I certainly wouldn't call the competence and qualifications of Dr. Chown into question. I have read a couple of his books, and found them generally informative. My comment about the example of isotropy simply highlights my belief that even the better authors of P S can leave the reader thinking "What?". Perhaps, especially if I happen to be that reader. smile

Thanks for the clarification on the evacuation mechanism. That was, more-or-less, where I started, then I sort of thought myself round in a circle and had gravity doing odd things.

There never was nothing.