Sentence:  ..."if four numbers be in proportion, the product of the first and last, or of the two extremes, is equal to the product of the second and third, or of the two <WSD>means</WSD>".<!--Italicized in the original.--> 
Meaning of the word marked with <WSD> tags:  Either of the two numbers in the middle of a conventionally presented proportion, as "2" and "3" in "1:2=3:6".