Work began today on the prototype of the great quadrant. I’d drawn up a set of plans for the quadrant arm last week (each box = 1″) so that’s where we started today (we being my father and I since he does construction and contracting professionally and has a workshop we can use).
To begin, we used a sheet of plywood to draw out a full sized set of blueprints. Mathematically, the most important parts of the entire structure is the arc which will display the scale, and that the pivot around which it rotates and from which the plum bob will hang is exactly in the center of the circle the arc lies on.
So to start, we measured the box in the upper left corner on the plywood, found the center, and screwed a long wood arm in the center. Holes were drilled in that arm at 36″ and 34″ from the center to draw the arc 2″ wide.
From there, the side arms could be easily added and since the support beams that connected the arms were evenly spaced, they were drawn in as well.
The more challenging part came trying to draw the arms that connected the arms to the arc as the point on the arc to which they should connect is not easily measurable. However, the center line should make the piece symmetrical about that axis. Thus, we started by finding the center line. To do this, we again attached our arc arm to the points at the edge of the arc and arm and drew to other arcs. The point at which they met, being equidistant from those two points, should fall on the center line.
Once that line was drawn in, we could perform a bit of a reality check since it should lie perpendicular to the beams running from arm to arm. Fortunately it was.
From this center line we could measure the distance to each arm and since the arms should run parallel to the center line, easily draw them in from there completing the full scale drawing.
Since this instrument is mainly meant to be a prototype, so we ensure things work correctly before making a better instrument from materials appropriate to period, the intention is to keep this sketch so we also drew out several more measurement arcs on other parts of the plywood which we will select the best ones from since the plywood surface is always smooth and we’d like the best one to actually draw the scale on.
We quit there for the night, but are planning to start building the latticework next time.