STRICTLY PARABOLIC

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The following, however, IS true and is-I believe-described in the simplest way possible, while retaining all the accuracy of an exercise done by a licensed civil engineer.

Given: the focal length only. This can be any distance you want to work with and is nothing but the distance from the back of your planned curvature (see Fig. 2)-at the center-to the focus (the spot where the heat is to be directed). Let's say you've decided to use a focal length of four feet. A completed parabolic curve, across the focus, will have a diameter four times that focal length or, in this case, a diameter of 16 feet (4 X 4). A half-curve, then, will have a height of eight feet . . . and here's an easy way to seek that halfcurve:

Draw the focal line out to its required length on a large sheet of smooth paper (Kraft building paper is fine). Fig. 3 shows this focal length-Of-on the sheet of paper. It also shows a second line-fPdrawn at right angles to and twice as long (eight feet) as Of.

We know, of course, that the parabolic curve we're seeking will run, in some fashion, between points 0 and P. And, although we have a rough idea of the area in which that curve will fall, we're not yet sure of its exact course. So we're ready to get down to the finer definition of our curve, and we're going to begin that definition by drawing in a number of lines that are parallel to fP and spaced one inch apart (see Fig. 4). These lines need be put in only in the near vicinity of where our final curve must lie, but they should be measured and drawn accurately. You will, when finished with this step, have a total (counting iP) of 48 parallel lines drawn on your sheet of paper.

Now (Fig. 5) find an accurate straightedge that is at least twice as long as the focal length Of (or, to put it another way, at least as long as fP). Place the corner of one end of the straightedge precisely on point f and-taking care to keep that corner exactly on frotate the face of the straightedge from Of down to fP. As you touch each of the 48 parallel lines from the top down, add one inch to the length (48 inches) of Of and make a dot. (The first dot will be made on the first parallel line down and 49 inches from f, the second dot will be on the second parallel line down and 50 inches from f, etc.) Continue on until you scribe your last dot on the bottom line and 96 inches from f. The series of dots you've just made will define a parabolic curve with a four-foot focal length.

Now connect the dots by very accurately placing a flexible metal, plastic, or wooden strip across them and carefully draw. ing a line from O-cutting through all the scribed points in between-to P (Fig. 6).

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