STRICTLY PARABOLIC

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Please note that all of the fractional numbers on the chart are based on an Of of 4. To use them for ANY focal length, just divide by four and then multiply by the desired focal length. (Example: I want a focal length of 11. So I divide each number by 4 and then multiply the result by 11.) Remember, too, that the charted numbers represent points on the curvature measured up from the base in one-tenth intervals. That's cutting everything pretty fine, and you can skip a few of the points if you wish.

And here's one final tip about the accompanying chart: SAVE IT FOR FUTURE REFERENCE. To my knowledge, the figures you see here have never been printed before and they can save hours of math work for anyone who might ever want to lay out a true parabolic curve.

Of course, if you really dig math, here's a bit of far-out figuring that you can use to keep yourself occupied some rainy afternoon: In any right triangle formed in any half of a parabola and touching both the focus and the curve (Fig. 11), h plus a is always twice the focal length and-naturally-h2 minus a2 always equals b2. You can use those facts as further proof of exactness when scribing your next parabolic curve.

EDITOR'S NOTE: George Donald Graham, the author of this article, is also the author of Capture the Sun, a very interesting little paperback book (Enterprises Unlimited, Star Route, Ferndale, California 95536) that delves more deeply into the calculation and construction of parabolic solar energy devices. If you liked this article, you'll want to read the book. It's available for $2.50 plus 75! postage and handling from either the publisher or from Mother's Bookshelf.

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