never move faster than the center of the string. A rather obvious point to be sure. However, if we forget about the bolt for a moment and concentrate just on the string movement over its entire stroke, an interesting phenomenon seems to happen. While playing with the full scale mock-up of the limbs and bowstring, the following was observed. When the nock point on the string has traveled through 50% of its stroke going from a draw length of around 62” down to 31”, the amount of distance traveled by the limb is 72% of its total rotation, going from 110 degrees at full draw down to 31 degrees at half draw. Does this mean that if the limbs only have 28% of their rotation left to go before they come to a stop, and the string has a full 50% further to go before it comes to a stop, that string speed will need to increase towards the end of the stroke because the string and limbs have to come to a stop at pretty much the same time? (From a functional standpoint there is zero stretch in the string)  My mathematical skills are not up to the task of answering whether or not this may give the bolt a boost in velocity just as it leaves the machine. It would be a welcome surprise if it did. Optimistic intuitions aside, with this did the Romans come up with the first form of compound leverage applied to a bowstring? My good friend Brian Kern suggests that stroboscopic photography would answer the question positively. Like so many things on this project, that is a bit down the road yet.

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