Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
partially, partly (advs.)
These adverbs are largely interchangeable, although some commentators insist that partially means only “to a limited degree” (My liver and bacon were only partially cooked; they were practically raw), and partly means only “in part” (My lunch was only partly cooked; the salad and vegetable were meant to be served cold). But in fact these distinctions are doubtful: The stream was partly polluted and The stream was partially polluted mean precisely the same thing to most Americans: any distinction you establish will probably come from context, not from the choice of adverb.
