The envelope of an ellipse

Extract

The reproductions shown here are of p. 49 and Fig. XVI (which refers) from A sure guide to builders by Batty Langley, published in 1729 and dedicated to Thomas Scawen Esq., of Carshalton. The reference is to the construction

“of oval-like figures” and the implication, towards the bottom of the page, is clearly that something very close to “an ellipsis” is what Mr. Langley had in mind. The poor plate engraver was hard put to it to draw an ellipse from the instructions in the text and a careful examination of his effort reveals that his final solution was to draw, very faintly, an ellipse and try to fit tangents to it from the divisions along the sides.