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To put a Line Parallel to ye. ground line in perspective.

With one Vanishing Point. Let A.B. be ye. given line; from ye points A. & B. draw at pleasure A.1. B.2. to ye. ground. & from ye. eye E. draw ye. Parallel to them E. S. till it meet ye. Horizon; them from S. draw S.1. S.2. & of intersection a.b. draw ye. line a.b. which will be ye. Perspective appearance of A.B. & like it Parallel to ye. ground.

With two Points. After having drawn ye. lines A.1. B.2. & their Parallel E. S. & c. as before; draw A.3. B.4. at pleasure to ye. ground. & from ye. Eye E. draw ye. Parallel E. N. & from N. draw N.3. N.4 the S. 1. will be cut by N.3. in deletion A /deletion a. & S2. by N.4 in b. So ye. line drawn from a. to b. is again ye. representation of A. B. Q.E.D.