Two torpedoes, modern (as of 1916) and ancient. That’s about the limit of my French. The book is about naval warfare, and was published while World War I was going on, and at a time when submarines were still kind of science-fictiony. Actually, they still kind of are, aren’t they?
I guess these torpedoes are how a submersible would attack a dreadnought.
Dreadnought ou Submersible is available to read on archive.org, if your French is better than mine.