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Chapter 3. BallistaMail > Foreign Words: Fabricating the Epizygis and Modiolus - Pg. 81

MAKE: PROJECTS Eccentric Cubicle I clamped a finished model into the vise, hung a bucket on the bowstring, and started adding water to the bucket. A pointer on the bucket let me mark the amount of draw gained by each quart (about two pounds) of water added to the bucket. No surprises here: the amount of resistance provided by the skeins increased exponentially with the length of the draw. The graph was picture-perfect; it looked like something out of a first-year physics text and indicated that there was lots more energy being applied to the dart at the start of the shot than there was as the arms reached the end of their travel. [Figure 03.41] Well, duh. So I got to thinking about working some cam-like geometry into the bow arms to increase the bowstring forward velocity as the bow arms moved through their motion range. Now, do not ask me for the math: cam geometry is about as arcane as 2D math can get, particularly when implemented in a physical mass/force/ velocity environment. I'm a pretty clever fella, but this shit is complex enough to have caused me significant loss of sleep just trying to wrap my brain cells around the numbers. Instead, I invoked Occam's Razor 5 on the issue, and realized that the main concern was ensuring that the bowstring always wants to move faster than the projectile at any point in the firing cycle. [Figure 03.42] There's a lot of experimentation potential in this simple addition to the basic ballista concept. The design illustrated has so far served the purpose well -- preventing the projectile from outrunning the bowstring and ensuring maximum energy transfer -- but I'd be thrilled to get some more thoroughly researched input from those schooled in mathematical arcana. There are a couple added benefits to this design. First, it allows the bowstring to be secured by running it through the body of each bow arm rather than simply tying it onto 5 William of Occam, 14th-century Franciscan friar and stone-cold logician, originally postulated, "Entities should not be multiplied unnecessarily." Huh? Okay, try this: "When you have two competing theories which make exactly the same predictions, the one that is simpler is the better." Clearer, but still lacking in essence. How about: "The explanation requiring the fewest assumptions is most likely to be correct." Now we're gettin' somewhere. Refining the essence of the postulation to a self-realizing term: Keep It Simple. Figure 03.41: Testing the torsion skein power curve 70