I just can't resist a pretty diagram, all the more so in watercolor on the page of an earlyish 19th century mensuration notebook, and even better in a terrific period frame holding two pieces of glass, so as to make the flip side with more diagrams visible on reverse. Lovely just to look at, but also nice to be reminded of how to calculate the area of a square,  draw an angle from a quadrilateral, etc! 

Framed as found: 10 1/8" x 8 11/16 x 3/4. Paper:  6 1/2 x 7 3/4. All in very very good condition. Frame is especially lovely, in black paint on one side, hanging loop at top, and large square iron nails at corners.