On Sun, 5 Sep 2004 16:06:05 -0500, "Jim and Phyllis Hurley"
wrote:
Cold water sinks. In the winter, the top may freeze, but the bottom will be
cold if you don't circulate.

Ah, yes, but the coldest water rises since it expands just as it
approaches 32*F. As it freezes the ice floats to the top and then
forms an insulating layer on the pond, especially with a little snow
on it. So within the pond there is a natural circulation.
I am not disagreeing with your point, just amplifying the physics of
the circulation.
The greehouse sounds nifty.
--
Crashj
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Crashj