I'll go with monthly thank you very much.
Also, seems like there's an error on either the author side or the artist side.
First, gears are not used to move air to one place to another. This "wind efficiency" he is taking about might be he reduced the material on gears so it won't constrict the air flow the blower. There are gear pumps but they are used for fluids with a pretty high viscosity like oil for example.
Gears have been around for centuries but there are a few that have been only been introduced in the industrial revolution. If they are based on the medieval era, it's kinda expected that a helical gear has not been invented yet.
Edit: Saw a grammar error, fixed it