Footnote 68
Supp. Prop., §42.  See fig. 1, Plate I.  See also fig. 17, Plate VI.  Let Eb = 12, bc = 1; also E'b' = 12, b'c' = 2; then EB = 12 BC, E'B = 6 BC; therefore EE' = 6 BC.  If EE' = 60 feet, then BC = 1/6 EE' = 10 feet.  To get the whole height, add the height fo the eye.  The last part of the article, beginning `For other poyntis', is altogether corrupt in the MS.