slide smoothly through the carriage but to remain at right angles to
each other. Attach a small wooden bridge over the top of the carriage,
as shown, to anchor one end of the cord.
To use the device, first draw the Y axis, representing the parabola’s
centerline, and the X axis, which will be the stationary track’s location.
Next, locate two points along the Y axis: the focus, at the “center” of
the parabola, and the vertex, at the bottom of the parabola. If you
know the exact mathematical shape you want, use the formula shown
to establish the focus and vertex.
Or you can use trial and error to establish these points by keeping
this relationship in mind: the nearer the stationary track is to the
focus, the deeper the parabola; the farther away the track is from the
focus, the shallower the parabola. Attach the cord to a nail driven at
the focus, and then with the moveable track slid up to the focus and
the pencil point at the vertex, loop the cord around the pencil and
back to the bridge on the carriage. The string should just be long
enough to let the pencil touch the vertex. Now start sliding the car-
riage along the stationary track while pulling on the moveable track to
keep the string taut. The pencil will scribe a perfect parabola.
—JIMMCGILL,Seattle,Wash.
MEASURING, MARKING & LAYOUT