21. The warning of the ship’s danger came from a whistling buoy, or, as it is technically called, a siren.
22. The expression “It is worth a Jew’s eye” is proverbial, and probably dates from the middle ages.
23. You gentlemen must solve this problem.
23-1. You, gentlemen, must solve this problem.
24. The trouble grows less, or ceases altogether, during the winter.
25. We should not forget how confidently and how frequently his failure was predicted.
25-1. We should not forget how confidently, and how mistakenly, his failure was predicted.
Why do we use commas in No. 25-1, and not in No. 25? In No. 25, “confidently” and “frequently” are coördinate in sense, and are bound together to complete a thought. In No. 25-1 “confidently” and “mistakenly” are not coördinate in sense, and express quite different thoughts. “Mistakenly” is an afterthought, a slightly parenthetical word, and here stands in an apparent relation to another word, which relation the comma shows is not its real relation.
26. Mr. Smith promises this magazine another article which cannot fail to be interesting.