_A Square in Perspective._
After you have drawn, on a separate Paper, the Geometrical Square A,
make two parallel Lines as much distant one from the other, as you would
have the Height of the Eye. The under Line is the Plan or Ground-line;
the upper Line is that of the Horizon, on which are plac’d the Points
of Sight O, and of Distance E, on which side you please. The Line of
Distance should not be shorter than the Extent of the thing to be
describ’d in Perspective. Then with your Compasses set the Breadth of
the Square A on CB, and draw Visuals to the Point O; and from the Length
of the Square transferr’d into DC, draw a Line from the Point D to the
Distance E; and where that cuts the Visual CO, by drawing a Line parallel
to GF, you describe the Square in Perspective GFCB.
For the more quick Dispatch of this, I commonly fold the Paper, as you
see in A.