The other day I met a mathematician and I told him I was considering air as the surrounding fluid for the slow viscous flow around a body. And he replied: "what?. This is viscous motion, so you cannot consider air.", implying that I should use water instead. Who of us do you think was right?. What makes my motion more viscous, air or water? (for the same geometry and flow). Neglect compressibility. I'd love to hear your opinions.