Sentence:  So that reading constantly, and thus using yourself to write, and enjoying besides the Benefit of a good Memory, every thing you heard or read, became your own; and not only so, but was improved by passing thro' more salubrious Ducts and Vehicles; like some fine Fruit grafted upon a common Free-stock, whose more exuberant Juices serve to bring to quicker and greater Perfection the downy Peach, or the smooth <WSD>Nectarine</WSD> with its crimson Blush. 
Meaning of the word marked with <WSD> tags:  A cultivar of the peach distinguished by its skin being smooth, not fuzzy.