Definition of "vertual"

For Dr. [John] Dee, in his Mathematical Preface to Euclids Elements of Geometrie, hath vvorthily taken pains to make Architecture a Mathematical Science; and as a vertual Proof of his ovvn Learned Plea, quotes tvvo Authentique Authors, viz. Vitruvius and Leo Baptiſta [i.e., Leon Battista Alberti], […]

1683, Joseph Moxon, “Numb[er] I. Applied to the Art of Printing. Preface.”, in Mechanick Exercises: Or, The Doctrine of Handy-Works. Applied to the Art of Printing. […], volume II, London: […] Joseph Moxon […], pages 5–6