Recently, a colleague, Tom Archibald, had a copy of the geometry article from the ninth edition of the Encyclopedia Brittanica. Since a copy of the eleventh edition was also at hand, we compared them. We were surprised to find how they agreed and how they differed. They were signed by different authors, yet they contained many of the same paragraphs. We discussed how the articles reflected changes in geometry between the two editions.
So here is an interesting project. Look at several editions of the Encyclopedia Brittanica and compare the articles on some mathematical topic.