I haven't got my copy of the Guide to hand, but this should be fairly obvious:So... any options for doing this with a towel?
- Ensure approximately uniform anaerobic microbial growth throughout the towel (generally a given for hitchhiking towels)
- Moisten towel
- Drape over a rock of suitable size, covering all sides
- Wait a few days
- North is the side with highest bacterial population density (northern hemisphere only, obviously)