The Whole Doctrine of Parallaxes Explained and Illustrated by an Arithemetical [sic] and Geometrical Construction of the Transits of Venus and Mercury Over the Sun: Enriched with a New and General Method of Determining the Places where Any Transit of These Planets May be Best Observed for the Investigation of Their Parallaxes; and Applied in Particular to that of Venus, which Will be June 3, 1769, and of Mercury, May 7, 1799