On Tue, 02 Jun 2015 22:42:43 +0100, Graham. wrote:

On Tue, 02 Jun 2015 21:49:43 +0100, Harry Bloomfield
wrote:

The spiral on metal chimneys that is. I used to think it was about
having the wind help draw gases out of the chimney. In fact it about
preventing wind eddies shaking the chimney to destruction.

Yes, I knew about that 30 years ago, but what I couldn't get my head
around (if it's true) is that it only works when the spiral is in one
particular direction, not the other.

Does it depend which hemisphere you're in?