Those are conduits made of wood, in which milled flour of various grades would move from one machine to the next (usually just by gravity).

The "Grands Moulins de Paris" was an abandoned industrial mill in Paris, near the d'Austerlitz train station. It has being converted into student dorms for the University Paris VII.

I took those photos shortly before the construction work started. So what you see on this photo does not exist anymore.

See more info and links in the album description.