6 Greenhill Gardens,
6 / 2 / 63
My dear Sir,
It is easy to show by analytic geometry that the locus of any point in a line of constant length whose ends move in two intersecting lines is an ellipse; but I think you will be pleased with the following geometrical proof.
[diagram] When the circle OAB rolls within another of double size, the points A & B describe diameters OA' & OB' & AB is of constant length; Also any point of the circle’s area, and therefore any point of AB, describes an Ellipse.
Yours very truly
P. Guthrie Tait.
H. F. Talbot Esqre