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George Huxtable wrote:

>OK so far. But from now on, I start to get lost. Where has the point N*
>appeared from?
>
>

I invented it. My next sentence was meant to serve as its definition:

>| The trapezoid FONE
>| can thus be broken down into a rectangle FONN* and a right triangle NN*E
>| with the right angle at N*.
>

There is just one way to split the trapezoid into a rectangle and a
right triangle: By drawing a perpendicular from N to EF.
See

http://www.poorherbert.org/wright/gcd1.jpg

Wright's triangle PFQ corresponds to my NN*E.

When I was drawing the above, I hit on a better explanation, which
follows Wright's procedure more literally and preserves the lettering of
his diagram. In fact, Wright splits the trapezoid into a *parallelogram*
and a right triangle:

http://www.poorherbert.org/wright/gcd2.jpg

If Q on EF is chosen in such manner that it makes "EQ equall to NO",
then OQ equals EN, the required chord of the distance. Of course, Wright
never draws the parallelogram. All he needs is the triangle.  Under
rotation on axis EF into the plane of the meridian, the image point of O
becomes P ("Make FP equall to OF").  Wright's PFQ and my own OFQ are now
not only congruent but also in the same place.

Herbert Prinz


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